Will the hominin from Kenya please stand up?

The origin of bipedalism, one of the classic traits popularly cited to separate humans from other primates, has long been a controversial area of research. A number of hypotheses have been floated over the years, but now that more fossil material from the time around the chimpanzee/human split has been uncovered researchers have been able to get a better idea of just how old bipedalism is. In a new paper published in Science, Brian Richmond and William Jungers suggest that the remains of the 6 million year old Orrorin tugenensis provide the earliest known postcranial evidence of hominin bipedalism.

Part of the problem with ancient hominins is that they are rare and often fragmentary, which can make studying problems associated with morphology and behavior quite difficult. In the case of the ~7 million year old Sahelanthropus tchadensis from Chad, only the skull and teeth were discovered. While the placement of the foramen magnum (the hole on the bottom, rear part of the skull that attaches to the spinal column) suggests that Sahelanthropus might have walked upright, the skull is somewhat distorted and the hypothesis cannot be fully refuted or denied until postcranial material can be found. In the case of the younger Orrorin, however, several fragments of femur have been discovered, and being that the femur is a major weight-bearing bone in our skeleton it's especially important for studies of bipedalism.

What Richmond and Jungers found in their analysis was that the femur known as BAR
1002'00 was more similar to the australopithecines than to living apes, fossil Homo, or our own species. Indeed, the femur of Orrorin appears to be intermediate between those of living great apes and Homo, having a femoral head intermediate of a size intermediate between Homo and chimpanzees, while have a femoral neck of about the same size as is seen in australopithecines. The areas of muscle attachment also seem to be closest to australopithecines as well, which leaves little doubt that Orrorin was walking upright.

What is even more interesting, though, is that like some of the early australopithecines, Orrorin has an upper body morphology that appears to be better adapted to an arboreal lifestyle. The authors argue, then, that Orrorin walked upright on the ground but perhaps moved into the trees to forage or build nests, being behaviorally closer to a chimpanzee than to an "arboreal specialist" like an orangutan. Admittedly I winced a little when it was mentioned that this mode of life is consistent with knuckle-walking (which I can't deny, although sometimes I feel that there is too much concern for making early hominins knuckle-walkers like chimpanzees), but the bipediality of Orrorin is important as it overturns part of the modern mythology surrounding human evolution.

The popular view is still that our ancestors first "stood up" to look over the hot African grasslands, perhaps carrying tools with them as they cautiously walked from refuge to refuge. Recent research (like the study mentioned here), however, seem to imply that bipedalism developed in forested habitats rather than as a result of life on the savanna. This will likely cause further controversy and contention in the larger debate about human origins, but hopefully more fossil evidence will come to light in the future that will help confirm or refute some hypotheses (if not inspire new ones altogether).

Richmond, B.G., Jungers, W.L. (2008). Orrorin tugenensis Femoral Morphology and the Evolution of Hominin Bipedalism. Science, 319(5870), 1662-1665. DOI: 10.1126/science.1154197