''Ardipithecus'' is a genus of an extinct

hominine 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 numerou ...

that lived during the Late Miocene and

Early Pliocene Early may refer to: History * The beginning or oldest part of a defined historical period, as opposed to middle or late periods, e.g.: ** Early Christianity ** Early modern Europe Places in the United States * Early, Iowa * Early, Texas * Early ...

epochs in the

Afar Depression 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 ...

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 ...

. Originally described as one of the earliest ancestors of humans after they diverged from the chimpanzees, the relation of this

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 nom ...

to human ancestors and whether it is a

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 ...

is now a matter of debate. Two fossil

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 appropriate s ...

are described in the literature: '' A. ramidus'', which lived about 4.4 million years ago during the early

Pliocene The Pliocene ( ; also Pleiocene) is the epoch in the geologic time scale that extends from 5.333 million to 2.58A. kadabba'', dated to approximately 5.6 million years ago (late

Miocene The Miocene ( ) is the first epoch (geology), geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and mea ...

). Initial behavioral analysis indicated that ''Ardipithecus'' could be very similar to chimpanzees, however more recent analysis based on canine size and lack of canine sexual dimorphism indicates that ''Ardipithecus'' was characterised by reduced aggression, and that they more closely resemble bonobos. Some analyses describe ''Australopithecus'' as being sister to ''Ardipithecus ramidus'' specifically. This means that ''

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 ...

'' is distinctly closer related to ''Ardipithecus ramidus'' than ''Ardipithecus kadabba''.

Cladistically Cladistics (; ) is an approach to biological classification in which organisms are categorized in groups ("clades") based on hypotheses of most recent common ancestry. The evidence for hypothesized relationships is typically shared derived char ...

, then, ''Australopithecus'' (and eventually ''

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, ...

'') indeed emerged within the ''Ardipithecus'' lineage, and this lineage is not literally extinct.

''Ardipithecus ramidus''

''A. ramidus'' was named in September 1994. The first fossil found was dated to 4.4 million years ago on the basis of its stratigraphic position between two volcanic strata: the basal Gaala Tuff Complex (G.A.T.C.) and the Daam Aatu Basaltic Tuff (D.A.B.T.). The name ''Ardipithecus ramidus'' stems mostly from the Afar language, in which ''Ardi'' means "ground/floor" and ''ramid'' means "root". The ''pithecus'' portion of the name is from the

Greek Greek may refer to: Greece Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group. *Greek language, a branch of the Indo-European language family. **Proto-Greek language, the assumed last common ancestor ...

word for "ape". Like most hominids, but unlike all previously recognized hominins, it had a grasping

hallux Toes are the digits (fingers) of the foot of a tetrapod. Animal species such as cats that walk on their toes are described as being '' digitigrade''. Humans, and other animals that walk on the soles of their feet, are described as being '' pl ...

or big toe adapted for locomotion in the trees. It is not confirmed how many other features of its skeleton reflect adaptation to

bipedalism 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' ...

on the ground as well. Like later hominins, ''Ardipithecus'' had reduced

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 howeve ...

and reduced canine sexual dimorphism. In 1992–1993 a research team headed by Tim White discovered the first ''A. ramidus'' fossils—seventeen fragments including skull, mandible, teeth and arm bones—from the

in the

Middle Awash The Middle Awash is a paleoanthropological research area in the Afar Region along the Awash River in Ethiopia's Afar Depression. It is a unique natural laboratory for the study of human origins and evolution and a number of fossils of the earliest ...

river valley of

. More fragments were recovered in 1994, amounting to 45% of the total skeleton. This fossil was originally described as a species of ''

'', but White and his colleagues later published a note in the same journal renaming the fossil under a new genus, ''Ardipithecus''. Between 1999 and 2003, a multidisciplinary team led by Sileshi Semaw discovered bones and teeth of nine ''A. ramidus'' individuals at

As Duma As, AS, A. S., A/S or similar may refer to: Art, entertainment, and media * A. S. Byatt (born 1936), English critic, novelist, poet and short story writer * "As" (song), by Stevie Wonder * , a Spanish sports newspaper * , an academic male voic ...

in the Gona area of Ethiopia's

Afar Region The Afar Region (; aa, Qafar Rakaakayak; am, አፋር ክልል), formerly known as Region 2, is a regional state in northeastern Ethiopia and the homeland of the Afar people. Its capital is the planned city of Semera, which lies on the pave ...

