| Ardipithecus | |
|---|---|
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| Ardipithecus ramidus specimen, nicknamedArdi | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Mammalia |
| Order: | Primates |
| Suborder: | Haplorhini |
| Infraorder: | Simiiformes |
| Family: | Hominidae |
| Subfamily: | Homininae |
| Tribe: | Hominini |
| Subtribe: | Hominina |
| Genus: | †Ardipithecus Whiteet al., 1995 |
| Species | |
Ardipithecus is a genus of an extincthominine that lived during theLate Miocene andEarly Pliocene epochs in theAfar Depression,Ethiopia. Originally described as one of the earliest ancestors ofhumans after they diverged from thechimpanzees, the relation of thisgenus to human ancestors and whether it is ahominin is now a matter of debate.[1] Two fossilspecies are described in the literature:A. ramidus, which lived about 4.4 million years ago[2] during the earlyPliocene, andA. kadabba, dated to approximately 5.6 million years ago (lateMiocene).[3] Initial behavioral analysis indicated thatArdipithecus could be very similar to chimpanzees;[1] however, more recent analysis based on canine size and lack of canine sexual dimorphism indicates thatArdipithecus was characterised by reduced aggression,[4] and that they more closely resemblebonobos.[5]
Some analyses describeAustralopithecus as being sister toArdipithecus ramidus specifically.[6] This means thatAustralopithecus is distinctly more closely related toArdipithecus ramidus thanArdipithecus kadabba.Cladistically, then,Australopithecus (and eventuallyHomo sapiens) indeed emerged within theArdipithecus lineage, and this lineage is not literally extinct.
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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 volcanicstrata: the basalGaala Tuff Complex (G.A.T.C.) and theDaam Aatu Basaltic Tuff (D.A.B.T.).[7] The nameArdipithecus ramidus stems mostly from theAfar language, in whichArdi means "ground/floor" andramid means "root". Thepithecus portion of the name is from theGreek word for "ape".[8]
Like mosthominids, but unlike all previously recognized hominins, it had a graspinghallux or big toe adapted for locomotion in the trees. It is not confirmed how many other features of its skeleton reflect adaptation tobipedalism on the ground as well. Like later hominins,Ardipithecus had reducedcanine teeth and reduced canine sexual dimorphism.[4]
In 1992–1993 a research team headed byTim White discovered the firstA. ramidus fossils—seventeen fragments including skull, mandible, teeth and arm bones—from theAfar Depression in theMiddle Awash river valley ofEthiopia. More fragments were recovered in 1994, amounting to 45% of the total skeleton. This fossil was originally described as a species ofAustralopithecus, 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 bySileshi Semaw discovered bones and teeth of nineA. ramidus individuals atAs Duma in theGona area of Ethiopia'sAfar Region.[9] The fossils were dated to between 4.35 and 4.45 million years old.[10]
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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 ofaustralopithecines like Lucy (~400 to 550 cm3) and roughly 20% the size of the modernHomo sapiens brain. Like common chimpanzees,A. ramidus was much moreprognathic than modern humans.[11]
The teeth ofA. ramidus lacked the specialization of other apes, and suggest that it was a generalizedomnivore andfrugivore (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 uppercanine tooth inA. 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 inA. ramidus contrast with thesexual dimorphism observed in common chimpanzees, where males have significantly larger and sharper upper canine teeth than females.[12] Of the living apes, bonobos have the smallest canine sexual dimorphism, although still greater than that displayed byA. ramidus.[4]
The less pronounced nature of the upper canine teeth inA. ramidus has been used to infer aspects of thesocial behavior of the species and more ancestral hominids. In particular, it has been used to suggest that thelast 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."[12] Bonobo canine size and canine sexual dimorphism more closely resembles that ofA. ramidus, and as a result, bonobos are now suggested as a behavioural model.[13]
A. ramidus existed more recently than themost recent common ancestor of humans and chimpanzees (CLCA orPan-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 ofA. 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.[3]
On October 1,2009, paleontologists formally announced the discovery of the relatively completeA. 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.[14] It was discovered in Ethiopia's harshAfar desert at a site calledAramis 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.[15]
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 beforeLucy (Australopithecus afarensis), the iconic early human ancestor candidate who lived 3.2 million years ago, and was discovered in 1974 just 74 km (46 mi) away from Ardi's discovery site. However, because the "Ardi" skeleton is no more than 200,000 years older than the earliest fossils ofAustralopithecus, and may in fact be younger than they are,[15] some researchers doubt that it can represent a direct ancestor ofAustralopithecus.
