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Humans (''Homo sapiens'') are the most abundant and widespread
species In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriat ...
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, and language. Humans are highly social and tend to live in complex social structures composed of many cooperating and competing groups, from families and kinship networks to political states. Social interactions between humans have established a wide variety of values, social norms, and rituals, which bolster human society. Its intelligence and its desire to understand and influence the environment and to explain and manipulate phenomena have motivated humanity's development of science,
philosophy Philosophy (from , ) is the systematized study of general and fundamental questions, such as those about existence, reason, knowledge, values, mind, and language. Such questions are often posed as problems to be studied or resolved. ...
,
mythology Myth is a folklore genre consisting of narratives that play a fundamental role in a society, such as foundational tales or origin myths. Since "myth" is widely used to imply that a story is not objectively true, the identification of a narra ...
,
religion Religion is usually defined as a social- cultural system of designated behaviors and practices, morals, beliefs, worldviews, texts, sanctified places, prophecies, ethics, or organizations, that generally relates humanity to supernatur ...
, and other fields of study. Although some scientists equate the term ''humans'' with all members of the genus '' Homo'', in common usage, it generally refers to ''Homo sapiens'', the only extant member. Anatomically modern humans emerged around 300,000 years ago in Africa, evolving from '' Homo heidelbergensis'' or a similar species and migrating out of Africa, gradually replacing or interbreeding with local populations of archaic humans. For most of history, humans were nomadic hunter-gatherers. Humans began exhibiting behavioral modernity about 160,000–60,000 years ago. The Neolithic Revolution, which began in Southwest Asia around 13,000 years ago (and separately in a few other places), saw the emergence of
agriculture Agriculture or farming is the practice of cultivating plants and livestock. Agriculture was the key development in the rise of sedentary human civilization, whereby farming of domesticated species created food surpluses that enabled people ...
and permanent human settlement. As populations became larger and denser, forms of governance developed within and between communities, and a number of
civilization A civilization (or civilisation) is any complex society characterized by the development of a state, social stratification, urbanization, and symbolic systems of communication beyond natural spoken language (namely, a writing system). ...
s have risen and fallen. Humans have continued to expand, with a global population of over 8 billion .
Gene In biology, the word gene (from , ; "...Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a b ...
s and the
environment Environment most often refers to: __NOTOC__ * Natural environment, all living and non-living things occurring naturally * Biophysical environment, the physical and biological factors along with their chemical interactions that affect an organism or ...
influence human biological variation in visible characteristics, physiology, disease susceptibility, mental abilities, body size, and life span. Though humans vary in many traits (such as genetic predispositions and physical features), any two humans are at least 99% genetically similar. Humans are sexually dimorphic: generally, males have greater body strength and females have a higher body fat percentage. At puberty, humans develop secondary sex characteristics. Females are capable of pregnancy, usually between puberty, at around 12 years, and menopause, around the age of 50. Humans are omnivorous, capable of consuming a wide variety of plant and animal material, and have used fire and other forms of heat to prepare and
cook Cook or The Cook may refer to: Food preparation * Cooking, the preparation of food * Cook (domestic worker), a household staff member who prepares food * Cook (professional), an individual who prepares food for consumption in the food industry * ...
food since the time of ''H. erectus''. Humans can survive for up to eight weeks without food and three or four days without water. Humans are generally diurnal, sleeping on average seven to nine hours per day. Childbirth is dangerous, with a high risk of complications and death. Often, both the mother and the father provide care for their children, who are helpless at birth. Humans have a large and highly developed prefrontal cortex, the region of the brain associated with higher cognition. They are highly intelligent, capable of episodic memory, have flexible facial expressions, self-awareness, and a theory of mind. The human mind is capable of introspection, private thought, imagination, volition, and forming views on existence. This has allowed great technological advancements and complex tool development possible through complex reasoning and the transmission of knowledge to subsequent generations. Language, art, and trade are defining characteristics of humans. Long-distance trade routes might have led to cultural explosions and resource distribution that gave humans an advantage over other similar species.


Etymology and definition

All modern humans are classified into the
species In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriat ...
''Homo sapiens'', coined 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 1735 work ''
Systema Naturae ' (originally in Latin written ' with the ligature æ) is one of the major works of the Swedish botanist, zoologist and physician Carl Linnaeus (1707–1778) and introduced the Linnaean taxonomy. Although the system, now known as binomial ...
''. The generic name "'' Homo''" is a learned 18th-century derivation from Latin , which refers to humans of either sex. The word ''human'' can refer to all members of the ''Homo'' genus, although in common usage it generally just refers to ''Homo sapiens,'' the only extant species. The name "''Homo'' ''sapiens''" means 'wise man' or 'knowledgeable man'. There is disagreement if certain extinct members of the genus, namely Neanderthals, should be included as a separate species of humans or as a subspecies of ''H. sapiens''. ''Human'' is a loanword of
Middle English Middle English (abbreviated to ME) is a form of the English language that was spoken after the Norman conquest of 1066, until the late 15th century. The English language underwent distinct variations and developments following the Old Englis ...
from
Old French Old French (, , ; Modern French: ) was the language spoken in most of the northern half of France from approximately the 8th to the 14th centuries. Rather than a unified language, Old French was a linkage of Romance dialects, mutually intel ...
, ultimately from
Latin Latin (, or , ) is a classical language belonging to the Italic languages, 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 adjectival form of ('man' — in the sense of humankind). The native English term '' man'' can refer to the species generally (a synonym for ''humanity'') as well as to human males. It may also refer to individuals of either sex, though this form is less common in contemporary English. Despite the fact that the word ''animal'' is colloquially used as an antonym for ''human'', and contrary to a common biological misconception, humans are animals. The word '' person'' is often used interchangeably with ''human'', but philosophical debate exists as to whether personhood applies to all humans or all sentient beings, and further if one can lose personhood (such as by going into a persistent vegetative state).


