| Ardipithecus ramidus | |
|---|---|
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| A. ramidus at theMuseo Nacional de Ciencias Naturales | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Mammalia |
| Order: | Primates |
| Suborder: | Haplorhini |
| Infraorder: | Simiiformes |
| Family: | Hominidae |
| Subfamily: | Homininae |
| Tribe: | Hominini |
| Genus: | †Ardipithecus |
| Species: | †A. ramidus |
| Binomial name | |
| †Ardipithecus ramidus | |
| Synonyms | |
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Ardipithecus ramidus is a species ofaustralopithecine from theAfar region ofEarly Pliocene Ethiopia 4.4 million years ago (mya).A. ramidus, unlike modernhominids, has adaptations for both walking on two legs (bipedality) and life in the trees (arboreality). However, it would not have been as efficient at bipedality ashumans, nor at arboreality as non-humangreat apes. Its discovery, along withMiocene apes, has reworked academic understanding of thechimpanzee–human last common ancestor from appearing much like modern-daychimpanzees,orangutans andgorillas to being a creature without a modern anatomical cognate.
The facial anatomy suggests thatA. ramidus males were less aggressive than those of modern chimps, which is correlated to increased parental care andmonogamy in primates. It has also been suggested that it was among the earliest of human ancestors to use some proto-language, possibly capable of vocalizing at the same level as a human infant. This is based on evidence of human-like skull architecture, cranial base angle and vocal tract dimensions, all of which inA. ramidus are paedomorphic when compared to chimpanzees and bonobos. This suggests the trend toward paedomorphic or juvenile-like form evident in human evolution, may have begun withA. ramidus. Given these unique features, it has been argued that inA. ramidus we may have the first evidence of human-like forms of social behaviour, vocally mediated sociality as well as increased levels of prosociality via the process of self-domestication—all of which seem to be associated with the same underlying changes in skull architecture.A. ramidus appears to have inhabitedwoodland andbushland corridors between savannas, and was a generalizedomnivore.
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The first remains were described in 1994 by American anthropologistTim D. White, Japanese paleoanthropologistGen Suwa, and Ethiopian paleontologistBerhane Asfaw. Theholotype specimen, ARA-VP-6/1, comprised an associated set of 10 teeth; and there were 16 otherparatypes identified, preserving also skull and arm fragments. These were unearthed in the 4.4-million-year-old (Ma) deposits of theAfar region inAramis, Ethiopia from 1992 to 1993, making them the oldest hominin remains at the time, surpassingAustralopithecus afarensis. They initially classified it asAustralopithecus ramidus, thespecies name deriving from theAfar languageramid "root".[1] In 1995, they made acorrigendum recommending it be split off into a separate genus,Ardipithecus; the name stems from Afarardi "ground" or "floor".[2] The 4.4-million-year-old female ARA-VP 6/500 ("Ardi") is the most complete specimen.[3]
Fossils from at least nineA. ramidus individuals atAs Duma,Gona Western Margin, Afar, were unearthed from 1993 to 2003. The fossils were dated to between 4.32 and 4.51 million years ago.[4]
In 2001, 6.5- to 5.5-million-year-old fossils from theMiddle Awash were classified as a subspecies ofA. ramidus by Ethiopian paleoanthropologistYohannes Haile-Selassie.[5] In 2004, Haile-Selassie, Suwa and White split it off into its own species,A. kadabba.[6]A. kadabba is considered to have been the direct ancestor ofA. ramidus, makingArdipithecus achronospecies.[7]
The exactaffinities ofArdipithecus have been debated. White, in 1994, consideredA. ramidus to have been more closely related tohumans than chimpanzees, though noting it to be the mostape-like fossil hominin to date.[1] In 2001, French paleontologistBrigitte Senut and colleagues aligned it more closely tochimpanzees,[8] but this has been refuted.[5] In 2009, White and colleagues reaffirmed the position ofArdipithecus as more closely related to modern humans based on dental similarity, a shortbase of the skull, and adaptations tobipedality.[9] In 2011, primatologistEsteban Sarmiento said that there is not enough evidence to assignArdipithecus toHominini (comprising both humans and chimps),[10] but its closer affinities to humans have been reaffirmed in following years.[11] White and colleagues consider it to have been closely related to or the ancestor of the temporally closeAustralopithecus anamensis, which was the ancestor toAu. afarensis.[3]
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Before the discovery ofArdipithecus and other pre-Australopithecus hominins, it was assumed that thechimpanzee–human last common ancestor and preceding apes appeared much like modern-daychimpanzees,orangutans andgorillas, which would have meant these three changed very little over millions of years. Their discovery led to the postulation that modern great apes, much like humans, evolved several specialized adaptations to their environment (have highly derivedmorphologies), and their ancestors were comparatively poorly adapted tosuspensory behavior or knuckle walking, and did not have such a specialized diet. Also, the origins of bipedality were thought to have occurred due to a switch from a forest to a savanna environment, but the presence of bipedal pre-Australopithecus hominins inwoodlands has called this into question,[12] though they inhabited wooded corridors near or between savannas. It is also possible thatArdipithecus and pre-Australopithecus were random offshoots of the hominin line.[13]
Assuming subsistence was primarily sourced from climbing in trees,A. ramidus may not have exceeded 35–60 kg (77–132 lb). "Ardi," a larger female specimen, was estimated to have stood 117–124 cm (3 ft 10 in – 4 ft 1 in) and weighed 51 kg (112 lb) based on comparisons with large-bodied female apes.[14] Unlike the laterAustralopithecus but much like chimps and humans, males and females were about the same size.[3]
