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Human evolution is the
evolution Evolution is change in the heritable characteristics of biological populations over successive generations. These characteristics are the expressions of genes, which are passed on from parent to offspring during reproduction. Variation ...
ary process within the
history History (derived ) is the systematic study and the documentation of the human activity. The time period of event before the History of writing#Inventions of writing, invention of writing systems is considered prehistory. "History" is an umbr ...
of
primate Primates are a diverse order (biology), order of mammals. They are divided into the Strepsirrhini, strepsirrhines, which include the lemurs, galagos, and lorisids, and the Haplorhini, haplorhines, which include the Tarsiiformes, tarsiers and ...
s that led to the emergence of ''
Homo sapiens Humans (''Homo sapiens'') are the most abundant and widespread species of primate, characterized by bipedalism and exceptional cognitive skills due to a large and complex brain. This has enabled the development of advanced tools, culture, ...
'' as a distinct species of the
hominid The Hominidae (), whose members are known as the great apes or hominids (), are a taxonomic family of primates that includes eight extant species in four genera: '' Pongo'' (the Bornean, Sumatran and Tapanuli orangutan); ''Gorilla'' (the e ...
family, which includes the great apes. This process involved the gradual development of traits such as human bipedalism and
language Language is a structured system of communication. The structure of a language is its grammar and the free components are its vocabulary. Languages are the primary means by which humans communicate, and may be conveyed through a variety of ...
, as well as interbreeding with other
hominins The Hominini form a taxonomic tribe of the subfamily Homininae ("hominines"). Hominini includes the extant genera ''Homo'' (humans) and '' Pan'' (chimpanzees and bonobos) and in standard usage excludes the genus ''Gorilla'' (gorillas). The t ...
, which indicate that human evolution was not linear but a web.Human Hybrids
(PDF). Michael F. Hammer. ''Scientific American'', May 2013.
The study of human evolution involves several scientific disciplines, including
physical anthropology Biological anthropology, also known as physical anthropology, is a scientific discipline concerned with the biological and behavioral aspects of human beings, their extinct hominin ancestors, and related non-human primates, particularly from an e ...
,
evolutionary anthropology Evolutionary anthropology, the interdisciplinary study of the evolution of human physiology and human behaviour and of the relation between hominids and non-hominid primates, builds on natural science and on social science. Various fields ...
,
primatology Primatology is the scientific study of primates. It is a diverse discipline at the boundary between mammalogy and anthropology, and researchers can be found in academic departments of anatomy, anthropology, biology, medicine, psychology, veter ...
,
archaeology Archaeology or archeology is the scientific study of human activity through the recovery and analysis of material culture. The archaeological record consists of Artifact (archaeology), artifacts, architecture, biofact (archaeology), biofacts ...
,
paleontology Paleontology (), also spelled palaeontology or palæontology, is the scientific study of life that existed prior to, and sometimes including, the start of the Holocene epoch (roughly 11,700 years before present). It includes the study of fos ...
,
neurobiology Neuroscience is the scientific study of the nervous system (the brain, spinal cord, and peripheral nervous system), its functions and disorders. It is a multidisciplinary science that combines physiology, anatomy, molecular biology, developmen ...
,
ethology Ethology is the scientific study of animal behaviour, usually with a focus on behaviour under natural conditions, and viewing behaviour as an evolutionarily adaptive trait. Behaviourism as a term also describes the scientific and objective ...
,
linguistics Linguistics is the scientific study of human language. It is called a scientific study because it entails a comprehensive, systematic, objective, and precise analysis of all aspects of language, particularly its nature and structure. Lingu ...
,
evolutionary psychology Evolutionary psychology is a theoretical approach in psychology that examines cognition and behavior from a modern evolutionary perspective. It seeks to identify human psychological adaptations with regards to the ancestral problems they evol ...
,
embryology Embryology (from Greek ἔμβρυον, ''embryon'', "the unborn, embryo"; and -λογία, '' -logia'') is the branch of animal biology that studies the prenatal development of gametes (sex cells), fertilization, and development of embryos an ...
, and
genetics Genetics is the study of genes, genetic variation, and heredity in organisms.Hartl D, Jones E (2005) It is an important branch in biology because heredity is vital to organisms' evolution. Gregor Mendel, a Moravian Augustinian friar worki ...
. Genetic studies show that primates diverged from other mammals about , in the
Late Cretaceous The Late Cretaceous (100.5–66 Ma) is the younger of two epochs into which the Cretaceous Period is divided in the geologic time scale. Rock strata from this epoch form the Upper Cretaceous Series. The Cretaceous is named after ''creta'', ...
period, and the earliest
fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...
s appear in the
Paleocene The Paleocene, ( ) or Palaeocene, is a geological epoch that lasted from about 66 to 56 million years ago (mya). It is the first epoch of the Paleogene Period in the modern Cenozoic Era. The name is a combination of the Ancient Greek ''pal ...
, around . Within the superfamily
Hominoidea Apes (collectively Hominoidea ) are a clade of Old World simians native to sub-Saharan Africa and Southeast Asia (though they were more widespread in Africa, most of Asia, and as well as Europe in prehistory), which together with its sister ...
, the family
Hominidae The Hominidae (), whose members are known as the great apes or hominids (), are a taxonomic family of primates that includes eight extant species in four genera: '' Pongo'' (the Bornean, Sumatran and Tapanuli orangutan); '' Gorilla'' (the ...
(great apes) diverged from the family
Hylobatidae Gibbons () are apes in the Family (biology), family Hylobatidae (). The family historically contained one genus, but now is split into four extant genera and 20 species. Gibbons live in subtropical and tropical rainforest from eastern Bangladesh ...
(
gibbon Gibbons () are apes in the family Hylobatidae (). The family historically contained one genus, but now is split into four extant genera and 20 species. Gibbons live in subtropical and tropical rainforest from eastern Bangladesh to Northeast Indi ...
s) some 15–20 million years ago; subfamily
Homininae Homininae (), also called "African hominids" or "African apes", is a subfamily of Hominidae. It includes two tribes, with their extant as well as extinct species: 1) the tribe Hominini (with the genus ''Homo'' including modern humans and numer ...
(African apes) diverged from
Ponginae Ponginae , also known as the Asian hominids, is a subfamily in the family Hominidae. Once a diverse lineage of Eurasian apes, the subfamily has only one extant genus, '' Pongo'' (orangutans), which contains three extant species; the Sumatran ora ...
(
orangutan Orangutans are great apes native to the rainforests of Indonesia and Malaysia. They are now found only in parts of Borneo and Sumatra, but during the Pleistocene they ranged throughout Southeast Asia and South China. Classified in the gen ...
s) about 14 million years ago; the tribe Hominini (including humans, ''Australopithecus'', and chimpanzees) parted from the tribe
Gorillini Gorillini is a taxonomic tribe containing three genera: ''Gorilla'' and the extinct ''Chororapithecus ''Chororapithecus'' is an extinct great ape from the Afar region of Ethiopia roughly 8 million years ago during the Late Miocene, comprising ...
(
gorilla Gorillas are herbivorous, predominantly ground-dwelling great apes that inhabit the tropical forests of equatorial Africa. The genus ''Gorilla'' is divided into two species: the eastern gorilla and the western gorilla, and either four ...
s) between 8–9 million years ago; and, in turn, the subtribes
Hominina Australopithecina or Hominina is a subtribe in the tribe Hominini. The members of the subtribe are generally ''Australopithecus'' ( cladistically including the genera ''Homo'', ''Paranthropus'', and '' Kenyanthropus''), and it typically include ...
(humans and extinct biped ancestors) and
Panina The genus ''Pan'' consists of two extant species: the chimpanzee and the bonobo. Taxonomically, these two ape species are collectively termed panins; however, both species are more commonly referred to collectively using the generalized ter ...
( chimpanzees) separated 4–7 million years ago.
Anatomically modern human Anatomy () is the branch of biology concerned with the study of the structure of organisms and their parts. Anatomy is a branch of natural science that deals with the structural organization of living things. It is an old science, having it ...
s appeared in Africa approximately 300,000 years ago.


Anatomical changes

Human evolution from its first separation from the last common ancestor of humans and chimpanzees is characterized by a number of morphological,
developmental Development of the human body is the process of growth to maturity. The process begins with fertilization, where an egg released from the ovary of a female is penetrated by a sperm cell from a male. The resulting zygote develops through mito ...
,
physiological Physiology (; ) is the scientific study of functions and mechanisms in a living system. As a sub-discipline of biology, physiology focuses on how organisms, organ systems, individual organs, cells, and biomolecules carry out the chemica ...
,
behavioral Behavior (American English) or behaviour (British English) is the range of actions and mannerisms made by individuals, organisms, systems or artificial entities in some environment. These systems can include other systems or organisms as we ...
, and environmental changes. Environmental (cultural) evolution discovered much later during the Pleistocene played a significant role in human evolution observed via human transitions between subsistence systems. The most significant of these adaptations are bipedalism, increased brain size, lengthened
ontogeny Ontogeny (also ontogenesis) is the origination and development of an organism (both physical and psychological, e.g., moral development), usually from the time of fertilization of the egg to adult. The term can also be used to refer to the stu ...
(gestation and infancy), and decreased
sexual dimorphism Sexual dimorphism is the condition where the sexes of the same animal and/or plant species exhibit different morphological characteristics, particularly characteristics not directly involved in reproduction. The condition occurs in most an ...
. The relationship between these changes is the subject of ongoing debate. Other significant morphological changes included the evolution of a power and precision grip, a change first occurring in '' H. erectus''.


Bipedalism

Bipedal Bipedalism is a form of terrestrial locomotion where an organism moves by means of its two rear limbs or legs. An animal or machine that usually moves in a bipedal manner is known as a biped , meaning 'two feet' (from Latin ''bis'' 'double' ...
ism is the basic adaptation of the hominid and is considered the main cause behind a suite of skeletal changes shared by all bipedal hominids. The earliest hominin, of presumably primitive bipedalism, is considered to be either ''
Sahelanthropus ''Sahelanthropus tchadensis'' is an extinct species of the Homininae (African apes) dated to about , during the Miocene epoch. The species, and its genus ''Sahelanthropus'', was announced in 2002, based mainly on a partial cranium, nicknamed '' ...
''} or ''
Orrorin ''Orrorin tugenensis'' is a postulated early species of Homininae, estimated at and discovered in 2000. It is not confirmed how ''Orrorin'' is related to modern humans. Its discovery was used to argue against the hypothesis that australopitheci ...
'', both of which arose some 6 to 7 million years ago. The non-bipedal knuckle-walkers, the gorillas and chimpanzees, diverged from the hominin line over a period covering the same time, so either ''Sahelanthropus'' or ''Orrorin'' may be our last shared ancestor. ''
Ardipithecus ''Ardipithecus'' is a genus of an extinct hominine that lived during the Late Miocene and Early Pliocene epochs in the Afar Depression, Ethiopia. Originally described as one of the earliest ancestors of humans after they diverged from the chi ...
'', a full biped, arose approximately 5.6 million years ago. The early bipeds eventually evolved into the australopithecines and still later into the genus ''Homo''. There are several theories of the adaptation value of bipedalism. It is possible that bipedalism was favored because it freed the hands for reaching and carrying food, saved energy during locomotion, enabled long-distance running and hunting, provided an enhanced field of vision, and helped avoid hyperthermia by reducing the surface area exposed to direct sun; features all advantageous for thriving in the new savanna and woodland environment created as a result of the East African Rift Valley uplift versus the previous closed forest habitat. A 2007 study provides support for the hypothesis that walking on two legs, or bipedalism, evolved because it used less energy than quadrupedal knuckle-walking. However, recent studies suggest that bipedality without the ability to use fire would not have allowed global dispersal. This change in gait saw a lengthening of the legs proportionately when compared to the length of the arms, which were shortened through the removal of the need for
brachiation Brachiation (from "brachium", Latin for "arm"), or arm swinging, is a form of arboreal locomotion in which primates swing from tree limb to tree limb using only their arms. During brachiation, the body is alternately supported under each forelimb ...
. Another change is the shape of the big toe. Recent studies suggest that australopithecines still lived part of the time in trees as a result of maintaining a grasping big toe. This was progressively lost in habilines. Anatomically, the evolution of bipedalism has been accompanied by a large number of skeletal changes, not just to the legs and pelvis, but also to the
vertebral column The vertebral column, also known as the backbone or spine, is part of the axial skeleton. The vertebral column is the defining characteristic of a vertebrate in which the notochord (a flexible rod of uniform composition) found in all chordate ...
, feet and ankles, and skull. The
femur The femur (; ), or thigh bone, is the proximal bone of the hindlimb in tetrapod vertebrates. The head of the femur articulates with the acetabulum in the pelvic bone forming the hip joint, while the distal part of the femur articulates wit ...
evolved into a slightly more angular position to move the center of gravity toward the geometric center of the body. The knee and ankle joints became increasingly robust to better support increased weight. To support the increased weight on each vertebra in the upright position, the human vertebral column became S-shaped and the
lumbar vertebrae The lumbar vertebrae are, in human anatomy, the five vertebrae between the rib cage and the pelvis. They are the largest segments of the vertebral column and are characterized by the absence of the foramen transversarium within the transverse ...
became shorter and wider. In the feet the big toe moved into alignment with the other toes to help in forward locomotion. The arms and forearms shortened relative to the legs making it easier to run. The
foramen magnum The foramen magnum ( la, great hole) is a large, oval-shaped opening in the occipital bone of the skull. It is one of the several oval or circular openings (foramina) in the base of the skull. The spinal cord, an extension of the medulla oblonga ...
migrated under the skull and more anterior. The most significant changes occurred in the pelvic region, where the long downward facing
iliac blade The crest of the ilium (or iliac crest) is the superior border of the wing of ilium and the superiolateral margin of the greater pelvis. Structure The iliac crest stretches posteriorly from the anterior superior iliac spine (ASIS) to the posterior ...
was shortened and widened as a requirement for keeping the center of gravity stable while walking; bipedal hominids have a shorter but broader, bowl-like pelvis due to this. A drawback is that the birth canal of bipedal apes is smaller than in knuckle-walking apes, though there has been a widening of it in comparison to that of australopithecine and modern humans, thus permitting the passage of newborns due to the increase in cranial size. This is limited to the upper portion, since further increase can hinder normal bipedal movement. The shortening of the pelvis and smaller birth canal evolved as a requirement for bipedalism and had significant effects on the process of human birth, which is much more difficult in modern humans than in other primates. During human birth, because of the variation in size of the pelvic region, the fetal head must be in a transverse position (compared to the mother) during entry into the birth canal and rotate about 90 degrees upon exit. The smaller birth canal became a limiting factor to brain size increases in early humans and prompted a shorter gestation period leading to the relative immaturity of human offspring, who are unable to walk much before 12 months and have greater
neoteny Neoteny (), also called juvenilization,Montagu, A. (1989). Growing Young. Bergin & Garvey: CT. is the delaying or slowing of the physiological, or somatic, development of an organism, typically an animal. Neoteny is found in modern humans compar ...
, compared to other primates, who are mobile at a much earlier age. The increased brain growth after birth and the increased dependency of children on mothers had a major effect upon the female reproductive cycle, and the more frequent appearance of
alloparenting Alloparenting (also referred to as alloparental care) is a term used to classify any form of parental care provided by an individual towards young that aren't its own direct offspring. These are often referred to as "non-descendant" young, even th ...
in humans when compared with other hominids. Delayed human sexual maturity also led to the evolution of
menopause Menopause, also known as the climacteric, is the time in women's lives when menstrual periods stop permanently, and they are no longer able to bear children. Menopause usually occurs between the age of 47 and 54. Medical professionals often d ...
with one explanation, the
grandmother hypothesis The grandmother hypothesis is a hypothesis to explain the existence of menopause in human life history by identifying the adaptive value of extended kin networking. It builds on the previously postulated " mother hypothesis" which states that as ...
, providing that elderly women could better pass on their genes by taking care of their daughter's offspring, as compared to having more children of their own.


