Ardipithecus ramidus was first discovered in 1992 by Gen Suwa, and is the most ancient hominid found thus far, living from 4.4 to 4 million years ago. The several isolated teeth, cranium fragments, and arm bones were uncovered at a site named Aramis, in the middle Awash region of Ethiopia. Dated at about 4.4 million years ago, these fossils are clearly more primitive than those of Australopithecus afarensis. The canines are larger, the size of the posterior teeth are comparatively small, there is a wider diastema, and the teeth have thin, ape-like enamel. Although the shape of the dental arcade resembles that of Australopithecus afarensis, the canines are lined up with the posterior teeth, which is a trait found in apes.

The arm and cranium pieces are not much informative. However, its discoverers reason that the former corresponds to upright locomotion. Recently, leg and pelvis bones have been discovered that belong to A. ramidus, plus a nearly complete skeleton. In light of this evidence, scientists have concluded that the creature exhibited a semi-bipedal mode of locomotion. Since the bones were found in a temperate or tropical rain forest environment, bipedalism was--by good chance--not related to the expansion of savanna habitats. Ian Tattersall stated that, "these early hominids were not necessarily (or even at all) savanna dwellers." Dick Rayner has also demonstrated that Makapansgat was also a wooded environment around 3 million years ago. Anyway, the arm has a mixture of both A. afarensis and simian features, but shows no adaptation for knuckle-walking.

It weighed roughly 40 kg. Its exact brain size is unknown, but it was approximately the size of a chimpanzee's. However, someone has suggested that, "The brain size is estimated to be around 450 cc[cubic centimeter]." The position of the foramen magnum is pretty much at the base of the skull, which further implies that Ardipithecus ramidus was a biped. Research done by biochemists Allan Wilson and Vincent Sarich have revealed that the hominid-ape line diverged between 5 to 7 million years ago, making Ardipithecus ramidus the possible "missing link" between humans and apes.

On to Australopithecus anamensis

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