Ardipithecus ramidus

Ardipithecus ramidus is a species of australopithecine from the Afar region of Early Pliocene Ethiopia 4.4 million years ago (Ma). The species A. ramidus is the type species for the genus Ardipithecus. There is an older species in this same genus, Ardipithecus kadabba that was discovered more recently.

A. ramidus, unlike modern hominids, has adaptations for both walking on two legs (bipedality) and life in the trees (arboreality), as it has a divergent big toe and evidence of bipedality. This combination of a big toe that would facilitate climbing suggests that Ardipithecus was not as efficient at bipedality as humans or even Australopithecus (a genus that did not have a divergent big toe), nor as good at arboreality as non-human great apes.

The discovery of Ardipithecus, along with Miocene apes, has reworked academic understanding of the chimpanzee–human last common ancestor. Historically, humans were thought to have evolved from a chimpanzee-like ancestor. However, Ardipithecus demonstrates that the last common ancestor was not like modern-day chimpanzees, orangutans or gorillas, but rather was an ape without a modern analogue.

In addition to the divergent big toe, another unusual feature of Ardipithecus is the dental anatomy, as it has reduced size and sexual dimorphism of the canines. This reduction in the size of the canines for both males and females suggests that A. ramidus males were less aggressive than those of modern chimps, a feature that is correlated with increased parental care and monogamy in primates.

A. ramidus appears to have inhabited woodland and bushland corridors between savannas, and was a generalized omnivore.

The first remains were described in 1994 by American anthropologist Tim D. White, Japanese paleoanthropologist Gen Suwa, and Ethiopian paleontologist Berhane Asfaw, as part of a larger research project done in collaboration with geologist Giday WoldeGabriel referred to as the Middle Awash Project.

The holotype specimen, ARA-VP-6/1, comprised an associated set of 10 teeth; and there were 16 other paratypes identified, preserving also skull and arm fragments. These were unearthed in the 4.4-million-year-old (Ma) deposits of the Afar region in Aramis, Ethiopia from 1992 to 1993, making them the oldest hominin remains at the time, surpassing Australopithecus afarensis. They initially classified it as Australopithecus ramidus, the species name deriving from the Afar language ramid "root". In 1995, they made a corrigendum recommending it be split off into a separate genus, Ardipithecus; the name stems from Afar ardi "ground" or "floor". The 4.4-million-year-old female ARA-VP 6/500 ("Ardi") is the most complete specimen.

Fossils from at least nine A. ramidus individuals at As Duma, in the Gona Western Margin, Afar, were unearthed from 1993 to 2003. The fossils were dated to between 4.32 and 4.51 million years ago.