Our knuckle-walking forebears

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Recent research challenges the long-held belief about how the ancestors of early humans moved about the ground and opens the door to explore why man began to walk upright.

The new findings, published in the science journal, Nature, focuses on the discovery of a bone in the wrist of early humans.

Brian Richmond and David Strait, paleoanthropologists at George Washington University made the discovery while analyzing the fossils of Australopithecus anamensis and Australopithecus afarensis, commonly known as Lucy. That bone prevented the wrists from moving backward allowing the animal to walk leaning on its knuckles the way that gorillas and chimpanzees move.

"The reason that chimpanzees and gorillas have this very stiff wrist is that if you're walking on your knuckles, you don't want your wrist to collapse from the weight of your body," Richmond said.

Both species lived in Africa between 4.1 million and 3 million years ago. But Richmond and Strait found a bony ridge in the wrists of those fossils that modern gorillas and chimpanzees have. Those species were bipedal, meaning they walked upright, and that bone ridge is considered a vestigial feature, like the tailbone or appendix is in modern man.

That means "the ancestor that gave rise to the earliest humans was a knuckle- walker," Strait said.

Their findings are significant for two reasons. It's long been thought that when the ancestors of early humans came down from the trees that they walked upright. These findings eliminate that perspective, Strait said. Second, there is now anatomical evidence that humans and chimpanzees are more closely related than chimpanzees and gorillas.

Scientists have always espoused that the ancestors of early humans moved among the trees in an upright fashion -- shimming up trunks or moving from branch to branch. When forests began to shrivel and those animals headed to the ground, they continued moving in that upright posture, experts contended.

But these new findings suggests that isn't necessarily true -- that those ancestors of early humans came to the ground as knuckle walkers and stood upright for another reason.

"This goes to 'Why did upright walking evolve?' Upright walking is a feature that defines humans," Strait said. "We can eliminate the perspective now that they came down from the trees and were upright and focus on what led humans to stand upright. Were they standing upright to forage for new food sources or does standing upright free their hands to carry things or manipulate objects?"

The study is also significant because until now there was only molecular evidence that indicated humans and chimpanzees are closer relatives than chimpanzees and gorillas. Anatomical evidence also suggested that the knuckle-walking gorillas and chimpanzees were more closely related.

Other scientists have praised the research that resolves that long-standing debate. "We are more closely related to chimps than either of us is to gorillas, yet they share the same specialization that doesn't appear in our ancestry," said John Fleagle, a professor of anatomical science at State University of New York at Stony Brook.

"There is now anatomical evidence to buttress that claim," Strait said, about the relationship between humans and chimpanzees.

"If the anatomical analyses are shown to be correct, I think it's a hugely important breakthrough in our understanding of early bipedalism," said Craig Stanford, a biological anthropologist at the University of Southern California.