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 had no "close analog among living monkeys or apes" (like modern-day chimpanzees, orangutans or gorillas), but rather represents an intermediate species between Australopithecus immediately (acting as a bridge in many respects to the yet to be found late Miocene African ape) and both humans and modern-day chimpanzees distantly.

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.