Orrorin tugenensis

The first bipedal hominid?

Fossils assigned to Orrorin tugenensis were found near Lake Baringo in western Kenya.

Location of Kenya Credit: Vardion

Lake Baringo in Kenya's Great Rift Valley (click to enlarge) Image: Doron

Panoramic view of the Great Rift Valley (click to enlarge)

The fragmentary remains include portions of an arm and thigh bones, lower jaws, and teeth. They date to between 6.1 and 5.8 mya and are therefore of Miocene age. They were discovered by a expedition led by Brigitte Senut and Martin Pickford of the Muséum national d'Histoire naturelle in 2000.

This hominid is the only member of the genus Orrorin. The scanty remains assigned to Orrorin tugenensis suggest it was bipedal (unlike Sahelanthropus tchadensis, which was once billed as the earliest hominid, but now considered a Miocene ape).

The limb bones, about 50 percent longer than those of Lucy, suggest that Orrorin tugenensis was about the size of a chimpanzee. Its discoverers have claimed Orrorin tugenensis was adapted to both bipedality and tree climbing, and that it was a direct human ancestor, with the australopithecines as an extinct offshoot not ancestral to modern humans. Other scientists are skeptical of these claims, due to the highly fragmentary nature of the remains (Aiello and Collard 2001). But an additional paper (Galik et al. 2004) has provided further evidence of bipedality in this form. The Orrorin femur is more similar to that of H. sapiens than is Lucy's. The brain size of this hominid is unknown, since there is no skull material allowing cranial capacity to be measured.

Fossils of other organisms from the Lukeino Formation show this hominid lived in a dry evergreen forest habitat, which suggests it probably had a diet similar to that of a modern ape. This contradicts the many theories depicting earliest humans as savanna hunters (Orrorin tugenensis remains long predate stone tools and the first use of fire). Thus, if the bipedality of O. tugenensis is confirmed, the only possible conclusion will be that human bipedalism actually arose in a forest-dwelling ancestor and not in the descendants of a quadrupedal form that moved out into the open savanna.