Ardipithecus ramidus (White et al. 1995; 4.4 and 5.6 mya early Pliocene, 4’11” tall) was preceded by Hylobates and succeed by Homo.

Ardipithecus was the first genus in human ancestry to walk upright, predating Australopithecus by a million years. It incorporated an arboreal grasping hallux or big toe, reduced canine teeth and a smaller brain size like that of the modern chimpanzee. The skull was noticeably smaller than either Proconsul or Australopithecus. The eyes (eye sockets) were relatively much larger.

The teeth of Ardipithecus lacked large canines. It was probably an omnivore, but did not eat plants, ripe fruit or hard or abrasive food. Without large display canines, Ardipithecus society probably had reduced male-to-male conflict, increased pair-bonding, and increased parental investment.

The pelvis was bowl-shaped, like that of an australopithecine or human. The knees were kept beneath the body, not bowed out like a chimp. Ardipithecus feet are suited for walking, but could still grasp trees, though not as well as a chimp.

When walking upright the female reproductive organs are hidden at the top of the legs. Now females would have to assume different postures to permit mating. With fertility signals now hidden, fertility may have changed to the current once a month cycle.