Encephalization

The human species eventually developed a much larger brain than that of other primates—typically in modern humans, nearly three times the size of a chimpanzee or gorilla brain. After a period of stasis with ''Australopithecus anamensis'' and ''Ardipithecus'', species which had smaller brains as a result of their bipedal locomotion, the pattern of
encephalization Encephalization quotient (EQ), encephalization level (EL), or just encephalization is a relative brain size measure that is defined as the ratio between observed to predicted brain mass for an animal of a given size, based on nonlinear regressi ...
started with ''Homo habilis'', whose brain was slightly larger than that of chimpanzees. This evolution continued in ''Homo erectus'' with , and reached a maximum in Neanderthals with , larger even than modern ''Homo sapiens''. This brain increase manifested during postnatal brain growth, far exceeding that of other apes (
heterochrony In evolutionary developmental biology, heterochrony is any genetically controlled difference in the timing, rate, or duration of a developmental process in an organism compared to its ancestors or other organisms. This leads to changes in the si ...
). It also allowed for extended periods of social learning and
language acquisition Language acquisition is the process by which humans acquire the capacity to perceive and comprehend language (in other words, gain the ability to be aware of language and to understand it), as well as to produce and use words and sentences to ...
in juvenile humans, beginning as much as 2 million years ago. Encephalization may be due to a dependency on calorie-dense, difficult-to-acquire food. Furthermore, the changes in the structure of
human brain The human brain is the central organ (anatomy), organ of the human nervous system, and with the spinal cord makes up the central nervous system. The brain consists of the cerebrum, the brainstem and the cerebellum. It controls most of the act ...
s may be even more significant than the increase in size. The
temporal lobe The temporal lobe is one of the four major lobes of the cerebral cortex in the brain of mammals. The temporal lobe is located beneath the lateral fissure on both cerebral hemispheres of the mammalian brain. The temporal lobe is involved in proc ...
s, which contain centers for language processing, have increased disproportionately, as has the
prefrontal cortex In mammalian brain anatomy, the prefrontal cortex (PFC) covers the front part of the frontal lobe of the cerebral cortex. The PFC contains the Brodmann areas BA8, BA9, BA10, BA11, BA12, BA13, BA14, BA24, BA25, BA32, BA44, BA45, BA ...
, which has been related to complex decision-making and moderating social behavior. Encephalization has been tied to increased starches and meat in the diet, however a 2022 meta study called into question the role of meat. Other factors are the development of cooking, and it has been proposed that intelligence increased as a response to an increased necessity for solving social problems as human society became more complex. Changes in skull morphology, such as smaller mandibles and mandible muscle attachments, allowed more room for the brain to grow. The increase in volume of the
neocortex The neocortex, also called the neopallium, isocortex, or the six-layered cortex, is a set of layers of the mammalian cerebral cortex involved in higher-order brain functions such as sensory perception, cognition, generation of motor commands, ...
also included a rapid increase in size of the
cerebellum The cerebellum (Latin for "little brain") is a major feature of the hindbrain of all vertebrates. Although usually smaller than the cerebrum, in some animals such as the mormyrid fishes it may be as large as or even larger. In humans, the cere ...
. Its function has traditionally been associated with balance and fine motor control, but more recently with
speech Speech is a human vocal communication using language. Each language uses phonetic combinations of vowel and consonant sounds that form the sound of its words (that is, all English words sound different from all French words, even if they are th ...
and
cognition Cognition refers to "the mental action or process of acquiring knowledge and understanding through thought, experience, and the senses". It encompasses all aspects of intellectual functions and processes such as: perception, attention, thoug ...
. The great apes, including hominids, had a more pronounced cerebellum relative to the neocortex than other primates. It has been suggested that because of its function of sensory-motor control and learning complex muscular actions, the cerebellum may have underpinned human technological adaptations, including the preconditions of speech. The immediate survival advantage of encephalization is difficult to discern, as the major brain changes from ''Homo erectus'' to ''Homo heidelbergensis'' were not accompanied by major changes in technology. It has been suggested that the changes were mainly social and behavioural, including increased empathic abilities, increases in size of social groups, and increased behavioral plasticity. Humans are unique in the ability to acquire information through social transmission and adapt that information. The emerging field of
cultural evolution Cultural evolution is an evolutionary theory of social change. It follows from the definition of culture as "information capable of affecting individuals' behavior that they acquire from other members of their species through teaching, imitation ...
studies human sociocultural change from an evolutionary perspective.


Sexual dimorphism

The reduced degree of sexual dimorphism in humans is visible primarily in the reduction of the male
canine tooth In mammalian oral anatomy, the canine teeth, also called cuspids, dog teeth, or (in the context of the upper jaw) fangs, eye teeth, vampire teeth, or vampire fangs, are the relatively long, pointed teeth. They can appear more flattened however ...
relative to other ape species (except gibbons) and reduced brow ridges and general robustness of males. Another important physiological change related to
sexuality Human sexuality is the way people experience and express themselves sexually. This involves biological, psychological, physical, erotic, emotional, social, or spiritual feelings and behaviors. Because it is a broad term, which has varied wi ...
in humans was the evolution of hidden estrus. Humans are the only hominoids in which the female is fertile year round and in which no special signals of fertility are produced by the body (such as genital swelling or overt changes in proceptivity during estrus). Nonetheless, humans retain a degree of sexual dimorphism in the distribution of body hair and subcutaneous fat, and in the overall size, males being around 15% larger than females. These changes taken together have been interpreted as a result of an increased emphasis on
pair bond In biology, a pair bond is the strong affinity that develops in some species between a mating pair, often leading to the production and rearing of offspring and potentially a lifelong bond. Pair-bonding is a term coined in the 1940s that is freque ...
ing as a possible solution to the requirement for increased parental investment due to the prolonged infancy of offspring.


Ulnar opposition

The ulnar opposition—the contact between the thumb and the tip of the
little finger The little finger, or pinkie, also known as the baby finger, fifth digit, or pinky finger, is the most ulnar and smallest digit of the human hand, and next to the ring finger. Etymology The word "pinkie" is derived from the Dutch word ''pi ...
of the same hand—is unique to the genus ''Homo'', including Neanderthals, the Sima de los Huesos
hominins The Hominini form a taxonomic tribe of the subfamily Homininae ("hominines"). Hominini includes the extant genera ''Homo'' (humans) and '' Pan'' (chimpanzees and bonobos) and in standard usage excludes the genus ''Gorilla'' (gorillas). The t ...
and
anatomically modern humans Early modern human (EMH) or anatomically modern human (AMH) are terms used to distinguish ''Homo sapiens'' (the only extant Hominina species) that are anatomically consistent with the range of phenotypes seen in contemporary humans from extin ...
. In other primates, the thumb is short and unable to touch the little finger. The ulnar opposition facilitates the precision grip and power grip of the human hand, underlying all the skilled manipulations.


Other changes

A number of other changes have also characterized the evolution of humans, among them an increased reliance on vision rather than smell (highly reduced
olfactory bulb The olfactory bulb (Latin: ''bulbus olfactorius'') is a neural structure of the vertebrate forebrain involved in olfaction, the sense of smell. It sends olfactory information to be further processed in the amygdala, the orbitofrontal cortex (O ...
); a longer juvenile developmental period and higher infant dependency; a smaller gut and small, misaligned teeth; faster basal metabolism; loss of body hair; evolution of sweat glands; a change in the shape of the dental arcade from u-shaped to parabolic; development of a
chin The chin is the forward pointed part of the anterior mandible ( mental region) below the lower lip. A fully developed human skull has a chin of between 0.7 cm and 1.1 cm. Evolution The presence of a well-developed chin is considered to be one ...
(found in ''Homo sapiens'' alone); styloid processes; and a descended larynx.


History of study


Before Darwin

The word ''homo'', the name of the biological genus to which humans belong, is
Latin Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through the power ...
for "human". It was chosen originally by
Carl Linnaeus Carl Linnaeus (; 23 May 1707 – 10 January 1778), also known after his ennoblement in 1761 as Carl von Linné Blunt (2004), p. 171. (), was a Swedish botanist, zoologist, taxonomist, and physician who formalised binomial nomenclature, ...
in his classification system. The word "human" is from the Latin ''humanus'', the adjectival form of ''homo''. The Latin "homo" derives from the
Indo-European The Indo-European languages are a language family native to the overwhelming majority of Europe, the Iranian plateau, and the northern Indian subcontinent. Some European languages of this family, English, French, Portuguese, Russian, ...
root *''dhghem'', or "earth". Linnaeus and other scientists of his time also considered the great apes to be the closest relatives of humans based on morphological and
anatomical Anatomy () is the branch of biology concerned with the study of the structure of organisms and their parts. Anatomy is a branch of natural science that deals with the structural organization of living things. It is an old science, having its ...
similarities.


Darwin

The possibility of linking humans with earlier apes by descent became clear only after 1859 with the publication of
Charles Darwin Charles Robert Darwin ( ; 12 February 1809 – 19 April 1882) was an English natural history#Before 1900, naturalist, geologist, and biologist, widely known for his contributions to evolutionary biology. His proposition that all speci ...
's ''
On the Origin of Species ''On the Origin of Species'' (or, more completely, ''On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life''),The book's full original title was ''On the Origin of Species by Me ...
'', in which he argued for the idea of the evolution of new species from earlier ones. Darwin's book did not address the question of human evolution, saying only that "Light will be thrown on the origin of man and his history." The first debates about the nature of human evolution arose between
Thomas Henry Huxley Thomas Henry Huxley (4 May 1825 – 29 June 1895) was an English biologist and anthropologist specialising in comparative anatomy. He has become known as "Darwin's Bulldog" for his advocacy of Charles Darwin's theory of evolution. The stor ...
and
Richard Owen Sir Richard Owen (20 July 1804 – 18 December 1892) was an English biologist, comparative anatomist and paleontologist. Owen is generally considered to have been an outstanding naturalist with a remarkable gift for interpreting fossils. Ow ...
. Huxley argued for human evolution from apes by illustrating many of the similarities and differences between humans and other apes, and did so particularly in his 1863 book ''
Evidence as to Man's Place in Nature ''Evidence as to Man's Place in Nature'' is an 1863 book by Thomas Henry Huxley, in which he gives evidence for the evolution of humans and apes from a common ancestor. It was the first book devoted to the topic of human evolution, and discusse ...
''. Many of Darwin's early supporters (such as Alfred Russel Wallace and
Charles Lyell Sir Charles Lyell, 1st Baronet, (14 November 1797 – 22 February 1875) was a Scottish geologist who demonstrated the power of known natural causes in explaining the earth's history. He is best known as the author of '' Principles of Geol ...
) did not initially agree that the origin of the mental capacities and the moral sensibilities of humans could be explained by
natural selection Natural selection is the differential survival and reproduction of individuals due to differences in phenotype. It is a key mechanism of evolution, the change in the heritable traits characteristic of a population over generations. Cha ...
, though this later changed. Darwin applied the theory of evolution and
sexual selection Sexual selection is a mode of natural selection in which members of one biological sex choose mates of the other sex to mate with (intersexual selection), and compete with members of the same sex for access to members of the opposite sex (in ...
to humans in his 1871 book ''
The Descent of Man, and Selection in Relation to Sex ''The Descent of Man, and Selection in Relation to Sex'' is a book by English naturalist Charles Darwin, first published in 1871, which applies evolutionary theory to human evolution, and details his theory of sexual selection, a form of biolo ...
''.


First fossils

A major problem in the 19th century was the lack of fossil intermediaries.
Neanderthal Neanderthals (, also ''Homo neanderthalensis'' and erroneously ''Homo sapiens neanderthalensis''), also written as Neandertals, are an extinct species or subspecies of archaic humans who lived in Eurasia until about 40,000 years ago. While ...
remains were discovered in a limestone quarry in 1856, three years before the publication of ''On the Origin of Species'', and Neanderthal fossils had been discovered in Gibraltar even earlier, but it was originally claimed that these were the remains of a modern human who had suffered some kind of illness. Despite the 1891 discovery by
Eugène Dubois Marie Eugène François Thomas Dubois (; 28 January 1858 – 16 December 1940) was a Dutch paleoanthropologist and geologist. He earned worldwide fame for his discovery of ''Pithecanthropus erectus'' (later redesignated ''Homo erectus''), or "Java ...
of what is now called ''Homo erectus'' at
Trinil Trinil is a palaeoanthropological site on the banks of the Bengawan Solo River in Ngawi Regency, East Java Province, Indonesia. It was at this site in 1891 that the Dutch anatomist Eugène Dubois discovered the first early hominin remains to ...
,
Java Java (; id, Jawa, ; jv, ꦗꦮ; su, ) is one of the Greater Sunda Islands in Indonesia. It is bordered by the Indian Ocean to the south and the Java Sea to the north. With a population of 151.6 million people, Java is the world's mo ...
, it was only in the 1920s when such fossils were discovered in Africa, that intermediate species began to accumulate. In 1925,
Raymond Dart Raymond Arthur Dart (4 February 1893 – 22 November 1988) was an Australian anatomist and anthropologist, best known for his involvement in the 1924 discovery of the first fossil ever found of ''Australopithecus africanus'', an extinct ho ...
described ''
Australopithecus africanus ''Australopithecus africanus'' is an extinct species of australopithecine which lived between about 3.3 and 2.1 million years ago in the Late Pliocene to Early Pleistocene of South Africa. The species has been recovered from Taung, Sterkfont ...
''. The
type specimen In biology, a type is a particular specimen (or in some cases a group of specimens) of an organism to which the scientific name of that organism is formally attached. In other words, a type is an example that serves to anchor or centralizes the ...
was the
Taung Child The Taung Child (or Taung Baby) is the fossilised skull of a young '' Australopithecus africanus''. It was discovered in 1924 by quarrymen working for the Northern Lime Company in Taung, South Africa. Raymond Dart described it as a new specie ...
, an australopithecine infant which was discovered in a cave. The child's remains were a remarkably well-preserved tiny skull and an
endocast An endocast is the internal cast of a hollow object, often referring to the cranial vault in the study of brain development in humans and other organisms. Endocasts can be artificially made for examining the properties of a hollow, inaccessible sp ...
of the brain. Although the brain was small (410 cm3), its shape was rounded, unlike that of chimpanzees and gorillas, and more like a modern human brain. Also, the specimen showed short
canine teeth In mammalian oral anatomy, the canine teeth, also called cuspids, dog teeth, or (in the context of the upper jaw) fangs, eye teeth, vampire teeth, or vampire fangs, are the relatively long, pointed teeth. They can appear more flattened however ...
, and the position of the
foramen magnum The foramen magnum ( la, great hole) is a large, oval-shaped opening in the occipital bone of the skull. It is one of the several oval or circular openings (foramina) in the base of the skull. The spinal cord, an extension of the medulla oblonga ...
(the hole in the skull where the spine enters) was evidence of
bipedal Bipedalism is a form of terrestrial locomotion where an organism moves by means of its two rear limbs or legs. An animal or machine that usually moves in a bipedal manner is known as a biped , meaning 'two feet' (from Latin ''bis'' 'double' ...
locomotion. All of these traits convinced Dart that the Taung Child was a bipedal human ancestor, a transitional form between apes and humans.