. The fossils were dated to between 4.35 and 4.45 million years old. ''Ardipithecus ramidus'' had a small brain, measuring between 300 and 350 cm³. This is slightly smaller than a modern bonobo or female chimpanzee brain, but much smaller than the brain of australopithecines like Lucy (~400 to 550 cm³) and roughly 20% the size of the modern ''Homo sapiens'' brain. Like common chimpanzees, ''A. ramidus'' was much more prognathic than modern humans. The teeth of ''A. ramidus'' lacked the specialization of other apes, and suggest that it was a generalized omnivore and frugivore (fruit eater) with a diet that did not depend heavily on foliage, fibrous plant material (roots, tubers, etc.), or hard and or abrasive food. The size of the upper

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 howeve ...

in ''A. ramidus'' males was not distinctly different from that of females. Their upper canines were less sharp than those of modern common chimpanzees in part because of this decreased upper canine size, as larger upper canines can be honed through wear against teeth in the lower mouth. The features of the upper canine in ''A. ramidus'' contrast with the

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 ...

observed in common chimpanzees, where males have significantly larger and sharper upper canine teeth than females. Of the living apes, bonobos have the smallest canine sexual dimorphism, although still greater than that displayed by ''A. ramidus''. The less pronounced nature of the upper canine teeth in ''A. ramidus'' has been used to infer aspects of the

social behavior Social behavior is behavior among two or more organisms within the same species, and encompasses any behavior in which one member affects the other. This is due to an interaction among those members. Social behavior can be seen as similar to an ...

of the species and more ancestral hominids. In particular, it has been used to suggest that the

last common ancestor In biology and genetic genealogy, the most recent common ancestor (MRCA), also known as the last common ancestor (LCA) or concestor, of a set of organisms is the most recent individual from which all the organisms of the set are descended. The ...

of hominids and African apes was characterized by relatively little aggression between males and between groups. This is markedly different from social patterns in common chimpanzees, among which intermale and intergroup aggression are typically high. Researchers in a 2009 study said that this condition "compromises the living chimpanzee as a behavioral model for the ancestral hominid condition." Bonobo canine size and canine sexual dimorphism more closely resembles that of ''A. ramidus'', and as a result, bonobos are now suggested as a behavioural model. ''A. ramidus'' existed more recently than the

most recent common ancestor In biology and genetic genealogy, the most recent common ancestor (MRCA), also known as the last common ancestor (LCA) or concestor, of a set of organisms is the most recent individual from which all the organisms of the set are descended. The ...

of humans and chimpanzees ( CLCA or ''Pan''-''Homo'' LCA) and thus is not fully representative of that common ancestor. Nevertheless, it is in some ways unlike chimpanzees, suggesting that the common ancestor differs from the modern chimpanzee. After the chimpanzee and human lineages diverged, both underwent substantial evolutionary change. Chimp feet are specialized for grasping trees; ''A. ramidus'' feet are better suited for walking. The canine teeth of ''A. ramidus'' are smaller, and equal in size between males and females, which suggests reduced male-to-male conflict, increased pair-bonding, and increased parental investment. "Thus, fundamental reproductive and social behavioral changes probably occurred in hominids long before they had enlarged brains and began to use stone tools," the research team concluded.

