Brain size and the ability to walk upright efficiently are two of the main attributes that separate humans from chimpanzees, gorillas and orangutans. Now researchers say learning to walk millions years ago was not without its trials.

A re-examination of anklebones from early hominids indicates their gaits were not as stable as previous research suggested — they were knock-kneed.

At least three different species, collectively known as therobust australopithecines — sometimes classified into their own genus, Paranthropus — lived about 2 million years ago.

Compared to us, the robust australopithecines had larger teeth, stronger chewing muscles, a stouter skull and a smaller brain.

But their feet were thought to be very much like ours, suggesting they had mastered bipedalism.

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

Human ancestors go back as far as 6 million years, fossils show. The genus Homo arose at least 1.8 million years ago, scientists believe, when a group of australopithecines likely evolved into Homo habilis, which had a larger brain than its ancestors but never grew larger than a 12-year-old child of today.

Schwartz and Dan Gebo from Northern Illinois University compared the ancient anklebones of various species with those of today's gorillas, chimps and people.

The results support the idea that bipedalism evolved only once.

"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 hominid lineages."

But that doesn't mean the transition was smooth.

"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."

The researchers have no idea how being knock-kneed might have been an advantage, but they plan to explore the question next.

The finding will be detailed in the April 2006 edition of the American Journal of Physical Anthropology.

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