Evolution

Humans are apes ( superfamily Hominoidea). The
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, populat ...
of apes that eventually gave rise to humans first split from gibbons (family Hylobatidae) and orangutans (genus ''Pongo''), then gorillas (genus ''Gorilla''), and finally, chimpanzees and bonobos (genus '' Pan''). The last split, between the human and chimpanzee–bonobo lineages, took place around 8–4 million years ago, in the late Miocene epoch. During this split, chromosome 2 was formed from the joining of two other chromosomes, leaving humans with only 23 pairs of chromosomes, compared to 24 for the other apes. Following their split with chimpanzees and bonobos, the hominins diversified into many species and at least two distinct genera. All but one of these lineages—representing the genus '' Homo'' and its sole extant species ''Homo sapiens''—are now extinct. The genus ''Homo'' evolved from '' Australopithecus''. Though fossils from the transition are scarce, the earliest members of ''Homo'' share several key traits with ''Australopithecus''. The earliest record of ''Homo'' is the 2.8 million-year-old specimen LD 350-1 from Ethiopia, and the earliest named species are '' Homo habilis'' and '' Homo rudolfensis'' which evolved by 2.3 million years ago. '' H. erectus'' (the African variant is sometimes called ''
H. ergaster ''Homo ergaster'' is an extinct species or subspecies of archaic humans who lived in Africa in the Early Pleistocene. Whether ''H. ergaster'' constitutes a species of its own or should be subsumed into ''H. erectus'' is an ongoing and unresolv ...
'') evolved 2 million years ago and was the first archaic human species to leave Africa and disperse across Eurasia. ''H. erectus'' also was the first to evolve a characteristically human body plan. ''Homo sapiens'' emerged in Africa around 300,000 years ago from a species commonly designated as either '' H. heidelbergensis'' or '' H. rhodesiensis'', the descendants of ''H. erectus'' that remained in Africa. ''H. sapiens'' migrated out of the continent, gradually replacing or interbreeding with local populations of archaic humans. Humans began exhibiting behavioral modernity about 160,000-70,000 years ago, and possibly earlier. The "out of Africa" migration took place in at least two waves, the first around 130,000 to 100,000 years ago, the second (
Southern Dispersal In the context of the recent African origin of modern humans, the Southern Dispersal scenario (also the coastal migration or great coastal migration hypothesis) refers to the early migration along the southern coast of Asia, from the Arabian Pen ...
) around 70,000 to 50,000 years ago. ''H. sapiens'' proceeded to colonize all the continents and larger islands, arriving in Eurasia 60,000 years ago, Australia around 65,000 years ago, the Americas around 15,000 years ago, and remote islands such as Hawaii, Easter Island, Madagascar, and New Zealand between the years 300 and 1280 CE. Human evolution was not a simple linear or branched progression but involved interbreeding between related species. Genomic research has shown that hybridization between substantially diverged lineages was common in human evolution. DNA evidence suggests that several genes of Neanderthal origin are present among all non sub-Saharan African populations, and Neanderthals and other hominins, such as Denisovans, may have contributed up to 6% of their genome to present-day non sub-Saharan African humans. Human evolution is characterized by a number of morphological, developmental, physiological, and behavioral changes that have taken place since the split between the last common ancestor of humans and chimpanzees. The most significant of these adaptations are obligate bipedalism, increased brain size and decreased sexual dimorphism ( neoteny). The relationship between all these changes is the subject of ongoing debate.


