premise: Without exception, living non-human primates habitually more around on all fours, or quadrupedally, when they are on the ground. Scientists generally assume therefore that the last common ancestor of humans and chimpanzees (our closest living relative) was also a quadrupted. Exactly when the last common ancestor lived is unknown, but clear indications of bipedalism, the trait that distinguishes ancient humans from other apes, are evident in the oldest known species of Australopithecus, which lived in Africa roughly four millions years ago.
hypothesis: Australopithecus is as closely related to ancient man as to the chimpanzee.