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Ardipithecus

Ardipithecus is a very early hominin genus (subfamily Homininae) which lived about 4.4 million years ago during the early Pliocene.

Because this genus shares several traits with the African great ape genera (genus Pan and genus Gorilla), it is considered by some to be on the chimpanzee rather than human branch, but most consider it a proto-human because of a likeness in teeth with Australopithecus.

Species

Two species have been described, Ardipithecus ramidus and Ardipithecus kadabba, which was initially described as a subspecies of A. ramidus, but on the basis of teeth recently discovered in Ethiopia has been raised to species rank. Remains from both species have been found in the Middle Awash.

Ardipithecus ramidus

A. ramidus was named in September 1994. The first fossil find was dated to 4.4 million years ago based on its interval between two volcanic strata: the basal Gaala Tuff Complex (GATC) and the Daam Aatu Basaltic Tuff (DABT). Subsequent fossil discoveries by Yohannes Haile-Selassie and Giday WoldeGabriel—if identified as A. ramidus—would push the date back as far as 5.8 million years ago.

In 1992-1993 a research team headed by Dr Timothy 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 Ethiopia. More fragments were recovered in 1994, amounting to 45 percent of the total skeleton. Features of the foramen magnum and leg fragments are indicative of bipedalism.

Ardipithecus kadabba

A. kadabba is dated to have lived between 5.8 million to 5.2 million years ago. The canine teeth show primitive features, shared with Sahelanthropus and Orrorin, that distinguish them from those of more recent hominins. It has been suggested that A. kadabba is the most recent common ancestor of Homo and Pan. Anthropologists Yohannes Haile-Selassie, Gen Suwa, and Tim D. White published an article suggesting that the presence of a "canine cutting complex" indicates a need for relocation in hominid evolutionary history. Since A. ramidus is lacking the canine cutting complex, the authors argue, it is reasonable to infer the canine cutting complex, which is present in modern day chimpanzees, is a primitive trait which was lost during hominin evolution. The specific name comes from the Afar word for "basal family ancestor".

Lifestyle

On the basis of bone sizes, Ardipithecus species are believed to have been about the size of a modern chimpanzee.

The toe structure of A. ramidus suggests that the creature walked upright, and this poses problems for current theories of the origins of hominid bipedalism: Ardipithecus is believed to have lived in shady forests rather than on the savannah, where the more energy efficient locomotion permitted by bipedalism would have been an advantage.