History

Until about 12,000 years ago, all humans lived as hunter-gatherers. The Neolithic Revolution (the invention of
agriculture Agriculture or farming is the practice of cultivating plants and livestock. Agriculture was the key development in the rise of sedentary human civilization, whereby farming of domesticated species created food surpluses that enabled people ...
) first took place in Southwest Asia and spread through large parts of the Old World over the following millennia. It also occurred independently in Mesoamerica (about 6,000 years ago), China,
Papua New Guinea Papua New Guinea (abbreviated PNG; , ; tpi, Papua Niugini; ho, Papua Niu Gini), officially the Independent State of Papua New Guinea ( tpi, Independen Stet bilong Papua Niugini; ho, Independen Stet bilong Papua Niu Gini), is a country i ...
, and the Sahel and West Savanna regions of Africa. Access to food surplus led to the formation of permanent human settlements, the domestication of animals and the use of metal tools for the first time in history. Agriculture and sedentary lifestyle led to the emergence of early civilizations. An urban revolution took place in the 4th millennium BCE with the development of city-states, particularly Sumerian cities located in Mesopotamia. It was in these cities that the earliest known form of writing, cuneiform script, appeared around 3000 BCE. Other major civilizations to develop around this time were Ancient Egypt and the Indus Valley Civilization. They eventually traded with each other and invented technology such as wheels, plows and sails. Astronomy and mathematics were also developed and the Great Pyramid of Giza was built. There is evidence of a severe drought lasting about a hundred years that may have caused the decline of these civilizations, with new ones appearing in the aftermath. Babylonians came to dominate Mesopotamia while others, such as Poverty Point cultures, Minoans and the Shang dynasty, rose to prominence in new areas. The Bronze Age suddenly collapsed around 1200 BCE, resulting in the disappearance of a number of civilizations and the beginning of the Greek Dark Ages. During this period iron started replacing bronze, leading to the Iron Age. In the 5th century BCE, history started being recorded as a discipline, which provided a much clearer picture of life at the time. Between the 8th and 6th century BCE, Europe entered the
classical antiquity Classical antiquity (also the classical era, classical period or classical age) is the period of cultural history between the 8th century BC and the 5th century AD centred on the Mediterranean Sea, comprising the interlocking civilizations of ...
age, a period when
ancient Greece Ancient Greece ( el, Ἑλλάς, Hellás) was a northeastern Mediterranean civilization, existing from the Greek Dark Ages of the 12th–9th centuries BC to the end of classical antiquity ( AD 600), that comprised a loose collection of cu ...
and
ancient Rome In modern historiography, ancient Rome refers to Roman people, Roman civilisation from the founding of the city of Rome in the 8th century BC to the collapse of the Western Roman Empire in the 5th century AD. It encompasses the Roman Kingdom ...
flourished. Around this time other civilizations also came to prominence. The Maya civilization started to build cities and create complex calendars. In Africa, the Kingdom of Aksum overtook the declining Kingdom of Kush and facilitated trade between India and the Mediterranean. In West Asia, the Achaemenid Empire's system of centralized governance became the precursor to many later empires, while the Gupta Empire in India and the
Han dynasty The Han dynasty (, ; ) was an Dynasties in Chinese history, imperial dynasty of China (202 BC – 9 AD, 25–220 AD), established by Emperor Gaozu of Han, Liu Bang (Emperor Gao) and ruled by the House of Liu. The dynasty was preceded by th ...
in China have been described as
golden ages A golden age is a period considered the apotheosis in the history of a country or people, a time period when the greatest achievements were made. The term originated from early Greek and Roman poets, who used it to refer to a time when mankin ...
in their respective regions. Following the fall of the Western Roman Empire in 476, Europe entered the
Middle Ages In the history of Europe, the Middle Ages or medieval period lasted approximately from the late 5th to the late 15th centuries, similar to the post-classical period of global history. It began with the fall of the Western Roman Empire ...
. During this period,
Christianity Christianity is an Abrahamic monotheistic religion based on the life and teachings of Jesus of Nazareth. It is the world's largest and most widespread religion with roughly 2.38 billion followers representing one-third of the global popula ...
and the
Church Church may refer to: Religion * Church (building), a building for Christian religious activities * Church (congregation), a local congregation of a Christian denomination * Church service, a formalized period of Christian communal worship * Chri ...
would provide centralized authority and education. In the Middle East,
Islam Islam (; ar, ۘالِإسلَام, , ) is an Abrahamic monotheistic religion centred primarily around the Quran, a religious text considered by Muslims to be the direct word of God (or '' Allah'') as it was revealed to Muhammad, the ...
became the prominent religion and expanded into North Africa. It led to an Islamic Golden Age, inspiring achievements in architecture, the revival of old advances in science and technology, and the formation of a distinct way of life. The Christian and Islamic worlds would eventually clash, with the Kingdom of England, the Kingdom of France and the Holy Roman Empire declaring a series of
holy wars A religious war or a war of religion, sometimes also known as a holy war ( la, sanctum bellum), is a war which is primarily caused or justified by differences in religion. In the modern period, there are frequent debates over the extent to ...
to regain control of the Holy Land from Muslims. In the Americas, complex Mississippian societies would arise starting around 800 CE, while further south, the Aztecs and Incas would become the dominant powers. The Mongol Empire would conquer much of Eurasia in the 13th and 14th centuries. Over this same time period, the Mali Empire in Africa grew to be the largest empire on the continent, stretching from Senegambia to Ivory Coast. Oceania would see the rise of the Tuʻi Tonga Empire which expanded across many islands in the South Pacific. The early modern period in Europe and the Near East (c.1450–1800) began with the final defeat of the Byzantine Empire, and the rise of the Ottoman Empire. Meanwhile, Japan entered the Edo period, the Qing dynasty rose in China and the Mughal Empire ruled much of India. Europe underwent the Renaissance, starting in the 15th century, and the Age of Discovery began with the exploring and colonizing of new regions. This includes the British Empire expanding to become the world's largest empire and the colonization of the Americas. This expansion led to the Atlantic slave trade and the genocide of Native American peoples. This period also marked the Scientific Revolution, with great advances in
mathematics Mathematics is an area of knowledge that includes the topics of numbers, formulas and related structures, shapes and the spaces in which they are contained, and quantities and their changes. These topics are represented in modern mathematics ...
, mechanics, astronomy and physiology. The late modern period (1800–present) saw the Technological and Industrial Revolution bring such discoveries as
imaging technology Imaging is the representation or reproduction of an object's form; especially a visual representation (i.e., the formation of an image). Imaging technology is the application of materials and methods to create, preserve, or duplicate images. ...
, major innovations in transport and energy development. The
United States of America The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 states, a federal district, five major unincorporated territo ...
underwent great change, going from a small group of colonies to one of the global superpowers. The
Napoleonic Wars The Napoleonic Wars (1803–1815) were a series of major global conflicts pitting the French Empire and its allies, led by Napoleon I, against a fluctuating array of European states formed into various coalitions. It produced a period of Fre ...
raged through Europe in the early 1800s, Spain lost most of its colonies in the New World, while Europeans continued expansion into Africa—where European control went from 10% to almost 90% in less than 50 years—and Oceania. A tenuous balance of power among European nations collapsed in 1914 with the outbreak of the
First World War World War I (28 July 1914 11 November 1918), often abbreviated as WWI, was List of wars and anthropogenic disasters by death toll, one of the deadliest global conflicts in history. Belligerents included much of Europe, the Russian Empire, ...
, one of the deadliest conflicts in history. In the 1930s, a worldwide economic crisis led to the rise of authoritarian regimes and a Second World War, involving almost all of the world's countries. Following its conclusion in 1945, the
Cold War The Cold War is a term commonly used to refer to a period of geopolitical tension between the United States and the Soviet Union and their respective allies, the Western Bloc and the Eastern Bloc. The term '' cold war'' is used because t ...
between the
USSR The Soviet Union,. officially the Union of Soviet Socialist Republics. (USSR),. was a transcontinental country that spanned much of Eurasia from 1922 to 1991. A flagship communist state, it was nominally a federal union of fifteen nati ...
and the United States saw a struggle for global influence, including a nuclear arms race and a space race. The current Information Age sees the world becoming increasingly globalized and interconnected.


