August 11, 2009

Humans May Not Have Evolved From Knuckle-Dragging Primates

A new study discovered that the wrist bones of some primate species challenge the theory that humans evolved their two-legged upright walking style from a knuckle-walking ancestor.

"We have the most robust data I've ever seen on this topic," said Daniel Schmitt, a Duke University associate professor of evolutionary anthropology. "This model should cause everyone to re-evaluate what they've said before."

The findings are published in this week's journal Proceedings of the National Academy of Sciences.

Tracy Kivell led the research with her team and says the debate over the origins of human bipedalism began during Charles Darwin's lifetime and continues to this day.

Researchers say the debate is commonly divided into two competing models.

One model "envisions the pre-human ancestor as a terrestrial knuckle-walker, a behavior frequently used by our closest living relatives, the African apes," they wrote in the PNAS report.

However, the second model traces our two-legged walking to earlier tree-climbing, a mode of locomotion that is used by all living apes.

The people who support the knuckle-walking origin think humans and African apes evolved from a common knuckle walking ancestor. They believe that connection is still shown in wrist and hand bone features shared by African apes and by fossil and living humans.

Kivell and her researchers found other data when comparing juvenile and adult wrist bones of more than 100 chimps and bonobos, our closest living primate kin, with those of gorillas.

Two key features associated with knuckle walking were present in only 6 percent of the gorilla specimens she studied. Yet, she found them in 96 percent of adult chimpanzees and 76 percent of bonobos.

The study examined specimens from 91 gorillas, 104 chimps and 43 bonobos.

Kivell noted one explanation for the absence of these features in gorillas is that they knuckle-walk in a fundamentally different way from chimps and bonobos.

Gorillas stride with their arms and wrists extended straight down and locked in what Kivell called "columnar" stances that resemble how elephants walk.

But compare that with chimps and bonobos walk more flexibly, "with their wrists in a bent position as opposed to being stacked-up," she said. "And with their wrists in bent positions there will be more stresses at those joints."

She noted that chimp and bonobo wrists have special features that gorillas don't have like ridges and concavities that serve as "bony stops" to keep their wrists from over-bending.

"When we first got together to work on this study that (difference) really jumped out in living color," Schmitt said.

"Then we sat down together and asked: 'What are the differences between them?' Schmitt said. "The answer is that chimps and bonobos spend a lot of time in the trees. And gorillas do not."

Chimpanzees and bonobos have a more extended-wrist way of knuckle-walking which gives them greater balance on branches, the researchers concluded.

In contrast, gorillas' "columnar" style of knuckle-walking is consistent with ground transport.

Indeed, "from what we know about knuckle-walking among wild populations, gorillas and adult chimpanzees will both knuckle-walk about 85 percent of the time that they're moving," Kivell said. "But chimpanzees and bonobos are more arboreal than gorillas. So they're doing a lot more of it in the trees."

Kivell thinks this points to an independent evolution of knuckle-walking behavior in the two African ape lineages.

Altogether, the evidence leans against the idea that our own bipedalism evolved from a knuckle-walking ancestor.

"Instead, our data support the opposite notion, that features of the hand and wrist found in the human fossil record that have traditionally been treated as indicators of knuckle-walking behavior in general are in fact evidence of arboreality."

In other words, a long-ago ancestor species that spent its time in the trees moved to the ground and began walking upright.


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