Ardi

On October 1, 2009, paleontologists formally announced the discovery of the relatively complete ''A. ramidus'' fossil skeleton first unearthed in 1994. The fossil is the remains of a small-brained 50-kilogram (110 lb) female, nicknamed "Ardi", and includes most of the skull and teeth, as well as the pelvis, hands, and feet. It was discovered in Ethiopia's harsh Afar desert at a site called

Aramis René d'Herblay, alias Aramis, is a fictional character in the novels ''The Three Musketeers'' (1844), '' Twenty Years After'' (1845), and '' The Vicomte de Bragelonne'' (1847-1850) by Alexandre Dumas, père. He and the other two musketeers, A ...

in the Middle Awash region. Radiometric dating of the layers of volcanic ash encasing the deposits suggest that Ardi lived about 4.3-4.5 million years ago. This date, however, has been questioned by others. Fleagle and Kappelman suggest that the region in which Ardi was found is difficult to date radiometrically, and they argue that Ardi should be dated at 3.9 million years. The fossil is regarded by its describers as shedding light on a stage of human evolution about which little was known, more than a million years before

Lucy Lucy is an English feminine given name derived from the Latin masculine given name Lucius with the meaning ''as of light'' (''born at dawn or daylight'', maybe also ''shiny'', or ''of light complexion''). Alternative spellings are Luci, Luce, Lu ...

(''Australopithecus afarensis''), the iconic early human ancestor candidate who lived 3.2 million years ago, and was discovered in 1974 just away from Ardi's discovery site. However, because the "Ardi" skeleton is no more than 200,000 years older than the earliest fossils of ''Australopithecus'', and may in fact be younger than they are, some researchers doubt that it can represent a direct ancestor of ''Australopithecus''. Some researchers infer from the form of her pelvis and limbs and the presence of her abductable

, that "Ardi" was a

facultative biped A facultative biped is an animal that is capable of walking or running on two legs (bipedal), as a response to exceptional circumstances (facultative), while normally walking or running on four limbs or more. In contrast, obligate bipedalism is w ...

biped 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' ...

al when moving on the ground, but

quadruped Quadrupedalism is a form of locomotion where four limbs are used to bear weight and move around. An animal or machine that usually maintains a four-legged posture and moves using all four limbs is said to be a quadruped (from Latin ''quattuor' ...

al when moving about in tree branches. ''A. ramidus'' had a more primitive walking ability than later hominids, and could not walk or run for long distances. The teeth suggest

omnivory An omnivore () is an animal that has the ability to eat and survive on both plant and animal matter. Obtaining energy and nutrients from plant and animal matter, omnivores digest carbohydrates, protein, fat, and fiber, and metabolize the nut ...

, and are more generalised than those of modern apes. File:Ardipithecus (finger bones).jpg, Casts of Ardi's finger bones.

''Ardipithecus kadabba''

''Ardipithecus kadabba'' is "known only from teeth and bits and pieces of skeletal bones", and is dated to approximately 5.6 million years ago. It has been described as a "probable

chronospecies A chronospecies is a species derived from a sequential development pattern that involves continual and uniform changes from an extinct ancestral form on an evolutionary scale. The sequence of alterations eventually produces a population that is p ...

" (i.e. ancestor) of ''A. ramidus''. Although originally considered a subspecies of ''A. ramidus'', in 2004 anthropologists

Yohannes Haile-Selassie Yohannes Haile-Selassie Ambaye (born 23 February 1961) is an Ethiopian paleoanthropologist. An authority on pre-''Homo sapiens'' hominids, he particularly focuses his attention on the East African Rift and Middle Awash valleys.Mangels, John (200 ...

Gen Suwa Gen Suwa (born 1954) is a Japanese paleoanthropologist. He is known for his contributions to the understanding of the evolution of early hominids, including the discovery of a tooth from a hominid that was more than one million years older than the ...

, and Tim D. White published an article elevating ''A. kadabba'' to species level on the basis of newly discovered teeth from

. These teeth show "primitive morphology and wear pattern" which demonstrate that ''A. kadabba'' is a distinct species from ''A. ramidus''. The specific name comes from the Afar word for "basal family ancestor".