Habitat and population

Early human settlements were dependent on proximity to
water Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as ...
and—depending on the lifestyle—other natural resources used for subsistence, such as populations of animal prey for hunting and arable land for growing crops and grazing livestock. Modern humans, however, have a great capacity for altering their habitats by means of technology, irrigation, urban planning, construction, deforestation and desertification. Human settlements continue to be vulnerable to natural disasters, especially those placed in hazardous locations and with low quality of construction. Grouping and deliberate habitat alteration is often done with the goals of providing protection, accumulating comforts or material wealth, expanding the available food, improving aesthetics, increasing knowledge or enhancing the exchange of resources. Humans are one of the most adaptable species, despite having a low or narrow tolerance for many of the earth's extreme environments. Through advanced tools, humans have been able to extend their tolerance to a wide variety of temperatures, humidity, and altitudes. As a result, humans are a
cosmopolitan Cosmopolitan may refer to: Food and drink * Cosmopolitan (cocktail), also known as a "Cosmo" History * Rootless cosmopolitan, a Soviet derogatory epithet during Joseph Stalin's anti-Semitic campaign of 1949–1953 Hotels and resorts * Cosmopoli ...
species found in almost all regions of the world, including tropical rainforest, arid desert, extremely cold arctic regions, and heavily polluted cities; in comparison, most other species are confined to a few geographical areas by their limited adaptability. The human population is not, however, uniformly distributed on the
Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's sur ...
's surface, because the population density varies from one region to another, and large stretches of surface are almost completely uninhabited, like Antarctica and vast swathes of the ocean. Most humans (61%) live in Asia; the remainder live in the Americas (14%), Africa (14%), Europe (11%), and Oceania (0.5%). Within the last century, humans have explored challenging environments such as Antarctica, the deep sea, and outer space. Human habitation within these hostile environments is restrictive and expensive, typically limited in duration, and restricted to scientific, military, or industrial expeditions. Humans have briefly visited the Moon and made their presence felt on other celestial bodies through human-made robotic spacecraft. Since the early 20th century, there has been continuous human presence in Antarctica through research stations and, since 2000, in space through habitation on the International Space Station. Estimates of the population at the time agriculture emerged in around 10,000 BC have ranged between 1 million and 15 million. Around 50–60 million people lived in the combined eastern and western
Roman Empire The Roman Empire ( la, Imperium Romanum ; grc-gre, Βασιλεία τῶν Ῥωμαίων, Basileía tôn Rhōmaíōn) was the post-Roman Republic, Republican period of ancient Rome. As a polity, it included large territorial holdings aro ...
in the 4th century AD. Bubonic plagues, first recorded in the 6th century AD, reduced the population by 50%, with the Black Death killing 75–200 million people in Eurasia and North Africa alone. Human population was believed to have reached one billion in 1800. It has since then increased exponentially, reaching two billion in 1930 and three billion in 1960, four in 1975, five in 1987 and six billion in 1999. It passed seven billion in 2011 and was expected to pass eight billion in November 2022. It took over two million years 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 ...
and history for the human population to reach one billion and only 207 years more to grow to 7 billion. The combined biomass of the carbon of all the humans on Earth in 2018 was estimated at 60 million tons, about 10 times larger than that of all non-domesticated mammals. In 2018, 4.2 billion humans (55%) lived in urban areas, up from 751 million in 1950. The most urbanized regions are Northern America (82%), Latin America (81%), Europe (74%) and Oceania (68%), with Africa and Asia having nearly 90% of the world's 3.4 billion rural population. Problems for humans living in cities include various forms of pollution and crime, especially in inner city and suburban slums. Humans have had a dramatic effect on the environment. They are apex predators, being rarely preyed upon by other species. Human population growth, industrialization, land development, overconsumption and combustion of fossil fuels have led to environmental destruction and
pollution Pollution is the introduction of contaminants into the natural environment that cause adverse change. Pollution can take the form of any substance (solid, liquid, or gas) or energy (such as radioactivity, heat, sound, or light). Pollutants, th ...
that significantly contributes to the ongoing mass extinction of other forms of life. They are the main contributor to global climate change, which may accelerate the Holocene extinction.