The East African fossils

During the 1960s and 1970s, hundreds of fossils were found in East Africa in the regions of the
Olduvai Gorge The Olduvai Gorge or Oldupai Gorge in Tanzania is one of the most important paleoanthropological localities in the world; the many sites exposed by the gorge have proven invaluable in furthering understanding of early human evolution. A steep-si ...
and
Lake Turkana Lake Turkana (), formerly known as Lake Rudolf, is a lake in the Kenyan Rift Valley, in northern Kenya, with its far northern end crossing into Ethiopia. It is the world's largest permanent desert lake and the world's largest alkaline lake ...
. These searches were carried out by the Leakey family, with
Louis Leakey Louis Seymour Bazett Leakey (7 August 1903 – 1 October 1972) was a Kenyan-British palaeoanthropologist and archaeologist whose work was important in demonstrating that humans evolved in Africa, particularly through discoveries made at Olduv ...
and his wife
Mary Leakey Mary Douglas Leakey, FBA (née Nicol, 6 February 1913 – 9 December 1996) was a British paleoanthropologist who discovered the first fossilised ''Proconsul'' skull, an extinct ape which is now believed to be ancestral to humans. She also disc ...
, and later their son
Richard Richard is a male given name. It originates, via Old French, from Old Frankish and is a compound of the words descending from Proto-Germanic ''*rīk-'' 'ruler, leader, king' and ''*hardu-'' 'strong, brave, hardy', and it therefore means 'stro ...
and daughter-in-law Meave, fossil hunters and
paleoanthropologists Paleoanthropology or paleo-anthropology is a branch of paleontology and anthropology which seeks to understand the early development of anatomically modern humans, a process known as hominization, through the reconstruction of evolutionary kinshi ...
. From the fossil beds of Olduvai and Lake Turkana they amassed specimens of the early hominins: the
australopithecines Australopithecina or Hominina is a subtribe in the tribe Hominini. The members of the subtribe are generally ''Australopithecus'' ( cladistically including the genera ''Homo'', ''Paranthropus'', and '' Kenyanthropus''), and it typically includ ...
and ''Homo'' species, and even ''Homo erectus''. These finds cemented Africa as the cradle of humankind. In the late 1970s and the 1980s,
Ethiopia Ethiopia, , om, Itiyoophiyaa, so, Itoobiya, ti, ኢትዮጵያ, Ítiyop'iya, aa, Itiyoppiya officially the Federal Democratic Republic of Ethiopia, is a landlocked country in the Horn of Africa. It shares borders with Eritrea to the Er ...
emerged as the new hot spot of
paleoanthropology Paleoanthropology or paleo-anthropology is a branch of paleontology and anthropology which seeks to understand the early development of anatomically modern humans, a process known as hominization, through the reconstruction of evolutionary kinshi ...
after "Lucy", the most complete fossil member of the species ''
Australopithecus afarensis ''Australopithecus afarensis'' is an extinct species of australopithecine which lived from about 3.9–2.9 million years ago (mya) in the Pliocene of East Africa. The first fossils were discovered in the 1930s, but major fossil finds would not ...
'', was found in 1974 by
Donald Johanson Donald Carl Johanson (born June 28, 1943) is an American paleoanthropologist. He is known for discovering, with Yves Coppens and Maurice Taieb, the fossil of a female hominin australopithecine known as "Lucy (Australopithecus), Lucy" in the Afar ...
near Hadar in the desertic
Afar Triangle The Afar Triangle (also called the Afar Depression) is a geological depression caused by the Afar Triple Junction, which is part of the Great Rift Valley in East Africa. The region has disclosed fossil specimens of the very earliest hominins; th ...
region of northern Ethiopia. Although the specimen had a small brain, the pelvis and leg bones were almost identical in function to those of modern humans, showing with certainty that these hominins had walked erect. Lucy was classified as a new species, ''
Australopithecus afarensis ''Australopithecus afarensis'' is an extinct species of australopithecine which lived from about 3.9–2.9 million years ago (mya) in the Pliocene of East Africa. The first fossils were discovered in the 1930s, but major fossil finds would not ...
'', which is thought to be more closely related to the genus ''
Homo ''Homo'' () is the genus that emerged in the (otherwise extinct) genus '' Australopithecus'' that encompasses the extant species ''Homo sapiens'' (modern humans), plus several extinct species classified as either ancestral to or closely relat ...
'' as a direct ancestor, or as a close relative of an unknown ancestor, than any other known hominid or hominin from this early time range; ''see'' terms "hominid" and "hominin". (The specimen was nicknamed "Lucy" after
the Beatles The Beatles were an English rock band, formed in Liverpool in 1960, that comprised John Lennon, Paul McCartney, George Harrison and Ringo Starr. They are regarded as the most influential band of all time and were integral to the developm ...
' song "
Lucy in the Sky with Diamonds "Lucy in the Sky with Diamonds" is a song by the English rock band the Beatles from their 1967 album ''Sgt. Pepper's Lonely Hearts Club Band''. It was written primarily by John Lennon and credited to the Lennon–McCartney songwriting partners ...
", which was played loudly and repeatedly in the camp during the excavations.) The
Afar Triangle The Afar Triangle (also called the Afar Depression) is a geological depression caused by the Afar Triple Junction, which is part of the Great Rift Valley in East Africa. The region has disclosed fossil specimens of the very earliest hominins; th ...
area would later yield discovery of many more hominin fossils, particularly those uncovered or described by teams headed by
Tim D. White Tim D. White (born August 24, 1950) is an American paleoanthropologist and Professor of Integrative Biology at the University of California, Berkeley. He is best known for leading the team which discovered Ardi, the type specimen of ''Ardipithecu ...
in the 1990s, including ''
Ardipithecus ramidus ''Ardipithecus ramidus'' is a species of australopithecine from the Afar region of Early Pliocene Ethiopia 4.4 million years ago (mya). ''A. ramidus'', unlike modern hominids, has adaptations for both walking on two legs (bipedality) and life in ...
'' and ''
Ardipithecus kadabba ''Ardipithecus kadabba'' is the scientific classification given to fossil remains "known only from teeth and bits and pieces of skeletal bones", originally estimated to be 5.8 to 5.2 million years old, and later revised to 5.77 to 5.54 million ye ...
''. In 2013, fossil skeletons of ''
Homo naledi '' Homo naledi'' is an extinct species of archaic human discovered in 2013 in the Rising Star Cave, Cradle of Humankind, South Africa dating to the Middle Pleistocene 335,000–236,000 years ago. The initial discovery comprises 1,550 specime ...
'', an
extinct species This page features lists of extinct species, organisms that have become extinct, either in the wild or completely disappeared from Earth. In actual theoretical practice, a species not definitely located in the wild in the last fifty years of cur ...
of
hominin The Hominini form a taxonomic tribe of the subfamily Homininae ("hominines"). Hominini includes the extant genera ''Homo'' (humans) and '' Pan'' (chimpanzees and bonobos) and in standard usage excludes the genus ''Gorilla'' (gorillas). The t ...
assigned (provisionally) to the
genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial n ...
''
Homo ''Homo'' () is the genus that emerged in the (otherwise extinct) genus '' Australopithecus'' that encompasses the extant species ''Homo sapiens'' (modern humans), plus several extinct species classified as either ancestral to or closely relat ...
'', were found in the
Rising Star Cave The Rising Star cave system (also known as Westminster or Empire cave) is located in the Malmani dolomites, in Bloubank River valley, about southwest of Swartkrans, part of the Cradle of Humankind World Heritage Site in South Africa. Recreati ...
system, a site in South Africa's
Cradle of Humankind The Cradle of Humankind is a paleoanthropological site and is located about northwest of Johannesburg, South Africa, in the Gauteng province. Declared a World Heritage Site by UNESCO in 1999, the site is home to the largest concentration ...
region in
Gauteng Gauteng ( ) is one of the nine provinces of South Africa. The name in Sotho-Tswana languages means 'place of gold'. Situated on the Highveld, Gauteng is the smallest province by land area in South Africa. Although Gauteng accounts for only ...
province near
Johannesburg Johannesburg ( , , ; Zulu language, Zulu and xh, eGoli ), colloquially known as Jozi, Joburg, or "The City of Gold", is the largest city in South Africa, classified as a Megacity#List of megacities, megacity, and is List of urban areas by p ...
. , fossils of at least fifteen individuals, amounting to 1,550 specimens, have been excavated from the cave. The species is characterized by a body mass and stature similar to small-bodied human populations, a smaller endocranial volume similar to ''
Australopithecus ''Australopithecus'' (, ; ) is a genus of early hominins that existed in Africa during the Late Pliocene and Early Pleistocene. The genus ''Homo'' (which includes modern humans) emerged within ''Australopithecus'', as sister to e.g. ''Austral ...
'', and a
cranial Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position prov ...
morphology Morphology, from the Greek and meaning "study of shape", may refer to: Disciplines *Morphology (archaeology), study of the shapes or forms of artifacts *Morphology (astronomy), study of the shape of astronomical objects such as nebulae, galaxies, ...
(skull shape) similar to early ''Homo'' species. The skeletal anatomy combines primitive features known from
australopithecine Australopithecina or Hominina is a subtribe in the tribe Hominini. The members of the subtribe are generally ''Australopithecus'' ( cladistically including the genera ''Homo'', ''Paranthropus'', and '' Kenyanthropus''), and it typically include ...
s with features known from early hominins. The individuals show signs of having been deliberately disposed of within the cave near the time of death. The fossils were dated close to 250,000 years ago, and thus are not a direct ancestor but a contemporary with the first appearance of larger-brained
anatomically modern humans Early modern human (EMH) or anatomically modern human (AMH) are terms used to distinguish ''Homo sapiens'' (the only extant Hominina species) that are anatomically consistent with the range of phenotypes seen in contemporary humans from extin ...
.


The genetic revolution

The genetic revolution in studies of human evolution started when
Vincent Sarich Vincent Matthew Sarich (December 13, 1934October 27, 2012) was an American anthropologist and biochemist. He was Professor Emeritus in anthropology at University of California, Berkeley. Sarich and his PhD advisor, Allan Wilson, used molecular da ...
and
Allan Wilson Alan, Allan, or Allen Wilson may refer to: Sports * Alan Wilson (cricketer, born 1920) (1920–2015), English cricketer * Alan Wilson (cricketer, born 1936), former English cricketer * Alan Wilson (Australian rules footballer) (born 1939), Austra ...
measured the strength of immunological cross-reactions of
blood serum Serum () is the fluid and solute component of blood which does not play a role in clotting. It may be defined as blood plasma without the clotting factors, or as blood with all cells and clotting factors removed. Serum includes all proteins not ...
albumin Albumin is a family of globular proteins, the most common of which are the serum albumins. All the proteins of the albumin family are water-soluble, moderately soluble in concentrated salt solutions, and experience heat denaturation. Albumins ...
between pairs of creatures, including humans and African apes (chimpanzees and gorillas). The strength of the reaction could be expressed numerically as an immunological distance, which was in turn proportional to the number of
amino acid Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha ...
differences between homologous proteins in different species. By constructing a calibration curve of the ID of species' pairs with known divergence times in the fossil record, the data could be used as a
molecular clock The molecular clock is a figurative term for a technique that uses the mutation rate of biomolecules to deduce the time in prehistory when two or more life forms diverged. The biomolecular data used for such calculations are usually nucleo ...
to estimate the times of divergence of pairs with poorer or unknown fossil records. In their seminal 1967 paper in ''
Science Science is a systematic endeavor that Scientific method, builds and organizes knowledge in the form of Testability, testable explanations and predictions about the universe. Science may be as old as the human species, and some of the earli ...
'', Sarich and Wilson estimated the divergence time of humans and apes as four to five million years ago, at a time when standard interpretations of the fossil record gave this divergence as at least 10 to as much as 30 million years. Subsequent fossil discoveries, notably "Lucy", and reinterpretation of older fossil materials, notably ''
Ramapithecus ''Sivapithecus'' () (syn: ''Ramapithecus)'' is a genus of extinct apes. Fossil remains of animals now assigned to this genus, dated from 12.2 million years old in the Miocene, have been found since the 19th century in the Siwalik Hills of the I ...
'', showed the younger estimates to be correct and validated the albumin method. Progress in DNA sequencing, specifically mitochondrial DNA (mtDNA) and then
Y-chromosome DNA In human genetics, a human Y-chromosome DNA haplogroup is a haplogroup defined by mutations in the non- recombining portions of DNA from the male-specific Y chromosome (called Y-DNA). Many people within a haplogroup share similar numbers of s ...
(Y-DNA) advanced the understanding of human origins. Application of the
molecular clock The molecular clock is a figurative term for a technique that uses the mutation rate of biomolecules to deduce the time in prehistory when two or more life forms diverged. The biomolecular data used for such calculations are usually nucleo ...
principle revolutionized the study of
molecular evolution Molecular evolution is the process of change in the sequence composition of cell (biology), cellular molecules such as DNA, RNA, and proteins across generations. The field of molecular evolution uses principles of evolutionary biology and popula ...
. On the basis of a separation from the orangutan between 10 and 20 million years ago, earlier studies of the molecular clock suggested that there were about 76 mutations per generation that were not inherited by human children from their parents; this evidence supported the divergence time between hominins and chimpanzees noted above. However, a 2012 study in Iceland of 78 children and their parents suggests a mutation rate of only 36 mutations per generation; this datum extends the separation between humans and chimpanzees to an earlier period greater than 7 million years ago ( Ma). Additional research with 226 offspring of wild chimpanzee populations in eight locations suggests that chimpanzees reproduce at age 26.5 years on average; which suggests the human divergence from chimpanzees occurred between 7 and 13 million years ago. And these data suggest that ''Ardipithecus'' (4.5 Ma), ''Orrorin'' (6 Ma) and ''Sahelanthropus'' (7 Ma) all may be on the hominid
lineage Lineage may refer to: Science * Lineage (anthropology), a group that can demonstrate its common descent from an apical ancestor or a direct line of descent from an ancestor * Lineage (evolution), a temporal sequence of individuals, populatio ...
, and even that the separation may have occurred outside the
East African Rift The East African Rift (EAR) or East African Rift System (EARS) is an active continental rift zone in East Africa. The EAR began developing around the onset of the Miocene, 22–25 million years ago. In the past it was considered to be part of a ...
region. Furthermore, analysis of the two species' genes in 2006 provides evidence that after human ancestors had started to diverge from chimpanzees, interspecies mating between "proto-human" and "proto-chimpanzees" nonetheless occurred regularly enough to change certain genes in the new
gene pool The gene pool is the set of all genes, or genetic information, in any population, usually of a particular species. Description A large gene pool indicates extensive genetic diversity, which is associated with robust populations that can surv ...
: : A new comparison of the human and chimpanzee genomes suggests that after the two lineages separated, they may have begun interbreeding... A principal finding is that the
X chromosome The X chromosome is one of the two sex-determining chromosomes ( allosomes) in many organisms, including mammals (the other is the Y chromosome), and is found in both males and females. It is a part of the XY sex-determination system and XO se ...
s of humans and chimpanzees appear to have diverged about 1.2 million years more recently than the other chromosomes. The research suggests: : There were in fact two splits between the human and chimpanzee lineages, with the first being followed by interbreeding between the two populations and then a second split. The suggestion of a hybridization has startled
paleoanthropologists Paleoanthropology or paleo-anthropology is a branch of paleontology and anthropology which seeks to understand the early development of anatomically modern humans, a process known as hominization, through the reconstruction of evolutionary kinshi ...
, who nonetheless are treating the new genetic data seriously.