Classification

Due to several shared characteristics with chimpanzees, its closeness to ape divergence period, and due to its fossil incompleteness, the exact position of ''Ardipithecus'' in the fossil record is a subject of controversy. Primatologist

Esteban Sarmiento Esteban Sarmiento is a primatologist and biologist. He is noted for his work in primate anthropology and for appearing on the '' Monster Quest'' television series. Biography Sarmiento earned a biological anthropology Ph.D. in 1985 and from then un ...

had systematically compared and concluded that there is not sufficient anatomical evidence to support an exclusively human lineage. Sarmiento noted that ''Ardipithecus'' does not share any characteristics exclusive to humans, and some of its characteristics (those in the wrist and basicranium) suggest it diverged from humans prior to the human–gorilla last common ancestor. His comparative (narrow

allometry Allometry is the study of the relationship of body size to shape, anatomy, physiology and finally behaviour, first outlined by Otto Snell in 1892, by D'Arcy Thompson in 1917 in '' On Growth and Form'' and by Julian Huxley in 1932. Overview Allom ...

) study in 2011 on the molar and body segment lengths (which included living primates of similar body size) noted that some dimensions including short upper limbs, and

metacarpals In human anatomy, the metacarpal bones or metacarpus form the intermediate part of the skeletal hand located between the phalanges of the fingers and the carpal bones of the wrist, which forms the connection to the forearm. The metacarpal bones ar ...

are reminiscent of humans, but other dimensions such as long toes and relative molar surface area are great ape-like. Sarmiento concluded that such length measures can change back and forth during evolution and are not very good indicators of relatedness (

homoplasy Homoplasy, in biology and phylogenetics, is the term used to describe a feature that has been gained or lost independently in separate lineages over the course of evolution. This is different from homology, which is the term used to characterize ...

). However, some later studies still argue for its classification in the human lineage. In 2014, it was reported that the hand bones of ''Ardipithecus'', '' Australopithecus sediba'' and '' A. afarensis'' have the third metacarpal styloid process, which is absent in other apes. Unique brain organisations (such as lateral shift of the carotid foramina, mediolateral abbreviation of the lateral tympanic, and a shortened, trapezoidal

basioccipital The basilar part of the occipital bone (also basioccipital) extends forward and upward from the foramen magnum, and presents in front an area more or less quadrilateral in outline. In the young skull this area is rough and uneven, and is joined t ...

element) in ''Ardipithecus'' are also found only in the ''Australopithecus'' and ''Homo''. Comparison of the tooth root morphology with those of the earlier ''

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 '' ...

'' also indicated strong resemblance, also pointing to inclusion to the human line.

Evolutionary tree 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 spec ...

according to a 2019 study:

Paleobiology

The ''Ardipithecus'' length measures are good indicators of function and together with dental isotope data and the fauna and flora from the fossil site indicate ''Ardipithecus'' was mainly a terrestrial quadruped collecting a large portion of its food on the ground. Its arboreal behaviors would have been limited and suspension from branches solely from the upper limbs rare. A comparative study in 2013 on carbon and oxygen stable isotopes within modern and fossil

tooth enamel Tooth enamel is one of the four major tissues that make up the tooth in humans and many other animals, including some species of fish. It makes up the normally visible part of the tooth, covering the crown. The other major tissues are dentin, ...