Biology


Anatomy and physiology

Most aspects of human physiology are closely homologous to corresponding aspects of animal physiology. The human body consists of the
legs A leg is a weight-bearing and locomotive anatomical structure, usually having a columnar shape. During locomotion, legs function as "extensible struts". The combination of movements at all joints can be modeled as a single, linear element c ...
, the torso, the arms, the neck, and the head. An adult human body consists of about 100 trillion (1014)
cells Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Locations * Monastic cell, a small room, hut, or cave in which a religious recluse lives, alternatively the small precursor of a monastery w ...
. The most commonly defined
body systems A biological system is a complex network which connects several biologically relevant entities. Biological organization spans several scales and are determined based different structures depending on what the system is. Examples of biological syst ...
in humans are the nervous, the cardiovascular, the digestive, the endocrine, the immune, the integumentary, the lymphatic, the musculoskeletal, the reproductive, the respiratory, and the urinary system. The dental formula of humans is: . Humans have proportionately shorter palates and much smaller teeth than other primates. They are the only primates to have short, relatively flush canine teeth. Humans have characteristically crowded teeth, with gaps from lost teeth usually closing up quickly in young individuals. Humans are gradually losing their third molars, with some individuals having them congenitally absent. Humans share with chimpanzees a vestigial tail,
appendix Appendix, or its plural form appendices, may refer to: __NOTOC__ In documents * Addendum, an addition made to a document by its author after its initial printing or publication * Bibliography, a systematic list of books and other works * Index (pu ...
, flexible shoulder joints, grasping fingers and opposable thumbs. Apart from bipedalism and brain size, humans differ from chimpanzees mostly in smelling, hearing and digesting proteins. While humans have a density of hair follicles comparable to other apes, it is predominantly vellus hair, most of which is so short and wispy as to be practically invisible. Humans have about 2 million sweat glands spread over their entire bodies, many more than chimpanzees, whose sweat glands are scarce and are mainly located on the palm of the hand and on the soles of the feet. It is estimated that the worldwide average height for an adult human male is about , while the worldwide average height for adult human females is about . Shrinkage of stature may begin in middle age in some individuals but tends to be typical in the extremely aged. Throughout history, human populations have universally become taller, probably as a consequence of better nutrition, healthcare, and living conditions. The average mass of an adult human is for females and for males. Like many other conditions, body weight and body type are influenced by both genetic susceptibility and environment and varies greatly among individuals. Humans have a far faster and more accurate
throw Throwing is an action which consists in accelerating a projectile and then releasing it so that it follows a ballistic trajectory, usually with the aim of impacting a remote target. This action is best characterized for animals with prehensile ...
than other animals. Humans are also among the best long-distance runners in the animal kingdom, but slower over short distances. Humans' thinner body hair and more productive sweat glands help avoid
heat exhaustion Heat exhaustion is a severe form of heat illness. It is a medical emergency. Heat exhaustion is caused by the loss of water and electrolytes through sweating. The United States Department of Labor makes the following recommendation, "Heat illness ...
while running for long distances.


Genetics

Like most animals, humans are a diploid and eukaryotic species. Each somatic cell has two sets of 23 chromosomes, each set received from one parent; gametes have only one set of chromosomes, which is a mixture of the two parental sets. Among the 23 pairs of chromosomes, there are 22 pairs of autosomes and one pair of sex chromosomes. Like other mammals, humans have an XY sex-determination system, so that females have the sex chromosomes XX and males have XY.
Gene In biology, the word gene (from , ; "...Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a b ...
s and
environment Environment most often refers to: __NOTOC__ * Natural environment, all living and non-living things occurring naturally * Biophysical environment, the physical and biological factors along with their chemical interactions that affect an organism or ...
influence human biological variation in visible characteristics, physiology, disease susceptibility and mental abilities. The exact influence of genes and environment on certain traits is not well understood. While no humans—not even monozygotic twins—are genetically identical, two humans on average will have a genetic similarity of 99.5%-99.9%. This makes them more homogeneous than other great apes, including chimpanzees. This small variation in human DNA compared to many other species suggests a population bottleneck during the Late Pleistocene (around 100,000 years ago), in which the human population was reduced to a small number of breeding pairs. The forces of natural selection have continued to operate on human populations, with evidence that certain regions of the genome display directional selection in the past 15,000 years. The human genome was first sequenced in 2001 and by 2020 hundreds of thousands of genomes had been sequenced. In 2012 the International HapMap Project had compared the genomes of 1,184 individuals from 11 populations and identified 1.6 million single nucleotide polymorphisms. African populations harbor the highest number of private genetic variants. While many of the common variants found in populations outside of Africa are also found on the African continent, there are still large numbers that are private to these regions, especially Oceania and the Americas. By 2010 estimates, humans have approximately 22,000 genes. By comparing mitochondrial DNA, which is inherited only from the mother, geneticists have concluded that the last female common ancestor whose genetic marker is found in all modern humans, the so-called mitochondrial Eve, must have lived around 90,000 to 200,000 years ago.