The quest for the earliest hominin

In the 1990s, several teams of paleoanthropologists were working throughout Africa looking for evidence of the earliest divergence of the hominin lineage from the great apes. In 1994, Meave Leakey discovered ''
Australopithecus anamensis ''Australopithecus anamensis'' is a hominin species that lived approximately between 4.2 and 3.8 million years ago and is the oldest known ''Australopithecus'' species, living during the Plio-Pleistocene era. Nearly one hundred fossil specimens ...
''. The find was overshadowed by Tim D. White's 1995 discovery of ''Ardipithecus ramidus'', which pushed back the fossil record to . In 2000,
Martin Pickford Martin Pickford was lecturer in the Chair of Paleoanthropology and Prehistory at the Collège de France and honorary affiliate at the Département Histoire de la Terre in the Muséum national d'Histoire. In 2001, Martin Pickford together wi ...
and Brigitte Senut discovered, in the
Tugen Hills The Tugen Hills (also known as ''Saimo'') are a series of hills in Baringo County, Kenya. They are located in the central-western portion of Kenya. The Tugen Hills represent one of the few areas in Africa preserving a succession of deposits from t ...
of
Kenya ) , national_anthem = " Ee Mungu Nguvu Yetu"() , image_map = , map_caption = , image_map2 = , capital = Nairobi , coordinates = , largest_city = Nairobi , ...
, a 6-million-year-old bipedal hominin which they named ''
Orrorin ''Orrorin tugenensis'' is a postulated early species of Homininae, estimated at and discovered in 2000. It is not confirmed how ''Orrorin'' is related to modern humans. Its discovery was used to argue against the hypothesis that australopitheci ...
tugenensis''. And in 2001, a team led by
Michel Brunet Michel Brunet may refer to: * Michel Brunet (historian) Michel Brunet (July 24, 1917 in Montreal – September 4, 1985 in Montreal) was a Quebec historian and essayist. He received his B.A. and M.A. from the Université de Montréal and received ...
discovered the skull of ''
Sahelanthropus ''Sahelanthropus tchadensis'' is an extinct species of the Homininae (African apes) dated to about , during the Miocene epoch. The species, and its genus ''Sahelanthropus'', was announced in 2002, based mainly on a partial cranium, nicknamed '' ...
tchadensis'' which was dated as , and which Brunet argued was a bipedal, and therefore a hominid—that is, a hominin ( Hominidae; terms "hominids" and hominins).


Human dispersal

Anthropologists in the 1980s were divided regarding some details of reproductive barriers and migratory dispersals of the genus ''Homo''. Subsequently, genetics has been used to investigate and resolve these issues. According to the
Sahara pump theory The Sahara pump theory is a hypothesis that explains how flora and fauna migrated between Eurasia and Africa via a land bridge in the Levant region. It posits that extended periods of abundant rainfall lasting many thousands of years ( pluvia ...
evidence suggests that the genus ''Homo'' have migrated out of Africa at least three and possibly four times (e.g. ''Homo erectus'', ''Homo heidelbergensis'' and two or three times for ''Homo sapiens''). Recent evidence suggests these dispersals are closely related to fluctuating periods of climate change. Recent evidence suggests that humans may have left Africa half a million years earlier than previously thought. A joint Franco-Indian team has found human artifacts in the Siwalk Hills north of New Delhi dating back at least 2.6 million years. This is earlier than the previous earliest finding of genus ''Homo'' at
Dmanisi Dmanisi ( ka, დმანისი, tr, , az, Başkeçid) is a town and archaeological site in the Kvemo Kartli region of Georgia approximately 93 km southwest of the nation’s capital Tbilisi in the river valley of Mashavera. The homini ...
, in
Georgia Georgia most commonly refers to: * Georgia (country), a country in the Caucasus region of Eurasia * Georgia (U.S. state), a state in the Southeast United States Georgia may also refer to: Places Historical states and entities * Related to t ...
, dating to 1.85 million years. Although controversial, tools found at a Chinese cave strengthen the case that humans used tools as far back as 2.48 million years ago. This suggests that the Asian "Chopper" tool tradition, found in Java and northern China may have left Africa before the appearance of the
Acheulian Acheulean (; also Acheulian and Mode II), from the French ''acheuléen'' after the type site of Saint-Acheul, is an archaeological industry of stone tool manufacture characterized by the distinctive oval and pear-shaped "hand axes" associate ...
hand axe.


Dispersal of modern ''Homo sapiens''

Up until the genetic evidence became available, there were two dominant models for the dispersal of modern humans. The
multiregional hypothesis The multiregional hypothesis, multiregional evolution (MRE), or polycentric hypothesis is a scientific model that provides an alternative explanation to the more widely accepted "Out of Africa" model of monogenesis for the pattern of human evoluti ...
proposed that the genus ''Homo'' contained only a single interconnected population as it does today (not separate species), and that its evolution took place worldwide continuously over the last couple of million years. This model was proposed in 1988 by Milford H. Wolpoff. In contrast, the "out of Africa" model proposed that modern ''H. sapiens'' speciated in Africa recently (that is, approximately 200,000 years ago) and the subsequent migration through
Eurasia Eurasia (, ) is the largest continental area on Earth, comprising all of Europe and Asia. Primarily in the Northern and Eastern Hemispheres, it spans from the British Isles and the Iberian Peninsula in the west to the Japanese archipelag ...
resulted in the nearly complete replacement of other ''Homo'' species. This model has been developed by Chris B. Stringer and Peter Andrews. Sequencing mtDNA and Y-DNA sampled from a wide range of indigenous populations revealed ancestral information relating to both male and female genetic heritage, and strengthened the "out of Africa" theory and weakened the views of multiregional evolutionism. Aligned in genetic tree differences were interpreted as supportive of a recent single origin. "Out of Africa" has thus gained much support from research using female mitochondrial DNA and the male
Y chromosome The Y chromosome is one of two sex chromosomes ( allosomes) in therian mammals, including humans, and many other animals. The other is the X chromosome. Y is normally the sex-determining chromosome in many species, since it is the presence or a ...
. After analysing genealogy trees constructed using 133 types of mtDNA, researchers concluded that all were descended from a female African progenitor, dubbed
Mitochondrial Eve In human genetics, the Mitochondrial Eve (also ''mt-Eve, mt-MRCA'') is the matrilineal most recent common ancestor (MRCA) of all living humans. In other words, she is defined as the most recent woman from whom all living humans descend in an u ...
. "Out of Africa" is also supported by the fact that mitochondrial genetic diversity is highest among African populations. A broad study of African genetic diversity, headed by
Sarah Tishkoff Sarah Anne Tishkoff (born December 26, 1965) is an American geneticist and the David and Lyn Silfen Professor in the Department of Genetics and Biology at the University of Pennsylvania. She also serves as a director for the American Society of Hu ...
, found the
San people The San peoples (also Saan), or Bushmen, are members of various Khoe, Tuu, or Kxʼa-speaking indigenous hunter-gatherer cultures that are the first cultures of Southern Africa, and whose territories span Botswana, Namibia, Angola, Zambia ...
had the greatest genetic diversity among the 113 distinct populations sampled, making them one of 14 "ancestral population clusters". The research also located a possible origin of modern human migration in southwestern Africa, near the coastal border of
Namibia Namibia (, ), officially the Republic of Namibia, is a country in Southern Africa. Its western border is the Atlantic Ocean. It shares land borders with Zambia and Angola to the north, Botswana to the east and South Africa to the south and e ...
and
Angola , national_anthem = "Angola Avante"() , image_map = , map_caption = , capital = Luanda , religion = , religion_year = 2020 , religion_ref = , coordina ...
. The fossil evidence was insufficient for archaeologist
Richard Leakey Richard Erskine Frere Leakey (19 December 1944 – 2 January 2022) was a Kenyan paleoanthropologist, conservationist and politician. Leakey held a number of official positions in Kenya, mostly in institutions of archaeology and wildlife cons ...
to resolve the debate about exactly where in Africa modern humans first appeared. Studies of
haplogroup A haplotype is a group of alleles in an organism that are inherited together from a single parent, and a haplogroup ( haploid from the el, ἁπλοῦς, ''haploûs'', "onefold, simple" and en, group) is a group of similar haplotypes that share ...
s in Y-chromosomal DNA and mitochondrial DNA have largely supported a recent African origin. All the evidence from autosomal DNA also predominantly supports a Recent African origin. However, evidence for archaic admixture in modern humans, both in Africa and later, throughout Eurasia has recently been suggested by a number of studies. Recent sequencing of
Neanderthal Neanderthals (, also ''Homo neanderthalensis'' and erroneously ''Homo sapiens neanderthalensis''), also written as Neandertals, are an extinct species or subspecies of archaic humans who lived in Eurasia until about 40,000 years ago. While ...
and
Denisovan The Denisovans or Denisova hominins ) are an extinct species or subspecies of archaic human that ranged across Asia during the Lower and Middle Paleolithic. Denisovans are known from few physical remains and consequently, most of what is know ...
genomes shows that some admixture with these populations has occurred. All modern human groups outside Africa have 1–4% or (according to more recent research) about 1.5–2.6% Neanderthal
allele An allele (, ; ; modern formation from Greek ἄλλος ''állos'', "other") is a variation of the same sequence of nucleotides at the same place on a long DNA molecule, as described in leading textbooks on genetics and evolution. ::"The chro ...
s in their genome, and some
Melanesians Melanesians are the predominant and indigenous inhabitants of Melanesia, in a wide area from Indonesia's New Guinea to as far East as the islands of Vanuatu and Fiji. Most speak either one of the many languages of the Austronesian language fam ...
have an additional 4–6% of Denisovan alleles. These new results do not contradict the "out of Africa" model, except in its strictest interpretation, although they make the situation more complex. After recovery from a
genetic bottleneck A population bottleneck or genetic bottleneck is a sharp reduction in the size of a population due to environmental events such as famines, earthquakes, floods, fires, disease, and droughts; or human activities such as specicide, widespread violen ...
that some researchers speculate might be linked to the Toba supervolcano catastrophe, a fairly small group left Africa and interbred with Neanderthals, probably in the Middle East, on the Eurasian steppe or even in North Africa before their departure. Their still predominantly African descendants spread to populate the world. A fraction in turn interbred with Denisovans, probably in southeastern Asia, before populating Melanesia. HLA haplotypes of Neanderthal and Denisova origin have been identified in modern Eurasian and Oceanian populations. The Denisovan
EPAS1 Endothelial PAS domain-containing protein 1 (EPAS1, also known as hypoxia-inducible factor-2alpha (HIF-2α)) is a protein that is encoded by the ''EPAS1'' gene in mammals. It is a type of hypoxia-inducible factor, a group of transcription factors ...
gene has also been found in Tibetan populations. Studies of the human genome using machine learning have identified additional genetic contributions in Eurasians from an "unknown" ancestral population potentially related to the Neanderthal-Denisovan lineage. There are still differing theories on whether there was a single exodus from Africa or several. A multiple dispersal model involves the Southern Dispersal theory, which has gained support in recent years from genetic, linguistic and archaeological evidence. In this theory, there was a coastal dispersal of modern humans from the
Horn of Africa The Horn of Africa (HoA), also known as the Somali Peninsula, is a large peninsula and geopolitical region in East Africa.Robert Stock, ''Africa South of the Sahara, Second Edition: A Geographical Interpretation'', (The Guilford Press; 2004), ...
crossing the Bab el Mandib to Yemen at a lower sea level around 70,000 years ago. This group helped to populate Southeast Asia and Oceania, explaining the discovery of early human sites in these areas much earlier than those in the
Levant The Levant () is an approximation, approximate historical geography, historical geographical term referring to a large area in the Eastern Mediterranean region of Western Asia. In its narrowest sense, which is in use today in archaeology an ...
. This group seems to have been dependent upon marine resources for their survival.
Stephen Oppenheimer Stephen Oppenheimer (born 1947) is a British paediatrician, geneticist, and writer. He is a graduate of Balliol College, Oxford and an honorary fellow of the Liverpool School of Tropical Medicine. In addition to his work in medicine and tropica ...
has proposed a second wave of humans may have later dispersed through the Persian Gulf oases, and the Zagros mountains into the Middle East. Alternatively it may have come across the
Sinai Peninsula The Sinai Peninsula, or simply Sinai (now usually ) (, , cop, Ⲥⲓⲛⲁ), is a peninsula in Egypt, and the only part of the country located in Asia. It is between the Mediterranean Sea to the north and the Red Sea to the south, and is a ...
into Asia, from shortly after 50,000 yrs BP, resulting in the bulk of the human populations of Eurasia. It has been suggested that this second group possibly possessed a more sophisticated "big game hunting" tool technology and was less dependent on coastal food sources than the original group. Much of the evidence for the first group's expansion would have been destroyed by the rising sea levels at the end of each
glacial maximum An ice age is a long period of reduction in the temperature of Earth's surface and atmosphere, resulting in the presence or expansion of continental and polar ice sheets and alpine glaciers. Earth's climate alternates between ice ages and gree ...
. The multiple dispersal model is contradicted by studies indicating that the populations of Eurasia and the populations of Southeast Asia and Oceania are all descended from the same mitochondrial DNA L3 lineages, which support a single migration out of Africa that gave rise to all non-African populations. On the basis of the early date of Badoshan Iranian Aurignacian, Oppenheimer suggests that this second dispersal may have occurred with a pluvial period about 50,000 years before the present, with modern human big-game hunting cultures spreading up the Zagros Mountains, carrying modern human genomes from Oman, throughout the Persian Gulf, northward into Armenia and Anatolia, with a variant travelling south into Israel and to Cyrenicia.Oppenheimer, Stephen (2012), "Out of Eden: The Peopling of the World" (Robinson; New Ed edition (March 1, 2012)) Recent genetic evidence suggests that all modern non-African populations, including those of Eurasia and Oceania, are descended from a single wave that left Africa between 65,000 and 50,000 years ago.


