Knuckle-walkers may have taken separate evolutionary paths

A Canadian scientist has spearheaded a study that could rewrite a key chapter in the story of human evolution, tracing the emergence of one of our defining traits – the ability to walk on two feet – to a tree-climbing primate ancestor rather than a ground-dwelling "knuckle-walker."

In a debate that has raged since the days of Charles Darwin, the 19th-century author of On The Origin of Species, experts have clashed over the sequence of anatomical adaptations that gave rise to "bipedal" primates and eventually – about 250,000 years ago – homo sapiens.

Alberta-born evolutionary anthropologist Tracy Kivell, lead author of a study of present-day and prehistoric primate wrist bones, carried out at the University of Toronto and Duke University, concludes that humans are more likely to have descended from "an ape that lived in the trees, and then came down to the ground to walk on two feet," she told Canwest News Service on Tuesday.

Kivell and Duke colleague Daniel Schmitt have published their research, which was funded by the Canadian government’s NSERC agency and the University of Toronto, in the latest issue of the Proceedings of the National Academy of Sciences (PNAS).

Their paper argues that many scientists have traditionally – and erroneously – viewed all forearm-assisted walking styles among apes as the inheritance of a single evolutionary adaptation that occurred on the ground and then spread into trees some 70 million years ago.

Instead, Kivell and Schmitt say "fundamental" differences between the wrist bones of gorillas and chimpanzees suggest there were two separate evolutionary pathways for knuckle-walking, with our own primate ancestors probably developing this ability in the branches of trees before taking it to the forest floor.

"We looked at these (wrist) features in African apes and found that many of the features are commonly found in chimps and bonobos, but not in gorillas," Kivell said in an e-mail. "This is particularly surprising since gorillas start to knuckle-walk earlier in life and do more knuckle-walking on the ground as adults. The absence of these knuckle-walking features in gorillas suggests to us that gorillas and chimps are knuckle-walking in different ways, and that maybe this behaviour evolved independently in each group."

In a summary of their research, Schmitt states: "We have the most robust data I’ve ever seen on this. This model should cause everyone to re-evaluate what they’ve said before."

The authors note in the PNAS paper that uncertainty over the origins of "bipedalism" has prompted "ongoing and often rancourous debate" among scientists "since Darwin first discussed pathways of human evolution in The Descent of Man."

Kivell, an Edmonton native who now teaches at Germany’s Max Planck Institute for Evolutionary Anthropology, explained in her e-mail that, in short, the PNAS study proposes a solution to the 150-year-old question: "Did we come down from the trees to walk on two feet or were we already on the ground and then stood up to walk on two feet?"

The differences in wrist structure between gorillas – which use a stiff-armed, "columnar" knuckle walk – and the more flexible chimps are a strong indication, she said, that even these two close cousins of humans gained their mobility skills in distinct ways from ancestors inhabiting separate ecological niches.

For chimps and humans, she says, "these wrist features are better thought of as adaptations to moving around in the trees. And therefore, when we find these features in living or fossil humans, it suggests that humans evolved from an ape that lived in the trees, and then came down to the ground to walk on two feet."

This discovery, the study concludes, sheds important new light on "understanding the evolution of ape and human locomotion."

In 2007, a team of British scientists reached a similar conclusion about the tree-dwelling origins of primate bipedalism after studying the mechanics of orangutans as they moved about in their forest habitat in Sumatra.