Some researchers infer from the form of her pelvis and limbs and the presence of herabductablehallux, that "Ardi" was afacultative biped:bipedal when moving on the ground, butquadrupedal when moving about in tree branches.[3][16][17]A. ramidus had a more primitive walking ability than later hominids, and could not walk or run for long distances.[18] The teeth suggestomnivory, and are more generalised than those of modern apes.[3]
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Ardipithecus kadabba is "known only from teeth and bits and pieces of skeletal bones",[14] and is dated to approximately 5.6 million years ago.[3] It has been described as a "probablechronospecies" (i.e. ancestor) ofA. ramidus.[3] Although originally considered a subspecies ofA. ramidus, in 2004 anthropologistsYohannes Haile-Selassie,Gen Suwa, andTim D. White published an article elevatingA. kadabba to species level on the basis of newly discovered teeth fromEthiopia. These teeth show "primitive morphology and wear pattern" which demonstrate thatA. kadabba is a distinct species fromA. ramidus.[19]
The specific name comes from theAfar word for "basal family ancestor".[20]
Due to several shared characteristics with chimpanzees, its closeness to ape divergence period, and due to its fossil incompleteness, the exact position ofArdipithecus in the fossil record is a subject of controversy.[21] PrimatologistEsteban Sarmiento had systematically compared and concluded that there is not sufficient anatomical evidence to support an exclusively human lineage. Sarmiento noted thatArdipithecus 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 thehuman–gorilla last common ancestor.[22] His comparative (narrowallometry) 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, andmetacarpals are reminiscent of humans, but other dimensions such as long toes and relative molar surface area aregreat ape-like. Sarmiento concluded that such length measures can change back and forth during evolution and are not very good indicators of relatedness (homoplasy).[23]
However, some later studies still argue for its classification in the human lineage. In 2014, it was reported that the hand bones ofArdipithecus,Australopithecus sediba andA. afarensis have the third metacarpalstyloid process, which is absent in other apes.[24] Unique brain organisations (such as lateral shift of the carotid foramina, mediolateral abbreviation of the lateraltympanic, and a shortened, trapezoidalbasioccipital element) inArdipithecus are also found only in theAustralopithecus andHomo.[25] Comparison of the tooth root morphology with those of the earlierSahelanthropus also indicated strong resemblance, also pointing to inclusion to the human line.[26]
Evolutionary tree according to a 2019 study:[27]
TheArdipithecus length measures are good indicators of function and together with dental isotope data and the fauna and flora from the fossil site indicateArdipithecus 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.[23] A comparative study in 2013 on carbon and oxygen stable isotopes within modern and fossiltooth enamel revealed thatArdipithecus fed botharboreally (on trees) and on the ground in a more open habitat, unlike chimpanzees.[28]
In 2015, Australian anthropologists Gary Clark and Maciej Henneberg said thatArdipithecus 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.[29] Previously, it was assumed that such ancient human ancestors behaved much like chimps, but this is no longer considered to be a viable comparison.[30] This view has yet to be corroborated by more detailed studies of the growth ofA. ramidus. The study also provides support forStephen Jay Gould's theory inOntogeny 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 thelarynx—as well as lordosis—allowing better movement of the larynx—increased vocal ability, significantly pushing back theorigin of language to well before the evolution ofHomo. They argued that self domestication was aided by the development of vocalization, living in a pro-social society. They conceded that chimps andA. ramidus likely had the same vocal capabilities, but said thatA. ramidus made use of more complex vocalizations, and vocalized at the same level as a human infant due toselective 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.[31][29]
According toScott Simpson, the Gona Project'sphysical anthropologist, the fossil evidence from theMiddle Awash indicates that bothA. kadabba andA. ramidus lived in "a mosaic of woodland and grasslands with lakes, swamps and springs nearby," but further research is needed to determine which habitatArdipithecus at Gona preferred.[9]
Indigenous people living where large boas and pythons occur are well within the prey size range of those species; an adult maleAeta would constitute greater than or equal to 60% of the mass of a large femalereticulated python, which is not a heavy meal by snake standards and especially not for a species whose natural diet includes pigs weighing up to 60 kg. Plio-Plestiocene hominins before the divergence ofHomo erectus and reaching back toArdipithecus ramidus averaged 30-52 kg in mass and thus, making them comparably susceptible to giantsnake predation.[32]
Another co-author is Tim D. White, a paleoanthropologist at UC–Berkeley who in 1994 discovered a pre-human fossil, named Ardipithecus ramidus, that was then the oldest known, at 4.4 million years.
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