Evidence

The evidence on which scientific accounts of human evolution are based comes from many fields of natural science. The main source of knowledge about the evolutionary process has traditionally been the fossil record, but since the development of genetics beginning in the 1970s, DNA analysis has come to occupy a place of comparable importance. The studies of ontogeny,
phylogeny A phylogenetic tree (also phylogeny or evolutionary tree Felsenstein J. (2004). ''Inferring Phylogenies'' Sinauer Associates: Sunderland, MA.) is a branching diagram or a tree showing the evolutionary relationships among various biological spe ...
and especially
evolutionary developmental biology Evolutionary developmental biology (informally, evo-devo) is a field of biological research that compares the developmental processes of different organisms to infer how developmental processes evolved. The field grew from 19th-century beginn ...
of both vertebrates and invertebrates offer considerable insight into the evolution of all life, including how humans evolved. The specific study of the origin and life of humans is
anthropology Anthropology is the scientific study of humanity, concerned with human behavior, human biology, cultures, societies, and linguistics, in both the present and past, including past human species. Social anthropology studies patterns of be ...
, particularly paleoanthropology which focuses on the study of human
prehistory Prehistory, also known as pre-literary history, is the period of human history between the use of the first stone tools by hominins 3.3 million years ago and the beginning of recorded history with the invention of writing systems. The use ...
.


Evidence from genetics

The closest living relatives of humans are
bonobo The bonobo (; ''Pan paniscus''), also historically called the pygmy chimpanzee and less often the dwarf chimpanzee or gracile chimpanzee, is an endangered great ape and one of the two species making up the genus '' Pan,'' the other being the co ...
s and chimpanzees (both genus ''Pan'') and
gorilla Gorillas are herbivorous, predominantly ground-dwelling great apes that inhabit the tropical forests of equatorial Africa. The genus ''Gorilla'' is divided into two species: the eastern gorilla and the western gorilla, and either four ...
s (genus ''Gorilla''). With the sequencing of both the human and chimpanzee genome, estimates of the similarity between their DNA sequences range between 95% and 99%. By using the technique called the
molecular clock The molecular clock is a figurative term for a technique that uses the mutation rate of biomolecules to deduce the time in prehistory when two or more life forms diverged. The biomolecular data used for such calculations are usually nucleo ...
which estimates the time required for the number of divergent mutations to accumulate between two lineages, the approximate date for the split between lineages can be calculated. The gibbons (family Hylobatidae) and then the
orangutan Orangutans are great apes native to the rainforests of Indonesia and Malaysia. They are now found only in parts of Borneo and Sumatra, but during the Pleistocene they ranged throughout Southeast Asia and South China. Classified in the gen ...
s (genus ''Pongo'') were the first groups to split from the line leading to the hominins, including humans—followed by gorillas (genus ''Gorilla''), and, ultimately, by the chimpanzees (genus ''Pan''). The splitting date between hominin and chimpanzee lineages is placed by some between , that is, during the
Late Miocene The Late Miocene (also known as Upper Miocene) is a sub-epoch of the Miocene Epoch made up of two stages. The Tortonian and Messinian stages comprise the Late Miocene sub-epoch, which lasted from 11.63 Ma (million years ago) to 5.333 Ma. The ...
.
Speciation Speciation is the evolutionary process by which populations evolve to become distinct species. The biologist Orator F. Cook coined the term in 1906 for cladogenesis, the splitting of lineages, as opposed to anagenesis, phyletic evolution withi ...
, however, appears to have been unusually drawn out. Initial divergence occurred sometime between , but ongoing hybridization blurred the separation and delayed complete separation during several millions of years. Patterson (2006) dated the final divergence at . Genetic evidence has also been employed to compare species within the genus ''Homo'', investigating gene flow between early modern humans and Neanderthals, and to enhance the understanding of the early human migration patterns and splitting dates. By comparing the parts of the genome that are not under natural selection and which therefore accumulate mutations at a fairly steady rate, it is possible to reconstruct a genetic tree incorporating the entire human species since the last shared ancestor. Each time a certain mutation (
single-nucleotide polymorphism In genetics, a single-nucleotide polymorphism (SNP ; plural SNPs ) is a germline substitution of a single nucleotide at a specific position in the genome. Although certain definitions require the substitution to be present in a sufficiently lar ...
) appears in an individual and is passed on to his or her descendants, a haplogroup is formed including all of the descendants of the individual who will also carry that mutation. By comparing mitochondrial DNA which is inherited only from the mother, geneticists have concluded that the last female common ancestor whose
genetic marker A genetic marker is a gene or DNA sequence with a known location on a chromosome that can be used to identify individuals or species. It can be described as a variation (which may arise due to mutation or alteration in the genomic loci) that can be ...
is found in all modern humans, the so-called
mitochondrial Eve In human genetics, the Mitochondrial Eve (also ''mt-Eve, mt-MRCA'') is the matrilineal most recent common ancestor (MRCA) of all living humans. In other words, she is defined as the most recent woman from whom all living humans descend in an u ...
, must have lived around 200,000 years ago.


Genetics

Human evolutionary genetics studies how
human genome The human genome is a complete set of nucleic acid sequences for humans, encoded as DNA within the 23 chromosome pairs in cell nuclei and in a small DNA molecule found within individual mitochondria. These are usually treated separately as the ...
s differ among individuals, the evolutionary past that gave rise to them, and their current effects. Differences between genomes have
anthropological Anthropology is the scientific study of humanity, concerned with human behavior, human biology, cultures, societies, and linguistics, in both the present and past, including past human species. Social anthropology studies patterns of behavi ...
, medical and
forensic Forensic science, also known as criminalistics, is the application of science to criminal and civil laws, mainly—on the criminal side—during criminal investigation, as governed by the legal standards of admissible evidence and crimin ...
implications and applications. Genetic data can provide important insight into human evolution.


Evidence from the fossil record

There is little fossil evidence for the divergence of the gorilla, chimpanzee and hominin lineages. The earliest fossils that have been proposed as members of the hominin lineage are ''Sahelanthropus tchadensis'' dating from , ''Orrorin tugenensis'' dating from , and ''
Ardipithecus kadabba ''Ardipithecus kadabba'' is the scientific classification given to fossil remains "known only from teeth and bits and pieces of skeletal bones", originally estimated to be 5.8 to 5.2 million years old, and later revised to 5.77 to 5.54 million ye ...
'' dating to . Each of these have been argued to be a
bipedal Bipedalism is a form of terrestrial locomotion where an organism moves by means of its two rear limbs or legs. An animal or machine that usually moves in a bipedal manner is known as a biped , meaning 'two feet' (from Latin ''bis'' 'double' ...
ancestor of later hominins but, in each case, the claims have been contested. It is also possible that one or more of these species are ancestors of another branch of African apes, or that they represent a shared ancestor between hominins and other apes. The question then of the relationship between these early fossil species and the hominin lineage is still to be resolved. From these early species, the australopithecines arose around and diverged into
robust Robustness is the property of being strong and healthy in constitution. When it is transposed into a system, it refers to the ability of tolerating perturbations that might affect the system’s functional body. In the same line ''robustness'' ca ...
(also called ''
Paranthropus ''Paranthropus'' is a genus of extinct hominin which contains two widely accepted species: '' P. robustus'' and '' P. boisei''. However, the validity of ''Paranthropus'' is contested, and it is sometimes considered to be synonymous with '' Au ...
'') and
gracile Gracility is slenderness, the condition of being gracile, which means slender. It derives from the Latin adjective ''gracilis'' ( masculine or feminine), or ''gracile'' (neuter), which in either form means slender, and when transferred for exam ...
branches, one of which (possibly ''
A. garhi ''Australopithecus garhi'' is a species of australopithecine from the Bouri Formation in the Afar Region of Ethiopia 2.6–2.5 million years ago (mya) during the Early Pleistocene. The first remains were described in 1999 based on several skele ...
'') probably went on to become ancestors of the genus ''Homo''. The australopithecine species that is best represented in the fossil record is ''Australopithecus afarensis'' with more than 100 fossil individuals represented, found from Northern Ethiopia (such as the famous "Lucy"), to Kenya, and South Africa. Fossils of robust australopithecines such as ''Au. robustus'' (or alternatively ''
Paranthropus robustus ''Paranthropus robustus'' is a species of robust australopithecine from the Early and possibly Middle Pleistocene of the Cradle of Humankind, South Africa, about 2.27 to 0.87 (or, more conservatively, 2 to 1) million years ago. It has been iden ...
'') and ''Au./P. boisei'' are particularly abundant in South Africa at sites such as
Kromdraai Kromdraai Conservancy is a protected conservation park located to the south-west of Gauteng province in north-east South Africa. It is in the Muldersdrift area not far from Krugersdorp. Etymology Its name is derived from Afrikaans meaning "Croo ...
and
Swartkrans Swartkrans is a fossil-bearing cave designated as a South African National Heritage Site, located about from Johannesburg. It is located in the Cradle of Humankind World Heritage Site and is notable for being extremely rich in archaeological ma ...
, and around
Lake Turkana Lake Turkana (), formerly known as Lake Rudolf, is a lake in the Kenyan Rift Valley, in northern Kenya, with its far northern end crossing into Ethiopia. It is the world's largest permanent desert lake and the world's largest alkaline lake ...
in Kenya. The earliest member of the genus ''Homo'' is ''Homo habilis'' which evolved around . ''Homo habilis'' is the first species for which we have positive evidence of the use of stone tools. They developed the
Oldowan The Oldowan (or Mode I) was a widespread stone tool archaeological industry (style) in prehistory. These early tools were simple, usually made with one or a few flakes chipped off with another stone. Oldowan tools were used during the Lower ...
lithic technology, named after the Olduvai Gorge in which the first specimens were found. Some scientists consider ''
Homo rudolfensis ''Homo rudolfensis'' is an extinct species of archaic human from the Early Pleistocene of East Africa about 2 million years ago (mya). Because ''H. rudolfensis'' coexisted with several other hominins, it is debated what specimens can be confiden ...
'', a larger bodied group of fossils with similar morphology to the original ''H. habilis'' fossils, to be a separate species, while others consider them to be part of ''H. habilis''—simply representing intraspecies variation, or perhaps even
sexual dimorphism Sexual dimorphism is the condition where the sexes of the same animal and/or plant species exhibit different morphological characteristics, particularly characteristics not directly involved in reproduction. The condition occurs in most an ...
. The brains of these early hominins were about the same size as that of a chimpanzee, and their main adaptation was bipedalism as an adaptation to terrestrial living. During the next million years, a process of encephalization began and, by the arrival (about ) of ''
Homo erectus ''Homo erectus'' (; meaning " upright man") is an extinct species of archaic human from the Pleistocene, with its earliest occurrence about 2 million years ago. Several human species, such as ''H. heidelbergensis'' and ''H. antecessor' ...
'' in the fossil record, cranial capacity had doubled. ''Homo erectus'' were the first of the hominins to emigrate from Africa, and, from , this species spread through Africa, Asia, and Europe. One population of ''H. erectus'', also sometimes classified as a separate species ''Homo ergaster'', remained in Africa and evolved into ''Homo sapiens''. It is believed that these species, ''H. erectus'' and ''H. ergaster'', were the first to use fire and complex tools. In Eurasia H. erectus evolved into species such as '' H. antecessor'', ''
H. heidelbergensis ''Homo heidelbergensis'' (also ''H. sapiens heidelbergensis''), sometimes called Heidelbergs, is an extinct species or subspecies of archaic human which existed during the Middle Pleistocene. It was subsumed as a subspecies of ''H. erectus'' in 1 ...
'' and ''
H. neanderthalensis Neanderthals (, also ''Homo neanderthalensis'' and erroneously ''Homo sapiens neanderthalensis''), also written as Neandertals, are an extinct species or subspecies of archaic humans who lived in Eurasia until about 40,000 years ago. While ...
''. The earliest fossils of
anatomically modern humans Early modern human (EMH) or anatomically modern human (AMH) are terms used to distinguish ''Homo sapiens'' (the only extant Hominina species) that are anatomically consistent with the range of phenotypes seen in contemporary humans from extin ...
are from the Middle Paleolithic, about 300–200,000 years ago such as the Herto and
Omo remains The Omo remains are a collection of homininThis article quotes historic texts that use the terms 'hominid' and 'hominin' with meanings that may be different from their modern usages. This is because several revisions in classifying the great apes ...
of Ethiopia,
Jebel Irhoud Jebel Irhoud or Adrar n Ighoud ( zgh, ⴰⴷⵔⴰⵔ ⵏ ⵉⵖⵓⴷ, Adrar n Iɣud; ar, جبل إيغود, žbəl iġud), is an archaeological site located just north of the locality known as Tlet Ighoud, approximately south-east of the cit ...
remains of Morocco, and Florisbad remains of South Africa; later fossils from
Es Skhul Es-Skhul (es-Skhūl, ar, السخول; meaning ''kid'', ''young goat'') or the Skhul Cave is a prehistoric cave site situated about south of the city of Haifa, Israel, and about from the Mediterranean Sea. Together with the nearby sites of T ...
cave in Israel and Southern Europe begin around 90,000 years ago (). As modern humans spread out from Africa, they encountered other hominins such as ''Homo neanderthalensis'' and the
Denisovans The Denisovans or Denisova hominins ) are an extinct species or subspecies of archaic human that ranged across Asia during the Lower and Middle Paleolithic. Denisovans are known from few physical remains and consequently, most of what is kn ...
, who may have evolved from populations of ''Homo erectus'' that had left Africa around . The nature of interaction between early humans and these sister species has been a long-standing source of controversy, the question being whether humans replaced these earlier species or whether they were in fact similar enough to interbreed, in which case these earlier populations may have contributed genetic material to modern humans. This migration out of Africa is estimated to have begun about 70–50,000 years BP and modern humans subsequently spread globally, replacing earlier hominins either through competition or hybridization. They inhabited Eurasia and Oceania by 40,000 years BP, and the Americas by at least 14,500 years BP.


