Ever since its fossil remains were discovered, scientists have wondered about the place of Orrorin tugenensis and its place in our family tree. Was it bipedal? And where were its closest relatives? One controversial suggestion was that Orrorin was directly ancestral to our own genus – leaving the australopiths completely out in the cold.
However, a study reported in Science last month suggests that Orrorin was in fact closely related to the australopithecines. Orrorin's fragmentary remains include portions of three femurs (thigh bones), and some features of these bones have been used to support the hypothesis that this species was a bipedal hominin. If correct, this would show that bipedalism was a very early development in hominin evolution, given that Orrorin is about 6 million years old.
Brian Richmond & William Jungers (2006) performed a series of very precise measurements of the femurs, and compared their results with data from early hominins (Australopithecus & Paranthropus), fossil & modern humans, & great apes. They found that the tugenensis remains – with their long, narrow neck and broad shaft – were more similar to those of the australopiths. These features are related to the mechanics of walking, & indicate that both tugenensis & the australopiths would have had a different gait from that of modern humans.
The authors suggested that adaptations for bipedalism arose early in hominin evolution, and were pretty much unchanged until the early members of our own genus, Homo, evolved new features of the hip and thigh around 2 million years ago. However, while there seems to be a consensus that tugenensis was indeed bipedal, not all scientists agree that all early hominins would have walked in the same way (Pennisi, 2006). Additional information from other parts of the postcranial skeleton (such as the pelvis, foot, & ankle) from a variety of early hominins might settle this debate.
E. Pennisi (2006) Millenium ancestor gets its walking papers. Science 319: 1599-1600
B.G. Richmond & W.L. Jungers (2006) Orrorin tugenensis femoral morphology and the evolution of hominin bipedalism. Science 319: 1662-1665