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 at www3.interscience.wiley.com/cgi-bin/abstract/112169244.
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."