''Ardipithecus'' is a genus of an extinct hominine that lived during the

Late Miocene The Late Miocene (also known as Upper Miocene) is a sub-epoch of the Miocene epoch (geology), Epoch made up of two faunal stage, stages. The Tortonian and Messinian stages comprise the Late Miocene sub-epoch, which lasted from 11.63 Ma (million ye ...

and

Early Pliocene Early may refer to: Places in the United States * Early, Iowa, a city * Early, Texas, a city * Early Branch, a stream in Missouri * Early County, Georgia * Fort Early, Georgia, an early 19th century fort Music * Early B, stage name of Jamaican d ...

epochs in the Afar Depression,

Ethiopia Ethiopia, officially the Federal Democratic Republic of Ethiopia, is a landlocked country located in the Horn of Africa region of East Africa. It shares borders with Eritrea to the north, Djibouti to the northeast, Somalia to the east, Ken ...

. Originally described as one of the earliest ancestors of

human Humans (''Homo sapiens'') or modern humans are the most common and widespread species of primate, and the last surviving species of the genus ''Homo''. They are Hominidae, great apes characterized by their Prehistory of nakedness and clothing ...

s after they diverged from the

chimpanzee The chimpanzee (; ''Pan troglodytes''), also simply known as the chimp, is a species of Hominidae, great ape native to the forests and savannahs of tropical Africa. It has four confirmed subspecies and a fifth proposed one. When its close rel ...

s, the relation of this

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

to human ancestors and whether it is a

hominin The Hominini (hominins) form a taxonomic tribe of the subfamily Homininae (hominines). They comprise two extant genera: ''Homo'' (humans) and '' Pan'' (chimpanzees and bonobos), and in standard usage exclude the genus '' Gorilla'' ( gorillas) ...

is now a matter of debate. Two fossil

species A species () is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of Taxonomy (biology), ...

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 (geology), epoch in the geologic time scale that extends from 5.33 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

bonobo The bonobo (; ''Pan paniscus''), also historically called the pygmy chimpanzee (less often the dwarf chimpanzee or gracile chimpanzee), is an endangered great ape and one of the two species making up the genus ''Pan (genus), Pan'' (the other bei ...

s. Some analyses describe ''Australopithecus'' as being sister to ''Ardipithecus ramidus'' specifically. This means that ''

Australopithecus ''Australopithecus'' (, ; or (, ) is a genus of early hominins that existed in Africa during the Pliocene and Early Pleistocene. The genera ''Homo'' (which includes modern humans), ''Paranthropus'', and ''Kenyanthropus'' evolved from some ''Aus ...

'' is distinctly more closely related to ''Ardipithecus ramidus'' than ''Ardipithecus kadabba''. Cladistically, then, ''Australopithecus'' (and eventually ''

Homo sapiens Humans (''Homo sapiens'') or modern humans are the most common and widespread species of primate, and the last surviving species of the genus ''Homo''. They are Hominidae, great apes characterized by their Prehistory of nakedness and clothing ...

'') 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 In geology and related fields, a stratum (: strata) is a layer of Rock (geology), rock or sediment characterized by certain Lithology, lithologic properties or attributes that distinguish it from adjacent layers from which it is separated by v ...

: 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 Afar is an Afroasiatic language belonging to the Cushitic branch, primarily spoken by the Afar people, native to parts of Djibouti, Eritrea and Ethiopia. It is an official language in Ethiopia; and a national language in Djibouti and Eritre ...

, in which ''Ardi'' means "ground/floor" and ''ramid'' means "root". The ''pithecus'' portion of the name is from the Greek word for "ape". Like most hominids, but unlike all previously recognized hominins, it had a grasping hallux 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 animal moves by means of its two rear (or lower) Limb (anatomy), limbs or legs. An animal or machine that usually moves in a bipedal manner is known as a biped , meaning 'two feet' (from ...

on the ground as well. Like later hominins, ''Ardipithecus'' had reduced canine teeth 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 Afar Depression in the Middle Awash 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 in the Gona area of Ethiopia's

Afar Region The Afar Region (; ; ), formerly known as Region 2, is a Regions of Ethiopia, regional state in northeastern Ethiopia and the homeland of the Afar people. Its capital is the planned city of Semera, which lies on the paved Awash, Ethiopia, Awash� ...

. 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, dogteeth, eye teeth, vampire teeth, or fangs, are the relatively long, pointed teeth. In the context of the upper jaw, they are also known as '' fangs''. They can appear more f ...

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 sexes of the same species exhibit different Morphology (biology), morphological characteristics, including characteristics not directly involved in reproduction. The condition occurs in most dioecy, di ...

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, it encompasses any behavior in which one member affects another. Social behavior can be seen as similar to an exchange of goods, with the expectation that when you ...

of the species and more ancestral hominids. In particular, it has been used to suggest that the last common ancestor 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 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 2009 was designated as the International Year of Astronomy by the United Nations to coincide with the 400th anniversary of Galileo Galilei's first known astronomical studies with a telescope and the publication of Astronomia Nova by Joha ...

, 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 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 in the Middle Awash region. Radiometric dating of the layers of volcanic ash encasing the deposits suggest that Ardi lived about 4.3 to 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 language, 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 ar ...

(''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 hallux, that "Ardi" was a facultative biped: bipedal when moving on the ground, but

quadruped Quadrupedalism is a form of locomotion in which animals have four legs that are used to bear weight and move around. An animal or machine that usually maintains a four-legged posture and moves using all four legs is said to be a quadruped (fr ...

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, 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" (i.e. ancestor) of ''A. ramidus''. Although originally considered a subspecies of ''A. ramidus'', in 2004 anthropologists Yohannes Haile-Selassie, Gen Suwa, 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 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) 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, also known as the "palm bones", are the appendicular skeleton, appendicular bones that form the intermediate part of the hand between the phalanges (fingers) and the carpal bones (wrist, wris ...

are reminiscent of humans, but other dimensions such as long toes and relative molar surface area are

great ape 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 ...

-like. Sarmiento concluded that such length measures can change back and forth during evolution and are not very good indicators of relatedness ( homoplasy). 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 ''Australopithecus sediba'' is an extinct species of australopithecine recovered from Malapa Fossil Site, Cradle of Humankind, Malapa Cave, Cradle of Humankind, South Africa. It is known from a partial juvenile skeleton, the holotype MH1, and a ...

'' 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 element) in ''Ardipithecus'' are also found only in the ''Australopithecus'' and ''Homo''. Comparison of the tooth root morphology with those of the earlier '' Sahelanthropus'' also indicated strong resemblance, also pointing to inclusion to the human line. Evolutionary tree 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 Tissue (biology), tissues that make up the tooth in humans and many animals, including some species of fish. It makes up the normally visible part of the tooth, covering the Crown (tooth), crown. The other ...

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 Stephen Jay Gould ( ; September 10, 1941 – May 20, 2002) was an American Paleontology, paleontologist, Evolutionary biology, evolutionary biologist, and History of science, historian of science. He was one of the most influential and widely re ...

'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 The larynx (), commonly called the voice box, is an organ (anatomy), organ in the top of the neck involved in breathing, producing sound and protecting the trachea against food aspiration. The opening of larynx into pharynx known as the laryngeal ...

—as well as lordosis—allowing better movement of the larynx—increased vocal ability, significantly pushing back the

origin of language The origin of language, its relationship with human evolution, and its consequences have been subjects of study for centuries. Scholars wishing to study the origins of language draw inferences from evidence such as the fossil record, archaeolog ...

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 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 Middle Awash 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.

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 (NMNH) 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. With 4.4 ...

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