by Michael Price
Some of our earliest ancestors possessed a rather unsteady stride due to subtle anatomical differences. They walked on the wild side as a result.
Gary Schwartz is an anthropologist at Arizona State University’s Institute of Human Origins. He and fellow anthropologist Dan Gebo from Northern Illinois University study the fossil anklebones of some early ancestors of modern humans.
During their latest study, the scientists looked at seven anklebones from a variety of early human ancestors found in eastern and southern Africa. They 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. Schwartz says that the finding confirms that these specimens were obligate bipeds. In simple terms, they most likely walked on two feet in a manner similar to how we walk today.
Although the samples were certainly from bipeds, the scientists also found structural differences in some of the anklebones. The differences indicate that these creatures 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.
“While looking at the specimens of robust australopithecines, we noticed that there were characteristics of the anklebone that would have affected its bipedal locomotion,” Schwartz explains. “We took a close look at the location where the shin bone rides across the anklebone. We found that the shin bones would have been angled inward.”
Robust australopithecines lived approximately 2 million years ago. They are distinct from modern humans in a number of ways. They had larger teeth, massive muscles for chewing, more heavily-built skulls, and a smaller brain size. But it was thought that their foot bones were not very different from our own, Schwartz says.
The new findings by Schwartz and Gebo suggest that was not the case at all. This contradicts the common wisdom that bipedalism was a rather stable, unwavering trait once it evolved in human ancestors.
“We know a lot about how teeth and facial structures changed over time,” Schwartz adds. “But it was thought that once our ancestors became bipedal, there were few, if any, changes in the ankle associated with walking on two legs. Now we know there were slightly different ways to be bipedal.”
Scientists have other questions as well. Did bipedalism evolve once and quickly became a dominant feature of hominids—humans, chimps, and their extinct ancestors? Or did bipedalism arise and evolve many times in different lineages?
Schwartz says the results from his 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 says. “It is unlikely that it could have evolved independently in multiple hominid 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 explains. “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 adds. 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 says. “Now we have evidence that they were interesting from the knee down as well.”
For more details, see the April 2006 edition of the American Journal of Physical Anthropology. Contact Gary Schwartz, Ph.D., Institute of Human Origins at 480.727.8684. Or send e-mail to Gary.Schwartz@asu.edu