A. ramidus had a small brain, measuring 300–350 cc (18–21 cu in). This is slightly smaller than a modernbonobo or chimp brain, but much smaller than the brain ofAustralopithecus—about 400–550 cc (24–34 cu in)—and roughly 20% the size of the modern human brain. Like chimps, theA. ramidus face was much more pronounced (prognathic) than modern humans.[15] The size of the uppercanine tooth inA. ramidus males was not distinctly different from that of females (only 12% larger), in contrast to thesexual dimorphism observed in chimps where males have significantly larger and sharper upper canines than females.[3][16]
A. ramidus feet are better suited for walking than chimps. However, like non-human great apes, but unlike all previously recognized human ancestors, it had a grasping big toe adapted for locomotion in the trees (anarboreal lifestyle), though it was likely not as specialized for grasping as it is in modern great apes.[9][17] Itstibial andtarsal lengths indicate a leaping ability similar to bonobos.[10] It lacks any characters suggestive of specializedsuspension, vertical climbing, orknuckle walking; and it seems to have used a method of locomotion unlike any modern great ape, which combined arborealpalm walking clambering and a form of bipedality more primitive thanAustralopithecus. The discovery of such unspecialized locomotion led American anthropologistOwen Lovejoy and colleagues to postulate that the chimpanzee–human last common ancestor used a similar method of locomotion.[9][18]
The upper pelvis (distance from thesacrum to thehip joint) is shorter than in any known ape. It is inferred to have had a longlumbar vertebral series, andlordosis (human curvature of the spine), which are adaptations for bipedality. However, the legs were not completely aligned with the torso (were anterolaterally displaced), andArdipithecus may have relied more on itsquadriceps thanhamstrings which is more effective for climbing than walking.[7][19] However, it lackedfoot arches and had to adopt a flat-footed stance. These would have made it less efficient at walking and running thanAustralopithecus andHomo. It may not have employed a bipedal gait for very long time intervals.[3] It may have predominantly used palm walking on the ground,[20] Nonetheless,A. ramidus still had specialized adaptations for bipedality, such as a robustfibularis longus muscle used in pushing the foot off the ground while walking (plantarflexion),[17] the big toe (though still capable of grasping) was used for pushing off, and the legs were aligned directly over the ankles instead of bowing out like in non-human great apes.[21]
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The reduced canine size and reduced skull robustness inA. ramidus males (about the same size in males and females) is typically correlated with reduced male–male conflict, increased parental investment, andmonogamy.[7][9] Because of this, it is assumed thatA. ramidus lived in a society similar to bonobos andateline monkeys[16] due to a process ofself domestication (becoming more and more docile which allows for a more gracile build). Because a similar process is thought to have occurred with the comparatively docile bonobos from more aggressive chimps,A. ramidus society may have seen an increase in maternal care and female mate selection compared to its ancestors.[22] Alternatively, it is possible that increased male size is a derived trait instead of basal (it evolved later rather than earlier), and is a specialized adaptation in modern great apes as a response to a different and more physically exerting lifestyle in males than females rather than being tied to interspecific conflict.[12]
Australian anthropologists Gary Clark andMaciej Henneberg 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, as a means of non-violently dealing with conflict. 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.[22][23]
American primatologistCraig Stanford postulated thatA. ramidus behaved similarly to chimps, which frequent both the trees and the ground, have apolygynous society, hunt cooperatively, and are the mosttechnologically advanced non-human.[24] However, Clark and Henneberg concluded thatArdipithecus cannot be compared to chimps, having been too similar to humans.[22] According to French paleoprimatologistJean-Renaud Boisserie, the hands ofArdipithecus would have been dextrous enough to handle basic tools, though it has not been associated with any tools.[25]
The teeth ofA. ramidus indicate that it was likely a generalizedomnivore andfruit eater which predominantly consumedC3 plants in woodlands orgallery forests. The teeth lacked adaptations for abrasive foods.[9][10][16] Lacking the speed and agility of chimps and baboons, meat intake byArdipithecus, if done, would have been sourced from only what could have been captured by limited pursuit, or from scavenging carcasses.[26]
The second-to-fourth digit ratios ofA. ramidus are low, consistent with high androgenisation and a disposition towards polygyny.[27]
Half of the large mammal species associated withA. ramidus at Aramis arespiral-horned antelope andcolobine monkeys (namelyKuseracolobus andPliopapio). There are a few specimens of primitivewhite andblack rhino species, and elephants, giraffes and hippo specimens are less abundant. These animals indicate that Aramis ranged from wooded grasslands to forests, butA. ramidus likely preferred the closed habitats,[28] specifically riverine areas as such water sources may have supported more canopy coverage.[29] Aramis as a whole generally had less than 25% canopy cover.[13] There were exceedingly high rates of scavenging, indicating a highly competitive environment somewhat likeNgorongoro Crater. Predators of the area were the hyenasIkelohyaena abronia andCrocuta dietrichi, the bearAgriotherium, the catsDinofelis andMegantereon, the dogEucyon, and crocodiles.[30]Bayberry,hackberry andpalm trees appear to have been common at the time from Aramis to theGulf of Aden; and botanical evidence suggests a cool, humid climate.[31] Conversely, annual water deficit (the difference between water loss byevapotranspiration and water gain by precipitation) at Aramis was calculated to have been about 1,500 mm (59 in), which is seen in some of the hottest, driest parts of East Africa.[13]
Carbon isotope analyses of the herbivore teeth from the Gona Western Margin associated withA. ramidus indicate that these herbivores fed mainly onC4 plants and grasses rather than forest plants. The area seems to have featuredbushland and grasslands.[32]
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