Life cycle

Most human reproduction takes place by internal fertilization via sexual intercourse, but can also occur through assisted reproductive technology procedures. The average gestation period is 38 weeks, but a normal pregnancy can vary by up to 37 days. Embryonic development in the human covers the first eight weeks of development; at the beginning of the ninth week the embryo is termed a fetus. Humans are able to induce early labor or perform a caesarean section if the child needs to be born earlier for medical reasons. In developed countries, infants are typically in weight and in height at birth. However,
low birth weight Low birth weight (LBW) is defined by the World Health Organization as a birth weight of an infant of or less, regardless of gestational age. Infants born with LBW have added health risks which require close management, often in a neonatal int ...
is common in developing countries, and contributes to the high levels of infant mortality in these regions. Compared with other species, human childbirth is dangerous, with a much higher risk of complications and death. The size of the fetus's head is more closely matched to the pelvis than other primates. The reason for this is not completely understood, but it contributes to a painful labor that can last 24 hours or more. The chances of a successful labor increased significantly during the 20th century in wealthier countries with the advent of new medical technologies. In contrast, pregnancy and natural childbirth remain hazardous ordeals in developing regions of the world, with
maternal death rates Maternal death or maternal mortality is defined in slightly different ways by several different health organizations. The World Health Organization (WHO) defines maternal death as the death of a pregnant mother due to complications related to pre ...
approximately 100 times greater than in developed countries. Both the mother and the father provide care for human offspring, in contrast to other primates, where parental care is mostly done by the mother. Helpless at birth, humans continue to grow for some years, typically reaching sexual maturity at 15 to 17 years of age. The human life span has been split into various stages ranging from three to twelve. Common stages include infancy, childhood, adolescence, adulthood and old age. The lengths of these stages have varied across cultures and time periods but is typified by an unusually rapid growth spurt during adolescence. Human females undergo menopause and become infertile at around the age of 50. It has been proposed that menopause increases a woman's overall reproductive success by allowing her to invest more time and resources in her existing offspring, and in turn their children (the grandmother hypothesis), rather than by continuing to bear children into old age. The life span of an individual depends on two major factors, genetics and lifestyle choices. For various reasons, including biological/genetic causes, women live on average about four years longer than men. , the global average
life expectancy at birth Life is a quality that distinguishes matter that has biological processes, such as signaling and self-sustaining processes, from that which does not, and is defined by the capacity for growth, reaction to stimuli, metabolism, energy tra ...
of a girl is estimated to be 74.9 years compared to 70.4 for a boy. There are significant geographical variations in human life expectancy, mostly correlated with economic development—for example, life expectancy at birth in Hong Kong is 87.6 years for girls and 81.8 for boys, while in the Central African Republic, it is 55.0 years for girls and 50.6 for boys. The developed world is generally aging, with the median age around 40 years. In the developing world, the median age is between 15 and 20 years. While one in five Europeans is 60 years of age or older, only one in twenty Africans is 60 years of age or older. In 2012, the United Nations estimated that there were 316,600 living centenarians (humans of age 100 or older) worldwide.


Diet

Humans are omnivorous, capable of consuming a wide variety of plant and animal material. Human groups have adopted a range of diets from purely vegan to primarily carnivorous. In some cases, dietary restrictions in humans can lead to
deficiency diseases Malnutrition occurs when an organism gets too few or too many nutrients, resulting in health problems. Specifically, it is "a deficiency, excess, or imbalance of energy, protein and other nutrients" which adversely affects the body's tissues ...
; however, stable human groups have adapted to many dietary patterns through both genetic specialization and cultural conventions to use nutritionally balanced food sources. The human diet is prominently reflected in human culture and has led to the development of food science. Until the development of agriculture approximately 10,000 years ago, ''Homo sapiens'' employed a hunter-gatherer method as their sole means of food collection. This involved combining stationary food sources (such as fruits, grains, tubers, and mushrooms, insect larvae and aquatic mollusks) with wild game, which must be hunted and captured in order to be consumed. It has been proposed that humans have used fire to prepare and
cook Cook or The Cook may refer to: Food preparation * Cooking, the preparation of food * Cook (domestic worker), a household staff member who prepares food * Cook (professional), an individual who prepares food for consumption in the food industry * ...
food since the time of '' Homo erectus''. Around ten thousand years ago, humans developed agriculture, which substantially altered their diet. This change in diet may also have altered human biology; with the spread of
dairy farming Dairy farming is a class of agriculture for long-term production of milk, which is processed (either on the farm or at a dairy plant, either of which may be called a dairy) for eventual sale of a dairy product. Dairy farming has a history t ...
providing a new and rich source of food, leading to the evolution of the ability to digest lactose in some adults. The types of food consumed, and how they are prepared, have varied widely by time, location, and culture. In general, humans can survive for up to eight weeks without food, depending on stored body fat. Survival without water is usually limited to three or four days, with a maximum of one week. In 2020 it is estimated 9 million humans die every year from causes directly or indirectly related to starvation. Childhood malnutrition is also common and contributes to the
global burden of disease Disease burden is the impact of a health problem as measured by financial cost, mortality, morbidity, or other indicators. It is often quantified in terms of quality-adjusted life years (QALYs) or disability-adjusted life years (DALYs). Both ...
. However, global food distribution is not even, and obesity among some human populations has increased rapidly, leading to health complications and increased mortality in some developed and a few developing countries. Worldwide, over one billion people are obese, while in the United States 35% of people are obese, leading to this being described as an "
obesity epidemic Obesity has been observed throughout human history. Many early depictions of the human form in art and sculpture appear obese. However, it was not until the 20th century that obesity became common — so much so that, in 1997, the World Heal ...
." Obesity is caused by consuming more calories than are expended, so excessive weight gain is usually caused by an energy-dense diet.


