It seems some of our earliest ancestors possessed a rather unsteady stride due to subtle anatomical differences. Schwartz and Gebo's findings will be published in the April 2006 edition of the American Journal of Physical Anthropology, but the article is available online here.

Schwartz and Gebo looked at seven anklebones from a variety of early human ancestors found in eastern and southern Africa and compared them to samples taken from modern humans, chimpanzees and gorillas. The research led them to two significant conclusions.

First, certain ancestral anklebones that were thought by some to be "half ape, half human" were found to be much more similar to humans, confirming these specimens were obligate bipeds--in other words, they most likely walked on two feet in a manner similar to how we walk today.

The second discovery was that although the samples were certainly from bipeds, there were structural differences in some of the anklebones that indicated they would have walked a little differently than modern humans. Specifically, an ancestral species commonly referred to as robust australopithecines appear to have been a little knock-kneed.

"We noticed that in the specimens of robust australopithecines, there were characteristics of the anklebone that would have affected its bipedal locomotion," Schwartz said. "By looking at the location where the shin bone rides across the anklebone, we found that the shin bones would have been angled inward."

Robust australopithecines, which lived approximately two million years ago, are distinct from modern humans in a number of ways, including larger teeth, more heavily-built skulls, massive muscles for chewing and smaller brain size, but it was thought that their foot bones were not very different from our own, Schwartz said. Schwartz and Gebo's findings suggest that was not the case at all, contradicting the common wisdom that bipedalism was a rather stable, unwavering trait once it evolved in human ancestors.

"While we know a lot about how teeth and facial structures changed over time, it was thought that once our ancestors became bipedal, there were few, if any, changes in the ankle associated with walking on two legs," Schwartz said. "Now we know there were slightly different ways to be bipedal."

Another question is whether bipedalism evolved once and quickly became a dominant feature of hominins--humans, chimps and their extinct ancestors--or if it arose many times in different lineages. Schwartz said the results from the new research supports the idea that it arose only once in an ancestral species.

"The skeletal modifications associated with bipedalism represent a phenomenal reorganization of one's anatomy," Schwartz said. "It is unlikely that it could have evolved independently in multiple hominin lineages."

Still, even if it only evolved once, the new research suggests there was a lot of tinkering within subsequent lineages.

"Think of the robust australopithecines as having developed a variation on the theme of bipedalism," Schwartz said. "Undoubtedly, it was not as efficient as the way we walk today, but it might have conferred some other evolutionary advantages."

Just what those advantages might have been remains a big unknown, Schwartz said, but finding out is the next big step for his research.

"Scientists have long been fascinated with robust australopithecines because they were so distinctive from the neck up," Schwartz said. "Now we have evidence that they were interesting from the knee down as well."

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