Inter-species breeding

The hypothesis of interbreeding, also known as hybridization, admixture or hybrid-origin theory, has been discussed ever since the discovery of Neanderthal remains in the 19th century. The linear view of human evolution began to be abandoned in the 1970s as different species of humans were discovered that made the linear concept increasingly unlikely. In the 21st century with the advent of molecular biology techniques and computerization,
whole-genome sequencing Whole genome sequencing (WGS), also known as full genome sequencing, complete genome sequencing, or entire genome sequencing, is the process of determining the entirety, or nearly the entirety, of the DNA sequence of an organism's genome at a ...
of Neanderthal and human
genome In the fields of molecular biology and genetics, a genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding ...
were performed, confirming recent admixture between different human species. In 2010, evidence based on molecular biology was published, revealing unambiguous examples of interbreeding between archaic and modern humans during the
Middle Paleolithic The Middle Paleolithic (or Middle Palaeolithic) is the second subdivision of the Paleolithic or Old Stone Age as it is understood in Europe, Africa and Asia. The term Middle Stone Age is used as an equivalent or a synonym for the Middle Paleo ...
and early
Upper Paleolithic The Upper Paleolithic (or Upper Palaeolithic) is the third and last subdivision of the Paleolithic or Old Stone Age. Very broadly, it dates to between 50,000 and 12,000 years ago (the beginning of the Holocene), according to some theories coi ...
. It has been demonstrated that interbreeding happened in several independent events that included Neanderthals and Denisovans, as well as several unidentified hominins. Today, approximately 2% of DNA from all non-African populations (including Europeans, Asians, and
Oceanians Oceania is a region centered on the islands of the tropical Pacific Ocean. Conceptions of what constitutes Oceania vary, with it being defined in various ways, often geopolitically or geographically. In the geopolitical conception used by the Un ...
) is Neanderthal, with traces of Denisovan heritage. Also, 4–6% of modern Melanesian genetics are Denisovan. Comparisons of the human genome to the genomes of Neandertals, Denisovans and apes can help identify features that set modern humans apart from other hominin species. In a 2016
comparative genomics Comparative genomics is a field of biological research in which the genomic features of different organisms are compared. The genomic features may include the DNA sequence, genes, gene order, regulatory sequences, and other genomic structural ...
study, a Harvard Medical School/UCLA research team made a world map on the distribution and made some predictions about where Denisovan and Neanderthal genes may be impacting modern human biology. For example, comparative studies in the mid-2010s found several traits related to neurological, immunological, developmental, and metabolic phenotypes, that were developed by archaic humans to European and Asian environments and inherited to modern humans through admixture with local hominins. Although the narratives of human evolution are often contentious, several discoveries since 2010 show that human evolution should not be seen as a simple linear or branched progression, but a mix of related species. In fact, genomic research has shown that hybridization between substantially diverged lineages is the rule, not the exception, in human evolution. Furthermore, it is argued that hybridization was an essential creative force in the emergence of modern humans.


Before ''Homo''


Early evolution of primates

The evolutionary history of the primates can be traced back 65 million years. One of the oldest known primate-like mammal species, the ''
Plesiadapis ''Plesiadapis'' is one of the oldest known primate-like mammal genera which existed about 58–55 million years ago in North America and Europe. ''Plesiadapis'' means "near-Adapis", which is a reference to the adapiform primate of the Eocene p ...
'', came from North America; another, ''
Archicebus ''Archicebus'' is a genus of fossil primates that lived in the early Eocene forests (~55.8–54.8 million years ago) of what is now Jingzhou in the Hubei Province in central China, discovered in 2003. The only known species, ''A. achil ...
'', came from China. Other similar basal primates were widespread in Eurasia and Africa during the tropical conditions of the Paleocene and
Eocene The Eocene ( ) Epoch is a geological epoch that lasted from about 56 to 33.9 million years ago (mya). It is the second epoch of the Paleogene Period in the modern Cenozoic Era. The name ''Eocene'' comes from the Ancient Greek (''ēṓs'', " ...
. David R. Begun concluded that early primates flourished in Eurasia and that a lineage leading to the African apes and humans, including to ''Dryopithecus'', migrated south from Europe or Western Asia into Africa. The surviving tropical population of primates—which is seen most completely in the Upper Eocene and lowermost Oligocene fossil beds of the Faiyum depression southwest of Cairo—gave rise to all extant primate species, including the lemurs of Madagascar, lorises of Southeast Asia, galagos or "bush babies" of Africa, and to the Simian, anthropoids, which are the New World monkey, Platyrrhines or New World monkeys, the Catarrhini, Catarrhines or Old World monkeys, and the great apes, including humans and other hominids. The earliest known Catarrhini, catarrhine is ''Kamoyapithecus'' from uppermost Oligocene at Eragaleit in the northern Great Rift Valley, Kenya, Great Rift Valley in Kenya, dated to 24 million years ago. Its ancestry is thought to be species related to ''Aegyptopithecus'', ''Propliopithecus'', and ''Parapithecus'' from the Faiyum, at around 35 million years ago. In 2010, ''Saadanius'' was described as a close relative of the last common ancestor of the Crown group, crown catarrhines, and tentatively dated to 29–28 million years ago, helping to fill an 11-million-year gap in the fossil record. In the Early Miocene, about 22 million years ago, the many kinds of Arboreal locomotion, arboreally adapted primitive catarrhines from East Africa suggest a long history of prior diversification. Fossils at 20 million years ago include fragments attributed to ''Victoriapithecus'', the earliest Old World monkey. Among the genera thought to be in the ape lineage leading up to 13 million years ago are ''Proconsul (mammal), Proconsul'', ''Rangwapithecus'', ''Dendropithecus'', ''Limnopithecus'', ''Nacholapithecus'', ''Equatorius'', ''Nyanzapithecus pickfordi, Nyanzapithecus'', ''Afropithecus'', ''Heliopithecus'', and ''Kenyapithecus'', all from East Africa. The presence of other generalized non-cercopithecids of Middle Miocene from sites far distant—''Otavipithecus'' from cave deposits in Namibia, and ''Pierolapithecus'' and ''Dryopithecus'' from France, Spain and Austria—is evidence of a wide diversity of forms across Africa and the Mediterranean basin during the relatively warm and equable climatic regimes of the Early and Middle Miocene. The youngest of the Miocene hominoids, ''Oreopithecus'', is from coal beds in Italy that have been dated to 9 million years ago. Molecular evidence indicates that the lineage of gibbons (family Hylobatidae) diverged from the line of great apes some 18–12 million years ago, and that of orangutans (subfamily Ponginae) diverged from the other great apes at about 12 million years; there are no fossils that clearly document the ancestry of gibbons, which may have originated in a so-far-unknown Southeast Asian hominoid population, but fossil proto-orangutans may be represented by ''Sivapithecus'' from India and ''Griphopithecus'' from Turkey, dated to around 10 million years ago.


Divergence of the human clade from other great apes

Species close to the last common ancestor of gorillas, chimpanzees and humans may be represented by ''Nakalipithecus'' fossils found in Kenya and ''Ouranopithecus'' found in Greece. Molecular evidence suggests that between 8 and 4 million years ago, first the gorillas, and then the chimpanzees (genus ''Pan'') split off from the line leading to the humans. Human DNA is approximately 98.4% identical to that of chimpanzees when comparing single nucleotide polymorphisms (see human evolutionary genetics). The fossil record, however, of gorillas and chimpanzees is limited; both poor preservation – rain forest soils tend to be acidic and dissolve bone – and sampling bias probably contribute to this problem. Other hominins probably adapted to the drier environments outside the equatorial belt; and there they encountered antelope, hyenas, dogs, pigs, elephants, horses, and others. The equatorial belt contracted after about 8 million years ago, and there is very little fossil evidence for the split—thought to have occurred around that time—of the hominin lineage from the lineages of gorillas and chimpanzees. The earliest fossils argued by some to belong to the human lineage are ''Sahelanthropus tchadensis'' (7 Ma) and ''Orrorin tugenensis'' (6 Ma), followed by ''Ardipithecus'' (5.5–4.4 Ma), with species ''Ar. kadabba'' and ''Ar. ramidus''. It has been argued in a study of the life history of ''Ar. ramidus'' that the species provides evidence for a suite of anatomical and behavioral adaptations in very early hominins unlike any species of extant great ape. This study demonstrated affinities between the skull morphology of ''Ar. ramidus'' and that of infant and juvenile chimpanzees, suggesting the species evolved a juvenalised or paedomorphic craniofacial morphology via Heterochrony, heterochronic dissociation of growth trajectories. It was also argued that the species provides support for the notion that very early hominins, akin to bonobos (''Pan paniscus'') the less aggressive species of the genus ''Pan'', may have evolved via the process of self-domestication. Consequently, arguing against the so-called "chimpanzee referential model" the authors suggest it is no longer tenable to use chimpanzee (''Pan troglodytes'') social and mating behaviors in models of early hominin social evolution. When commenting on the absence of aggressive canine morphology in ''Ar. ramidus'' and the implications this has for the evolution of hominin social psychology, they wrote: The authors argue that many of the basic human adaptations evolved in the ancient forest and woodland ecosystems of late Miocene and early Pliocene Africa. Consequently, they argue that humans may not represent evolution from a chimpanzee-like ancestor as has traditionally been supposed. This suggests many modern human adaptations represent Phylogeny, phylogenetically deep traits and that the behavior and morphology of chimpanzees may have evolved subsequent to the split with the common ancestor they share with humans.


Genus ''Australopithecus''

The genus ''Australopithecus'' evolved in eastern Africa around 4 million years ago before spreading throughout the continent and eventually becoming extinct 2 million years ago. During this time period various forms of australopiths existed, including ''
Australopithecus anamensis ''Australopithecus anamensis'' is a hominin species that lived approximately between 4.2 and 3.8 million years ago and is the oldest known ''Australopithecus'' species, living during the Plio-Pleistocene era. Nearly one hundred fossil specimens ...
'', ''Australopithecus afarensis, Au. afarensis'', ''Australopithecus sediba, Au. sediba'', and ''Australopithecus africanus, Au. africanus''. There is still some debate among academics whether certain African hominid species of this time, such as ''Australopithecus robustus, Au. robustus'' and ''Australopithecus boisei, Au. boisei'', constitute members of the same genus; if so, they would be considered to be ''Au. robust australopiths'' whilst the others would be considered ''Au. gracile australopiths''. However, if these species do indeed constitute their own genus, then they may be given their own name, ''Paranthropus''. * ''
Australopithecus ''Australopithecus'' (, ; ) is a genus of early hominins that existed in Africa during the Late Pliocene and Early Pleistocene. The genus ''Homo'' (which includes modern humans) emerged within ''Australopithecus'', as sister to e.g. ''Austral ...
'' (4–1.8 Ma), with species ''Australopithecus anamensis, Au. anamensis'', ''Australopithecus afarensis, Au. afarensis'', ''Australopithecus africanus, Au. africanus'', ''Australopithecus bahrelghazali, Au. bahrelghazali'', ''Australopithecus garhi, Au. garhi'', and ''Australopithecus sediba, Au. sediba''; * ''Kenyanthropus'' (3–2.7 Ma), with species ''Kenyanthropus, K. platyops''; * ''
Paranthropus ''Paranthropus'' is a genus of extinct hominin which contains two widely accepted species: '' P. robustus'' and '' P. boisei''. However, the validity of ''Paranthropus'' is contested, and it is sometimes considered to be synonymous with '' Au ...
'' (3–1.2 Ma), with species ''Paranthropus aethiopicus, P. aethiopicus'', ''Paranthropus boisei, P. boisei'', and ''Paranthropus robustus, P. robustus'' A new proposed species ''Australopithecus deyiremeda'' is claimed to have been discovered living at the same time period of ''Australopithecus afarensis, Au. afarensis''. There is debate if Australopithecus afarensis, Au. Australopithecus deyiremeda, deyiremeda is a new species or is ''Australopithecus afarensis, Au. afarensis.'' ''Australopithecus prometheus'', otherwise known as Little Foot has recently been dated at 3.67 million years old through a new dating technique, making the genus ''Australopithecus'' as old as ''afarensis''. Given the opposable big toe found on Little Foot, it seems that the specimen was a good climber. It is thought given the night predators of the region that he built a nesting platform at night in the trees in a similar fashion to chimpanzees and gorillas.


Evolution of genus ''Homo''

The earliest documented representative of the genus ''Homo'' is ''Homo habilis'', which evolved around , and is arguably the earliest species for which there is positive evidence of the use of stone tools. The brains of these early hominins were about the same size as that of a chimpanzee, although it has been suggested that this was the time in which the human SRGAP2 gene doubled, producing a more rapid wiring of the frontal cortex. During the next million years a process of rapid
encephalization Encephalization quotient (EQ), encephalization level (EL), or just encephalization is a relative brain size measure that is defined as the ratio between observed to predicted brain mass for an animal of a given size, based on nonlinear regressi ...
occurred, and with the arrival of ''
Homo erectus ''Homo erectus'' (; meaning " upright man") is an extinct species of archaic human from the Pleistocene, with its earliest occurrence about 2 million years ago. Several human species, such as ''H. heidelbergensis'' and ''H. antecessor' ...
'' and ''Homo ergaster'' in the fossil#Dating, fossil record, cranial capacity had doubled to 850 cm3. (Such an increase in human brain size is equivalent to each generation having 125,000 more neurons than their parents.) It is believed that ''Homo erectus'' and ''Homo ergaster'' were the first to use fire and complex tools, and were the first of the hominin line to leave Africa, spreading throughout Africa, Asia, and Europe between . According to the recent African origin of modern humans theory, modern humans evolved in Africa possibly from ''Homo heidelbergensis'', ''Homo rhodesiensis'' or ''Homo antecessor'' and migrated out of the continent some 50,000 to 100,000 years ago, gradually replacing local populations of ''Homo erectus'', Denisovan, Denisova hominins, ''Homo floresiensis'', ''Homo luzonensis'' and ''Neanderthal, Homo neanderthalensis''. Archaic humans, Archaic ''Homo sapiens'', the forerunner of
anatomically modern humans Early modern human (EMH) or anatomically modern human (AMH) are terms used to distinguish ''Homo sapiens'' (the only extant Hominina species) that are anatomically consistent with the range of phenotypes seen in contemporary humans from extin ...
, evolved in the
Middle Paleolithic The Middle Paleolithic (or Middle Palaeolithic) is the second subdivision of the Paleolithic or Old Stone Age as it is understood in Europe, Africa and Asia. The term Middle Stone Age is used as an equivalent or a synonym for the Middle Paleo ...
between 400,000 and 250,000 years ago. Recent DNA evidence suggests that several haplotypes of
Neanderthal Neanderthals (, also ''Homo neanderthalensis'' and erroneously ''Homo sapiens neanderthalensis''), also written as Neandertals, are an extinct species or subspecies of archaic humans who lived in Eurasia until about 40,000 years ago. While ...
origin are present among all non-African populations, and Neanderthals and other hominins, such as
Denisovan The Denisovans or Denisova hominins ) are an extinct species or subspecies of archaic human that ranged across Asia during the Lower and Middle Paleolithic. Denisovans are known from few physical remains and consequently, most of what is know ...
s, may have contributed up to 6% of their
genome In the fields of molecular biology and genetics, a genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding ...
to present-day humans, suggestive of a Archaic human admixture with modern humans, limited interbreeding between these species. The transition to behavioral modernity with the development of symbolic culture, language, and specialized lithic technology happened around 50,000 years ago, according to some anthropologists, although others point to evidence that suggests that a gradual change in behavior took place over a longer time span. ''Homo sapiens'' is the only Extant taxon, extant species of its genus, ''Homo''. While some (extinct) ''Homo'' species might have been ancestors of ''Homo sapiens'', many, perhaps most, were likely "cousins", having speciated away from the ancestral hominin line. There is yet no consensus as to which of these groups should be considered a separate species and which should be a subspecies; this may be due to the dearth of fossils or to the slight differences used to classify species in the genus ''Homo''. The
Sahara pump theory The Sahara pump theory is a hypothesis that explains how flora and fauna migrated between Eurasia and Africa via a land bridge in the Levant region. It posits that extended periods of abundant rainfall lasting many thousands of years ( pluvia ...
(describing an occasionally passable Wet Sahara, "wet" Sahara desert) provides one possible explanation of the early variation in the genus ''Homo''. Based on archaeological and paleontological evidence, it has been possible to infer, to some extent, the ancient dietary practices of various ''Homo'' species and to study the role of diet in physical and behavioral evolution within ''Homo''. Some anthropologists and archaeologists subscribe to the Toba catastrophe theory, which posits that the Supervolcano, supereruption of Lake Toba on Sumatran island in Indonesia some 70,000 years ago caused global consequences, killing the majority of humans and creating a population bottleneck that affected the genetic inheritance of all humans today. The genetic and archaeological evidence for this remains in question however.