Biological variation

There is biological variation in the human species—with traits such as blood type, genetic diseases, cranial features, facial features, organ systems, eye color, hair color and texture, height and build, and skin color varying across the globe. The typical height of an adult human is between , although this varies significantly depending on sex, ethnic origin, and family bloodlines. Body size is partly determined by genes and is also significantly influenced by environmental factors such as diet, exercise, and
sleep pattern Sleep is a sedentary state of mind and body. It is characterized by altered consciousness, relatively inhibited sensory activity, reduced muscle activity and reduced interactions with surroundings. It is distinguished from wakefulness by a de ...
s. There is evidence that populations have adapted genetically to various external factors. The genes that allow adult humans to digest lactose are present in high frequencies in populations that have long histories of cattle domestication and are more dependent on
cow milk Milk is a white liquid food produced by the mammary glands of mammals. It is the primary source of nutrition for young mammals (including breastfed human infants) before they are able to digest solid food. Immune factors and immune-modulating ...
. Sickle cell anemia, which may provide increased resistance to malaria, is frequent in populations where malaria is endemic. Populations that have for a very long time inhabited specific climates tend to have developed specific
phenotype In genetics, the phenotype () is the set of observable characteristics or traits of an organism. The term covers the organism's morphology (biology), morphology or physical form and structure, its Developmental biology, developmental proc ...
s that are beneficial for those environments— short stature and stocky build in cold regions, tall and lanky in hot regions, and with high lung capacities or other adaptations at high altitudes. Some populations have evolved highly unique adaptations to very specific environmental conditions, such as those advantageous to ocean-dwelling lifestyles and
freediving Freediving, free-diving, free diving, breath-hold diving, or skin diving is a form of underwater diving that relies on breath-holding until resurfacing rather than the use of breathing apparatus such as scuba gear. Besides the limits of breath- ...
in the Bajau. Human hair ranges in color from red to blond to brown to black, which is the most frequent. Hair color depends on the amount of
melanin Melanin (; from el, μέλας, melas, black, dark) is a broad term for a group of natural pigments found in most organisms. Eumelanin is produced through a multistage chemical process known as melanogenesis, where the oxidation of the amino ...
, with concentrations fading with increased age, leading to grey or even white hair. Skin color can range from darkest brown to lightest peach, or even nearly white or colorless in cases of albinism. It tends to vary clinally and generally correlates with the level of ultraviolet radiation in a particular geographic area, with darker skin mostly around the equator. Skin darkening may have evolved as protection against ultraviolet solar radiation. Light skin pigmentation protects against depletion of
vitamin D Vitamin D is a group of Lipophilicity, fat-soluble secosteroids responsible for increasing intestinal absorption of calcium, magnesium, and phosphate, and many other biological effects. In humans, the most important compounds in this group ar ...
, which requires sunlight to make. Human skin also has a capacity to darken (tan) in response to exposure to ultraviolet radiation. There is relatively little variation between human geographical populations, and most of the variation that occurs is at the individual level. Much of human variation is continuous, often with no clear points of demarcation. Genetic data shows that no matter how population groups are defined, two people from the same population group are almost as different from each other as two people from any two different population groups. Dark-skinned populations that are found in Africa, Australia, and South Asia are not closely related to each other. Genetic research has demonstrated that human populations native to the African continent are the most genetically diverse and genetic diversity decreases with migratory distance from Africa, possibly the result of bottlenecks during human migration. These non-African populations acquired new genetic inputs from local admixture with archaic populations and have much greater variation from Neanderthals and Denisovans than is found in Africa, though Neanderthal admixture into African populations may be underestimated. Furthermore, recent studies have found that populations in sub-Saharan Africa, and particularly West Africa, have ancestral genetic variation which predates modern humans and has been lost in most non-African populations. Some of this ancestry is thought to originate from admixture with an unknown archaic hominin that diverged before the split of Neanderthals and modern humans. Humans are a gonochoric species, meaning they are divided into male and female sexes. The greatest degree of genetic variation exists between males and females. While the nucleotide genetic variation of individuals of the same sex across global populations is no greater than 0.1%–0.5%, the genetic difference between males and females is between 1% and 2%. Males on average are 15% heavier and taller than females. On average, men have about 40–50% more upper body strength and 20–30% more lower body strength than women at the same weight, due to higher amounts of muscle and larger muscle fibers. Women generally have a higher body fat percentage than men. Women have lighter skin than men of the same population; this has been explained by a higher need for vitamin D in females during pregnancy and lactation. As there are chromosomal differences between females and males, some X and Y chromosome-related conditions and disorders only affect either men or women. After allowing for body weight and volume, the male voice is usually an octave deeper than the female voice. Women have a longer life span in almost every population around the world.


