New Study: Ardipithecus ramidus Derived In Human Direction

According to Bill Kimbel and colleagues, the skull of Ardipithecus ramidus shows tell-tale signs of being derived in the human direction away from great apes.  From Science Daily:

White's field-research team has been recovering fossil remains of Ardipithecus ramidus in the Middle Awash research area, Ethiopia since the 1990s. The most recent study of the Ardi skull, led by Suwa, was published in Science in 2009, whose work (with the Middle Awash team) first revealed humanlike aspects of its base. Kimbel co-leads the team that recovered the earliest known Australopithecus skulls from the Hadar site, home of the "Lucy" skeleton, in Ethiopia.

"Given the very tiny size of the Ardi skull, the similarity of its cranial base to a human's is astonishing," says Kimbel.

The cranial base is a valuable resource for studying phylogenetic, or natural evolutionary relationships, because its anatomical complexity and association with the brain, posture and chewing system have provided numerous opportunities for adaptive evolution over time. The human cranial base, accordingly, differs profoundly from that of apes and other primates.

In humans, the structures marking the articulation of the spine with the skull are more forwardly located than in apes, where the base is shorter from front to back and the openings on each side for passage of blood vessels and nerves are more widely separated.

Given that the post-cranial remains from Ardipithecus suggest facultative bipedalism, this is not so surprising. If you are going to walk upright at all, you need to be able to see where you are going. This information, in conjunction with the new appraisal of the Orrorin tugenensis remains strongly suggests that, while Orrorin was at or near the junction of the last common ancestor, Ardipithecus, at 1.6 million years later, is quite a bit beyond it. It also reinforces what a terrible model the modern apes are for early human morphology.