''H. habilis'' and ''H. gautengensis''

''Homo habilis'' lived from about 2.8 to 1.4 Ma. The species evolved in South and East Africa in the Piacenzian, Late Pliocene or Early Pleistocene, 2.5–2 Ma, when it diverged from the australopithecines with the development of smaller molars and larger brains. One of the first known hominins, it made stone tool, tools from stone and perhaps animal bones, leading to its name ''homo'' ''habilis'' (Latin 'handy man') bestowed by discoverer
Louis Leakey Louis Seymour Bazett Leakey (7 August 1903 – 1 October 1972) was a Kenyan-British palaeoanthropologist and archaeologist whose work was important in demonstrating that humans evolved in Africa, particularly through discoveries made at Olduv ...
. Some scientists have proposed moving this species from ''Homo'' into ''Australopithecus'' due to the morphology of its skeleton being more adapted to Arboreal locomotion, living in trees rather than Bipedalism, walking on two legs like later hominins. In May 2010, a new species, ''Homo gautengensis'', was discovered in South Africa.


''H. rudolfensis'' and ''H. georgicus''

These are proposed species names for fossils from about 1.9–1.6 Ma, whose relation to ''Homo habilis'' is not yet clear. * ''Homo rudolfensis'' refers to a single, incomplete skull from Kenya. Scientists have suggested that this was a specimen of ''Homo habilis'', but this has not been confirmed. * ''Homo georgicus'', from
Georgia Georgia most commonly refers to: * Georgia (country), a country in the Caucasus region of Eurasia * Georgia (U.S. state), a state in the Southeast United States Georgia may also refer to: Places Historical states and entities * Related to t ...
, may be an intermediate form between ''Homo habilis'' and ''Homo erectus'', or a subspecies of ''Homo erectus''.


''H. ergaster'' and ''H. erectus''

The first fossils of ''Homo erectus'' were discovered by Dutch physician Eugene Dubois in 1891 on the Indonesian island of Java. He originally named the material ''Anthropopithecus erectus'' (1892–1893, considered at this point as a chimpanzee-like fossil primate) and ''Pithecanthropus erectus'' (1893–1894, changing his mind as of based on its morphology, which he considered to be intermediate between that of humans and apes). Years later, in the 20th century, the German physician and Paleoanthropology, paleoanthropologist Franz Weidenreich (1873–1948) compared in detail the characters of Dubois' Java Man, then named ''Pithecanthropus erectus'', with the characters of the Peking Man, then named ''Sinanthropus pekinensis''. Weidenreich concluded in 1940 that because of their anatomical similarity with modern humans it was necessary to gather all these specimens of Java and China in a single species of the genus ''
Homo ''Homo'' () is the genus that emerged in the (otherwise extinct) genus '' Australopithecus'' that encompasses the extant species ''Homo sapiens'' (modern humans), plus several extinct species classified as either ancestral to or closely relat ...
'', the species ''
Homo erectus ''Homo erectus'' (; meaning " upright man") is an extinct species of archaic human from the Pleistocene, with its earliest occurrence about 2 million years ago. Several human species, such as ''H. heidelbergensis'' and ''H. antecessor' ...
''. ''Homo erectus'' lived from about 1.8 Ma to about 70,000 years ago – which would indicate that they were probably wiped out by the Toba catastrophe; however, nearby ''Homo floresiensis'' survived it. The early phase of ''Homo erectus'', from 1.8 to 1.25 Ma, is considered by some to be a separate species, ''Homo ergaster'', or as ''Homo erectus ergaster'', a subspecies of ''Homo erectus''. Many paleoanthropologists now use the term ''Homo ergaster'' for the non-Asian forms of this group, and reserve ''Homo erectus'' only for those fossils that are found in Asia and meet certain skeletal and dental requirements which differ slightly from ''H. ergaster''. In Africa in the Early Pleistocene, 1.5–1 Ma, some populations of ''Homo habilis'' are thought to have evolved larger brains and to have made more elaborate stone tools; these differences and others are sufficient for anthropologists to classify them as a new species, ''Homo erectus''—in Africa. The evolution of locking knees and the movement of the foramen magnum are thought to be likely drivers of the larger population changes. This species also may have used fire to cook meat. Catching Fire: How Cooking Made Us Human, Richard Wrangham notes that Homo seems to have been ground dwelling, with reduced intestinal length, smaller dentition, and "brains [swollen] to their current, horrendously fuel-inefficient size", and hypothesizes that control of fire and cooking, which released increased nutritional value, was the key adaptation that separated Homo from tree-sleeping Australopithecines.


''H. cepranensis'' and ''H. antecessor''

These are proposed as species intermediate between ''H. erectus'' and ''H. heidelbergensis''. * ''H. antecessor'' is known from fossils from Spain and England that are dated 1.2 Ma–500 Year#SI prefix multipliers, ka. * ''Homo cepranensis, H. cepranensis'' refers to a single skull cap from Italy, estimated to be about 800,000 years old.


''H. heidelbergensis''

''H. heidelbergensis'' ("Heidelberg Man") lived from about 800,000 to about 300,000 years ago. Also proposed as ''Homo sapiens heidelbergensis'' or ''Homo sapiens paleohungaricus''.


''H. rhodesiensis'', and the Gawis cranium

* ''H. rhodesiensis'', estimated to be 300,000–125,000 years old. Most current researchers place Rhodesian Man within the group of ''Homo heidelbergensis'', though other designations such as archaic ''Homo sapiens'' and ''Homo sapiens rhodesiensis'' have been proposed. * In February 2006 a fossil, the Gawis cranium, was found which might possibly be a species intermediate between ''H. erectus'' and ''H. sapiens'' or one of many evolutionary dead ends. The skull from Gawis, Ethiopia, is believed to be 500,000–250,000 years old. Only summary details are known, and the finders have not yet released a peer-reviewed study. Gawis man's facial features suggest its being either an intermediate species or an example of a "Bodo man" female.


Neanderthal and Denisovan

''Homo neanderthalensis'', alternatively designated as ''Homo sapiens neanderthalensis'', lived in Europe and Asia from 400,000 to about 28,000 years ago. There are a number of clear anatomical differences between
anatomically modern humans Early modern human (EMH) or anatomically modern human (AMH) are terms used to distinguish ''Homo sapiens'' (the only extant Hominina species) that are anatomically consistent with the range of phenotypes seen in contemporary humans from extin ...
(AMH) and Neanderthal specimens, many relating to the superior Neanderthal adaptation to cold environments. Neanderthal surface to volume ratio was even lower than that among modern Inuit populations, indicating superior retention of body heat. Neanderthals also had significantly larger brains, as shown from brain endocasts, casting doubt on their intellectual inferiority to modern humans. However, the higher body mass of Neanderthals may have required larger brain mass for body control. Also, recent research by Pearce, Chris Stringer, Stringer, and Dunbar has shown important differences in brain architecture. The larger size of the Neanderthal orbital chamber and occipital lobe suggests that they had a better visual acuity than modern humans, useful in the dimmer light of glacial Europe. Neanderthals may have had less Dunbar's number, brain capacity available for social functions. Inferring social group size from endocranial volume (minus occipital lobe size) suggests that Neanderthal groups may have been limited to 120 individuals, compared to 144 possible relationships for modern humans. Larger social groups could imply that modern humans had less risk of inbreeding within their clan, trade over larger areas (confirmed in the distribution of stone tools), and faster spread of social and technological innovations. All these may have all contributed to modern Homo sapiens replacing Neanderthal populations by 28,000 BP. Earlier evidence from sequencing mitochondrial DNA suggested that no significant gene flow occurred between ''H. neanderthalensis'' and ''H. sapiens'', and that the two were separate species that shared a common ancestor about 660,000 years ago. However, a sequencing of the Neanderthal genome in 2010 indicated that Neanderthals did indeed interbreed with anatomically modern humans c. 45,000-80,000 years ago, around the time modern humans migrated out from Africa, but before they dispersed throughout Europe, Asia and elsewhere. The genetic sequencing of a 40,000-year-old Peștera cu Oase, human skeleton from Romania showed that 11% of its genome was Neanderthal, implying the individual had a Neanderthal ancestor 4–6 generations previously, in addition to a contribution from earlier interbreeding in the Middle East. Though this interbred Romanian population seems not to have been ancestral to modern humans, the finding indicates that interbreeding happened repeatedly. All modern non-African humans have about 1% to 4% (or 1.5% to 2.6% by more recent data) of their DNA derived from Neanderthals. This finding is consistent with recent studies indicating that the divergence of some human alleles dates to one Ma, although this interpretation has been questioned. Neanderthals and AMH ''Homo sapiens'' could have co-existed in Europe for as long as 10,000 years, during which AMH populations exploded, vastly outnumbering Neanderthals, possibly outcompeting them by sheer numbers. In 2008, archaeologists working at the site of Denisova Cave in the Altai Mountains of Siberia uncovered a small bone fragment from the fifth finger of a juvenile member of another human species, the Denisovans. Artifacts, including a bracelet, excavated in the cave at the same level were Radiocarbon dating, carbon dated to around 40,000 BP. As DNA had survived in the fossil fragment due to the cool climate of the Denisova Cave, both mtDNA and nuclear DNA were sequenced. While the divergence point of the mtDNA was unexpectedly deep in time, the full genomic sequence suggested the Denisovans belonged to the same lineage as Neanderthals, with the two diverging shortly after their line split from the lineage that gave rise to modern humans. Modern humans are known to have overlapped with Neanderthals in Europe and the Near East for possibly more than 40,000 years, and the discovery raises the possibility that Neanderthals, Denisovans, and modern humans may have co-existed and interbred. The existence of this distant branch creates a much more complex picture of humankind during the Late Pleistocene than previously thought. Evidence has also been found that as much as 6% of the DNA of some modern
Melanesians Melanesians are the predominant and indigenous inhabitants of Melanesia, in a wide area from Indonesia's New Guinea to as far East as the islands of Vanuatu and Fiji. Most speak either one of the many languages of the Austronesian language fam ...
derive from Denisovans, indicating limited interbreeding in Southeast Asia. Alleles thought to have originated in Neanderthals and Denisovans have been identified at several genetic loci in the genomes of modern humans outside Africa. HLA haplotypes from Denisovans and Neanderthal represent more than half the HLA alleles of modern Eurasians, indicating strong positive selection for these introgression, introgressed alleles. Corinne Simoneti at Vanderbilt University, in Nashville and her team have found from medical records of 28,000 people of European descent that the presence of Neanderthal DNA segments may be associated with a higher rate of depression. The flow of genes from Neanderthal populations to modern humans was not all one way. Sergi Castellano of the Max Planck Institute for Evolutionary Anthropology reported in 2016 that while Denisovan and Neanderthal genomes are more related to each other than they are to us, Siberian Neanderthal genomes show more similarity to modern human genes than do European Neanderthal populations. This suggests Neanderthal populations interbred with modern humans around 100,000 years ago, probably somewhere in the Near East. Studies of a Neanderthal child at Gibraltar show from brain development and tooth eruption that Neanderthal children may have matured more rapidly than Homo sapiens.


''H. floresiensis''

''H. floresiensis'', which lived from approximately 190,000 to 50,000 years before present (BP), has been nicknamed the ''hobbit'' for its small size, possibly a result of insular dwarfism. ''H. floresiensis'' is intriguing both for its size and its age, being an example of a recent species of the genus ''Homo'' that exhibits derived traits not shared with modern humans. In other words, ''H. floresiensis'' shares a common ancestor with modern humans, but split from the modern human lineage and followed a distinct evolutionary path. The main find was a skeleton believed to be a woman of about 30 years of age. Found in 2003, it has been dated to approximately 18,000 years old. The living woman was estimated to be one meter in height, with a brain volume of just 380 cm3 (considered small for a chimpanzee and less than a third of the ''H. sapiens'' average of 1400 cm3). However, there is an ongoing debate over whether ''H. floresiensis'' is indeed a separate species. Some scientists hold that ''H. floresiensis'' was a modern ''H. sapiens'' with pathological dwarfism. This hypothesis is supported in part, because some modern humans who live on Flores, the Indonesian island where the skeleton was found, are Pygmy peoples, pygmies. This, coupled with pathological dwarfism, could have resulted in a significantly diminutive human. The other major attack on ''H. floresiensis'' as a separate species is that it was found with tools only associated with ''H. sapiens''. The hypothesis of pathological dwarfism, however, fails to explain additional Homo floresiensis#Anatomy, anatomical features that are unlike those of modern humans (diseased or not) but much like those of ancient members of our genus. Aside from cranial features, these features include the form of bones in the wrist, forearm, shoulder, knees, and feet. Additionally, this hypothesis fails to explain the find of multiple examples of individuals with these same characteristics, indicating they were common to a large population, and not limited to one individual. In 2016, fossil teeth and a partial jaw from hominins assumed to be ancestral to ''H. floresiensis'' were discovered at Mata Menge, about from Liang Bua. They date to about 700,000 years ago and are noted by Australian archaeologist Gerrit van den Bergh for being even smaller than the later fossils.


''H. luzonensis''

A small number of specimens from the island of Luzon, dated 50,000 to 67,000 years ago, have recently been assigned by their discoverers, based on dental characteristics, to a novel human species, ''H. luzonensis''.