Psychology

The human brain, the focal point of the central nervous system in humans, controls the peripheral nervous system. In addition to controlling "lower," involuntary, or primarily autonomic activities such as respiration and digestion, it is also the locus of "higher" order functioning such as thought, reasoning, and abstraction. These cognitive processes constitute the mind, and, along with their behavioral consequences, are studied in the field of psychology. Humans have a larger and more developed prefrontal cortex than other primates, the region of the brain associated with higher cognition. This has led humans to proclaim themselves to be more intelligent than any other known species. Objectively defining intelligence is difficult, with other animals adapting senses and excelling in areas that humans are unable to. There are some traits that, although not strictly unique, do set humans apart from other animals. Humans may be the only animals who have episodic memory and who can engage in " mental time travel". Even compared with other social animals, humans have an unusually high degree of flexibility in their facial expressions. Humans are the only animals known to cry emotional tears. Humans are one of the few animals able to self-recognize in mirror tests and there is also debate over to what extent humans are the only animals with a theory of mind.


Sleep and dreaming

Humans are generally diurnal. The average sleep requirement is between seven and nine hours per day for an adult and nine to ten hours per day for a child; elderly people usually sleep for six to seven hours. Having less sleep than this is common among humans, even though sleep deprivation can have negative health effects. A sustained restriction of adult sleep to four hours per day has been shown to correlate with changes in physiology and mental state, including reduced memory, fatigue, aggression, and bodily discomfort. During sleep humans dream, where they experience sensory images and sounds. Dreaming is stimulated by the pons and mostly occurs during the
REM phase of sleep Rapid eye movement sleep (REM sleep or REMS) is a unique phase of sleep in mammals and birds, characterized by random rapid movement of the eyes, accompanied by low muscle tone throughout the body, and the propensity of the sleeper to dream viv ...
. The length of a dream can vary, from a few seconds up to 30 minutes. Humans have three to five dreams per night, and some may have up to seven; however most dreams are immediately or quickly forgotten. They are more likely to remember the dream if awakened during the REM phase. The events in dreams are generally outside the control of the dreamer, with the exception of lucid dreaming, where the dreamer is self-aware. Dreams can at times make a
creative Creative may refer to: *Creativity, phenomenon whereby something new and valuable is created * "Creative" (song), a 2008 song by Leon Jackson * Creative class, a proposed socioeconomic class * Creative destruction, an economic term * Creative dir ...
thought occur or give a sense of
inspiration Inspiration, inspire, or inspired often refers to: * Artistic inspiration, sudden creativity in artistic production * Biblical inspiration, the doctrine in Judeo-Christian theology concerned with the divine origin of the Bible * Creative inspirat ...
.


Consciousness and thought

Human consciousness, at its simplest, is sentience or awareness of internal or external existence. Despite centuries of analyses, definitions, explanations and debates by philosophers and scientists, consciousness remains puzzling and controversial, being "at once the most familiar and most mysterious aspect of our lives". The only widely agreed notion about the topic is the intuition that it exists. Opinions differ about what exactly needs to be studied and explained as consciousness. Some philosophers divide consciousness into phenomenal consciousness, which is sensory experience itself, and access consciousness, which can be used for reasoning or directly controlling actions. It is sometimes synonymous with 'the mind', and at other times, an aspect of it. Historically it is associated with introspection, private thought, imagination and volition. It now often includes some kind of experience, cognition, feeling or perception. It may be 'awareness', or ' awareness of awareness', or self-awareness. There might be different levels or orders of consciousness, or different kinds of consciousness, or just one kind with different features. The process of acquiring knowledge and understanding through thought, experience, and the senses is known as cognition. The human brain perceives the external world through the senses, and each individual human is influenced greatly by his or her experiences, leading to subjective views of existence and the passage of time. The nature of thought is central to psychology and related fields. Cognitive psychology studies cognition, the
mental processes 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, thought, ...
underlying behavior. Largely focusing on the development of the human mind through the life span, developmental psychology seeks to understand how people come to perceive, understand, and act within the world and how these processes change as they age. This may focus on intellectual, cognitive, neural, social, or moral development. Psychologists have developed intelligence tests and the concept of intelligence quotient in order to assess the relative intelligence of human beings and study its
distribution Distribution may refer to: Mathematics * Distribution (mathematics), generalized functions used to formulate solutions of partial differential equations *Probability distribution, the probability of a particular value or value range of a vari ...
among population.


Motivation and emotion

Human motivation is not yet wholly understood. From a psychological perspective, Maslow's hierarchy of needs is a well-established theory that can be defined as the process of satisfying certain needs in ascending order of complexity. From a more general, philosophical perspective, human motivation can be defined as a commitment to, or withdrawal from, various goals requiring the application of human ability. Furthermore, incentive and preference are both factors, as are any perceived links between incentives and preferences. Volition may also be involved, in which case willpower is also a factor. Ideally, both motivation and volition ensure the selection, striving for, and realization of goals in an optimal manner, a function beginning in childhood and continuing throughout a lifetime in a process known as socialization. Emotions are biological states associated with the nervous system brought on by neurophysiological changes variously associated with thoughts, feelings, behavioral responses, and a degree of pleasure or
displeasure {{Short pages monitor