Thursday, March 06, 2008

Two feet good...

Along with a big brain, walking upright on two feet has often been taken to be a defining feature of the human line. In this week’s New Scientist I have a feature article on some recent ideas about why, and where, bipedality first arose. The ‘where’ question relates not to which part of the globe walking on two feet got going, but whether it was on the ground or in the trees.

What? Walking in the trees? It might sound counter-intuitive, but some researchers have recently been arguing for just this possibility, based on observations of the locomotor behaviour of orangutans. In the wild, orangutans not only move through the branches suspended by their hands, but occasionally ‘walk’ along branches while stabilising themselves by holding onto braches overhead. This ‘hand-assisted bipedalism’, the suggestion goes, could have been the precursor to bipedality in the human line.

It’s a controversial theory. Many other experts in human evolution argue that the fossil record clearly shows that the earliest humans show features of knuckle-walking ancestry. This, the counter-argument goes, points to a knuckle-walking ancestor of humans, chimps and gorillas: while the latter species retained this trait, the human line evolved a commitment to bipedalism as it increasingly abandoned life in, and among, the trees.

As is so often the case in debates about the course of human evolution, more fossils are needed. It would also be good if researchers could arrive at some sort of consensus about what the existing fossils tell us. I suspect that academic rivalries, and prior theoretical commitments, make this prospect unlikely in the short term. But keep your eyes open.

There is an idea even more provocative than ‘tree-walking’ that I was unable to cover in my New Scientist piece because of space constraints. Aaron Filler, a spinal expert at Harvard University with a fascination with human evolution, has recently proposed that the evolution of bipedality has a much older, and much simpler, origin than previous account allow.

By studying the spines of many living and extinct mammals, including apes, Filer claims to have documented a series of changes leading to the upright spine typical of humans. And some of these are astonishingly old. Filler argues that the lumbar vertebrae of Morotopithecus bishopi, an ancient ape that lived more than 20 million years ago (some 7 million years before the split between orangutans and the other great apes), shows tell-tale signs that its owner was an upright biped.

And for Filler, this has little to do with trees or savannahs. Rather, changes to the spine might have arisen by mutations in ‘homeotic’ genes that orchestrate developmental processes. Small changes to such genes can produce big changes by affecting entire developmental cascades. Filler speculates that an ancient mutation may have produced an individual in a single generation with a lumbar region causing an upright posture – the first bipedal ape. And so orangutan tree-walking is derivative, not innovative. “Bipedalism in the arboreal orangutans is a vestige of their ancestry and not so much a harbinger of the human locomotor style,” says Filler. (Filler has a website based on his book expounding this theory at Filler also has a video describing various forms of primate locomotion online here).

While I’m at it, here’s a cool video of gibbons showing off their arboreal parkour antics (this really needs to be watched with sound, as the mixed in music (Welcome To The Jungle, by Guns'n'Roses) really adds to the film).