''H. sapiens''

''H. sapiens'' (the adjective ''wikt:sapiens, sapiens'' is Latin for "wise" or "intelligent") emerged in Africa around 300,000 years ago, likely derived from ''Homo heidelbergensis'' or a related lineage. In September 2019, scientists reported the computerized determination, based on 260 CT scans, of a virtual Human skull, skull shape of the last common human ancestor to modern humans/''H. sapiens'', representative of the earliest modern humans, and suggested that modern humans arose between 260,000 and 350,000 years ago through a merging of populations in East Africa, East and South Africa. Between 400,000 years ago and the second interglacial period in the Middle Pleistocene, around 250,000 years ago, the trend in Brain size#Cranial capacity, intra-cranial volume expansion and the elaboration of stone tool technologies developed, providing evidence for a transition from ''H. erectus'' to ''H. sapiens''. The direct evidence suggests there was a migration of ''H. erectus'' out of Africa, then a further speciation of ''H. sapiens'' from ''H. erectus'' in Africa. A subsequent migration (both within and out of Africa) eventually replaced the earlier dispersed ''H. erectus''. This migration and origin theory is usually referred to as the "recent single-origin hypothesis" or "out of Africa" theory. ''H. sapiens'' Archaic human admixture with modern humans, interbred with archaic humans both in Africa and in Eurasia, in Eurasia notably with Neanderthals and
Denisovans The Denisovans or Denisova hominins ) are an extinct species or subspecies of archaic human that ranged across Asia during the Lower and Middle Paleolithic. Denisovans are known from few physical remains and consequently, most of what is kn ...
. The Toba catastrophe theory, which postulates a population bottleneck for ''H. sapiens'' about 70,000 years ago, was controversial from its first proposal in the 1990s and by the 2010s had very little support. Distinctive human genetic variability has arisen as the result of the founder effect, by Archaic human admixture with modern humans, archaic admixture and by Recent human evolution, recent evolutionary pressures.


Use of tools

The use of tools has been interpreted as a sign of intelligence, and it has been theorized that tool use may have stimulated certain aspects of human evolution, especially the continued expansion of the human brain. Paleontology has yet to explain the expansion of this organ over millions of years despite being extremely demanding in terms of energy consumption. The brain of a modern human consumes about 13 watts (260 kilocalories per day), a fifth of the body's resting power consumption. Increased tool use would allow hunting for energy-rich meat products, and would enable processing more energy-rich plant products. Researchers have suggested that early hominins were thus under evolutionary pressure to increase their capacity to create and use tools. Precisely when early humans started to use tools is difficult to determine, because the more primitive these tools are (for example, sharp-edged stones) the more difficult it is to decide whether they are natural objects or human artifacts. There is some evidence that the australopithecines (4 Ma) Osteodontokeratic culture, may have used broken bones as tools, but this is debated. Tool use by animals, Many species make and use tools, but it is the human genus that dominates the areas of making and using more complex tools. The oldest known tools are flakes from West Turkana, Kenya, which date to 3.3 million years ago. The next oldest stone tools are from Gona, Ethiopia, and are considered the beginning of the Oldowan technology. These tools date to about 2.6 million years ago. A ''Homo'' fossil was found near some
Oldowan The Oldowan (or Mode I) was a widespread stone tool archaeological industry (style) in prehistory. These early tools were simple, usually made with one or a few flakes chipped off with another stone. Oldowan tools were used during the Lower ...
tools, and its age was noted at 2.3 million years old, suggesting that maybe the ''Homo'' species did indeed create and use these tools. It is a possibility but does not yet represent solid evidence. The third metacarpal styloid process enables the hand bone to lock into the wrist bones, allowing for greater amounts of pressure to be applied to the wrist and hand from a grasping thumb and fingers. It allows humans the dexterity and strength to make and use complex tools. This unique anatomical feature separates humans from apes and other nonhuman primates, and is not seen in human fossils older than 1.8 million years. Bernard Wood noted that ''Paranthropus'' co-existed with the early ''Homo'' species in the area of the "Oldowan Industrial Complex" over roughly the same span of time. Although there is no direct evidence which identifies ''Paranthropus'' as the tool makers, their anatomy lends to indirect evidence of their capabilities in this area. Most paleoanthropologists agree that the early ''Homo'' species were indeed responsible for most of the Oldowan tools found. They argue that when most of the Oldowan tools were found in association with human fossils, ''Homo'' was always present, but ''Paranthropus'' was not. In 1994, Randall Susman used the anatomy of opposable thumbs as the basis for his argument that both the ''Homo'' and ''Paranthropus'' species were toolmakers. He compared bones and muscles of human and chimpanzee thumbs, finding that humans have 3 muscles which are lacking in chimpanzees. Humans also have thicker metacarpals with broader heads, allowing more precise grasping than the chimpanzee hand can perform. Susman posited that modern anatomy of the human opposable thumb is an evolutionary response to the requirements associated with making and handling tools and that both species were indeed toolmakers.


Stone tools

Stone tools are first attested around 2.6 million years ago, when hominins in Eastern Africa used so-called core Oldowan, tools, Chopper (archaeology), choppers made out of round cores that had been split by simple strikes. This marks the beginning of the Paleolithic, or Old Stone Age; its end is taken to be the end of the last Last glacial period, Ice Age, around 10,000 years ago. The Paleolithic is subdivided into the Lower Paleolithic (Early Stone Age), ending around 350,000–300,000 years ago, the
Middle Paleolithic The Middle Paleolithic (or Middle Palaeolithic) is the second subdivision of the Paleolithic or Old Stone Age as it is understood in Europe, Africa and Asia. The term Middle Stone Age is used as an equivalent or a synonym for the Middle Paleo ...
(Middle Stone Age), until 50,000–30,000 years ago, and the
Upper Paleolithic The Upper Paleolithic (or Upper Palaeolithic) is the third and last subdivision of the Paleolithic or Old Stone Age. Very broadly, it dates to between 50,000 and 12,000 years ago (the beginning of the Holocene), according to some theories coi ...
, (Late Stone Age), 50,000–10,000 years ago. Archaeologists working in the Great Rift Valley in Kenya have discovered the oldest known stone tools in the world. Dated to around 3.3 million years ago, the implements are some 700,000 years older than stone tools from Ethiopia that previously held this distinction. The period from 700,000 to 300,000 years ago is also known as the Acheulean, when ''H. ergaster'' (or ''erectus'') made large stone hand axes out of flint and quartzite, at first quite rough (Early Acheulian), later "retouch (lithics), retouched" by additional, more-subtle strikes at the sides of the Lithic flake, flakes. After 350,000 BP the more refined so-called Levallois technique was developed, a series of consecutive strikes, by which scrapers, slicers ("racloirs"), needles, and flattened needles were made. Finally, after about 50,000 BP, ever more refined and specialized flint tools were made by the Neanderthals and the immigrant Cro-Magnons (knives, blades, skimmers). Bone tools were also made by ''H. sapiens'' in Africa by 90–70,000 years ago and are also known from early ''H. sapiens'' sites in Eurasia by about 50,000 years ago.


Transition to behavioral modernity

Anthropologists describe modern human behavior to include cultural and behavioral traits such as specialization of tools, use of jewellery and images (such as cave drawings), organization of living space, rituals (such as grave gifts), specialized hunting techniques, exploration of less hospitable geographical areas, and barter trade networks, as well as more general traits such as language and complex symbolic thinking. Debate continues as to whether a "revolution" led to modern humans ("big bang of human consciousness"), or whether the evolution was more gradual. Until about 50,000–40,000 years ago, the use of stone tools seems to have progressed stepwise. Each phase (''H. habilis'', ''H. ergaster'', ''H. neanderthalensis'') marked a new technology, followed by very slow development until the next phase. Currently paleoanthropologists are debating whether these ''Homo'' species possessed some or many modern human behaviors. They seem to have been culturally conservative, maintaining the same technologies and foraging patterns over very long periods. Around 50,000 BP, human culture started to evolve more rapidly. The transition to behavioral modernity has been characterized by some as a "Great Leap Forward", or as the "Upper Palaeolithic Revolution", due to the sudden appearance in the archaeological record of distinctive signs of modern behavior and big game hunting. Evidence of behavioral modernity significantly earlier also exists from Africa, with older evidence of abstract imagery, widened subsistence strategies, more sophisticated tools and weapons, and other "modern" behaviors, and many scholars have recently argued that the transition to modernity occurred sooner than previously believed. Some other scholars consider the transition to have been more gradual, noting that some features had already appeared among archaic African ''Homo sapiens'' 300,000–200,000 years ago. Recent evidence suggests that the Australian Aboriginal population separated from the African population 75,000 years ago, and that they made a 160 km sea journey 60,000 years ago, which may diminish the significance of the Upper Paleolithic Revolution. Modern humans started burying their dead, making clothing from animal hides, hunting with more sophisticated techniques (such as using trapping pit, pit traps or driving animals off cliffs), and cave painting. As human culture advanced, different populations innovated existing technologies: artifacts such as fish hooks, buttons, and bone needles show signs of cultural variation, which had not been seen prior to 50,000 BP. Typically, the older ''H. neanderthalensis'' populations did not vary in their technologies, although the Chatelperronian assemblages have been found to be Neanderthal imitations of ''Homo sapiens'' Aurignacian technologies.


Recent and ongoing human evolution

Anatomically modern human Anatomy () is the branch of biology concerned with the study of the structure of organisms and their parts. Anatomy is a branch of natural science that deals with the structural organization of living things. It is an old science, having it ...
populations continue to evolve, as they are affected by both natural selection and genetic drift. Although selection pressure on some traits, such as resistance to smallpox, has decreased in the modern age, humans are still undergoing natural selection for many other traits. Some of these are due to specific environmental pressures, while others are related to lifestyle changes since the development of agriculture (10,000 years ago), urbanization (5,000), and industrialization (250 years ago). It has been argued that human evolution has accelerated since the development of agriculture 10,000 years ago and civilization some 5,000 years ago, resulting, it is claimed, in substantial genetic differences between different current human populations, and more recent research indicates that for some traits, the developments and innovations of human culture have driven a new form of selection that coexists with, and in some cases has largely replaced, natural selection. Particularly conspicuous is variation in superficial characteristics, such as Afro-textured hair, or the recent evolution of light skin and blond hair in some populations, which are attributed to differences in climate. Particularly strong selective pressures have resulted in high-altitude adaptation in humans, with different ones in different isolated populations. Studies of the High-altitude adaptation in humans#Genetic basis, genetic basis show that some developed very recently, with Tibetans evolving over 3,000 years to have high proportions of an allele of
EPAS1 Endothelial PAS domain-containing protein 1 (EPAS1, also known as hypoxia-inducible factor-2alpha (HIF-2α)) is a protein that is encoded by the ''EPAS1'' gene in mammals. It is a type of hypoxia-inducible factor, a group of transcription factors ...
that is adaptive to high altitudes. Other evolution is related to endemic diseases: the presence of malaria selects for sickle cell trait (the heterozygote, heterozygous form of sickle cell gene), while in the absence of malaria, the health effects of sickle-cell anemia select against this trait. For another example, the population at risk of the severe debilitating disease kuru (disease), kuru has significant over-representation of an immune variant of the PRNP, prion protein gene G127V versus non-immune alleles. The frequency of this Mutation, genetic variant is due to the survival of immune persons. Some reported trends remain unexplained and the subject of ongoing research in the novel field of evolutionary medicine: polycystic ovary syndrome (PCOS) reduces fertility and thus is expected to be subject to extremely strong negative selection, but its relative commonality in human populations suggests a counteracting selection pressure. The identity of that pressure remains the subject of some debate. Recent human evolution related to agriculture includes genetic resistance to infectious disease that has appeared in human populations by crossing the species barrier from domesticated animals, as well as changes in metabolism due to changes in diet, such as lactase persistence. Culturally-driven evolution can defy the expectations of natural selection: while human populations experience some pressure that drives a selection for producing children at younger ages, the advent of effective contraception, higher education, and changing social norms have driven the observed selection in the opposite direction. However, culturally-driven selection need not necessarily work counter or in opposition to natural selection: some proposals to explain the high rate of recent human brain expansion indicate a kind of feedback whereupon the brain's increased social learning efficiency encourages cultural developments that in turn encourage more efficiency, which drive more complex cultural developments that demand still-greater efficiency, and so forth. Culturally-driven evolution has an advantage in that in addition to the genetic effects, it can be observed also in the archaeological record: the development of stone tools across the Palaeolithic period connects to culturally-driven cognitive development in the form of skill acquisition supported by the culture and the development of increasingly complex technologies and the cognitive ability to elaborate them. In contemporary times, since industrialization, some trends have been observed: for instance, menopause is evolving to occur later. Other reported trends appear to include lengthening of the human reproductive period and reduction in cholesterol levels, blood glucose and blood pressure in some populations.


Species list

This list is in chronological order across the table by ''
genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial n ...
''. Some species/subspecies names are well-established, and some are less established – especially in genus ''Homo''. Please see articles for more information.


See also

* Adaptive evolution in the human genome * Amity-enmity complex * Archaeogenetics * Dual inheritance theory * Dysgenics * Evolution of human intelligence * Evolution of morality * Evolutionary medicine * Evolutionary neuroscience * Evolutionary origin of religions * Human behavioral ecology * Human evolution (origins of society and culture) * Human origins (disambiguation), Human origins * Human vestigiality * List of human evolution fossils * Molecular paleontology * Noogenesis * Obstetrical dilemma * Origin of language * Origin of speech * Sexual selection in humans * Transgenerational trauma


Notes


References


Sources

* * * * (9th edition 2021) * * * * * * * * * * * * * * "The Conference on the Comparative Reception of Darwinism was held in Austin, Texas, on April 22 and 23, 1972, under the joint sponsorship of the American Council of Learned Societies and the University of Texas at Austin" * * * * * * * * * * * * * * * * * * * * "Contributions from the Third Stony Brook Human Evolution Symposium and Workshop October 3–7, 2006."


Further reading

* * * * * * * * * * * – two ancestral ape chromosomes fused to give rise to human chromosome 2 * * (Note: this book contains very useful, information dense chapters on primate evolution in general, and human evolution in particular, including fossil history). * * * * * * * * * * * * * * * * * (Note: this book contains very accessible descriptions of human and non-human primates, their evolution, and fossil history). *


External links

*
Race, Evolution and the Science of Human Origins
by Allison Hopper, ''Scientific American'' (July 5, 2021). * * * * – Illustrations from the book ''Evolution'' (2007) * *
"Human Trace" video 2015 Normandy University
] UNIHAVRE, Centre national de la recherche scientifique, CNRS, IDEES, E.Laboratory on Human Trace Unitwin Complex System Digital Campus UNESCO. *
Shaping Humanity Video
2013 Yale University
Human Timeline (Interactive)
– Smithsonian Institution, Smithsonian, National Museum of Natural History (August 2016).
Human Evolution
BBC Radio 4 discussion with Steve Jones, Fred Spoor & Margaret Clegg (''In Our Time'', February 16, 2006)
Evolutionary Timeline of Home Sapiens
− Smithsonian (magazine), Smithsonian (February 2021)
History of Human Evolution in the United States
– Salon.com, Salon (August 24, 2021) {{Authority control Human evolution, Anthropology