revealed that ''Ardipithecus'' fed both arboreally (on trees) and on the ground in a more open habitat, unlike chimpanzees. In 2015, Australian anthropologists Gary Clark and Maciej Henneberg said that ''Ardipithecus'' adults have a facial anatomy more similar to chimpanzee subadults than adults, with a less-projecting face and smaller canines (large canines in primate males are used to compete within mating hierarchies), and attributed this to a decrease in craniofacial growth in favour of brain growth. This is only seen in humans, so they argued that the species may show the first trend towards human social, parenting and sexual psychology. Previously, it was assumed that such ancient human ancestors behaved much like chimps, but this is no longer considered to be a viable comparison. This view has yet to be corroborated by more detailed studies of the growth of ''A.ramidus''. The study also provides support for Stephen Jay Gould's theory in ''Ontogeny and Phylogeny'' that the paedomorphic (childlike) form of early hominin craniofacial morphology results from dissociation of growth trajectories. Clark and Henneberg also argued that such shortening of the skull—which may have caused a descension of the larynx—as well as lordosis—allowing better movement of the larynx—increased vocal ability, significantly pushing back the origin of language to well before the evolution of ''Homo''. They argued that self domestication was aided by the development of vocalization, living in a pro-social society. They conceded that chimps and ''A. ramidus'' likely had the same vocal capabilities, but said that ''A. ramidus'' made use of more complex vocalizations, and vocalized at the same level as a human infant due to

selective pressure Any cause that reduces or increases reproductive success in a portion of a population potentially exerts evolutionary pressure, selective pressure or selection pressure, driving natural selection. It is a quantitative description of the amount of ...

to become more social. This would have allowed their society to become more complex. They also noted that the base of the skull stopped growing with the brain by the end of juvenility, whereas in chimps it continues growing with the rest of the body into adulthood; and considered this evidence of a switch from a gross skeletal anatomy trajectory to a neurological development trajectory due to selective pressure for sociability. Nonetheless, their conclusions are highly speculative. According to Scott Simpson, the Gona Project's physical anthropologist, the fossil evidence from the

indicates that both ''A. kadabba'' and ''A. ramidus'' lived in "a mosaic of woodland and grasslands with lakes, swamps and springs nearby," but further research is needed to determine which habitat ''Ardipithecus'' at Gona preferred. Anatomically, Ardi is closer to 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 genu ...

than to the chimpanzee, bonobo or

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 or fi ...

. All three are adapted for knuckle walking on the forest floor and climbling trees. The orangutan spends virtually its entire life in the forest canopy. Its anatomy allows for maximum reach by its upper limbs for overhead branches while its lower limbs have a narrow stance that enables it to balance while walking "foot over foot" on lower branches and vines. Although not as closely related genetically as the knuckle walking

ape 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 g ...

s, this would indicate that Ardi was behaviourly more similar to the orangutan than to the other three great apes. In nature, the orangutan is widely dispersed. There is one dominant male in a territory with wide cheek flaps who maintains contact with others in his territory by vocalizations. The female raises offspring one at a time with a maturation period of about eight years. This is the longest maturation period of any of the apes. During this period, the offspring learns the intricate task of weaving a sleeping platform. Rescue centers are now raising young orangutans that have lost their mother typically from deforestation. They are raised in age cohorts by these centers to learn the necessary skills to be able to be released into the forest. While in these cohorts, they show social behavior reminiscent of age appropriate behavior of

human 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, cultu ...

children. This suggests that Ardi was physically preadapted for bipedal locomotion as well as social behavior.

References

External links

Science Magazine: ''Ardipithecus'' special
(requires free registration) * The Smithsonian Institution's Human Origins Program: *
Ardipithecus kadabba
*
Ardipithecus ramidus

at Archaeology info
''Explore Ardipithecus''
at NationalGeographic.com
''Ardipithecus ramidus'' - Science Journal Article

Human Timeline (Interactive)
– Smithsonian,

National Museum of Natural History The National Museum of Natural History is a natural history museum administered by the Smithsonian Institution, located on the National Mall in Washington, D.C., United States. It has free admission and is open 364 days a year. In 2021, with 7 ...

(August 2016). {{Authority control Hominini Prehistoric primate genera Pliocene primates Transitional fossils Fossil taxa described in 1995 Prehistoric Ethiopia Miocene genus first appearances Zanclean extinctions