Bipedal humans came down from the trees, not up from the ground (w/ Video)

A detailed examination of the wrist bones of several primate species challenges the notion that humans evolved their two-legged upright walking style from a knuckle-walking ancestor.

The same lines of evidence also suggest that knuckle-walking evolved at least two different times, making distinct from chimpanzees and bonobos.

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

Upright human ancestors came down from trees, not up from ground, according to this interview with Tracy Kivell of Duke University. Credit: Duke University

A report on the findings will appear online during the week of Aug. 10 in the research journal .

The research, led by post-doctoral research associate Tracy Kivell, was supported by the Natural Sciences and Engineering Research Council in her native Canada, General Motors' Women in Science and Mathematics, and the University of Toronto, where Kivell did her Ph.D. work.

The debate over the origins of human bipedalism began during Charles Darwin's lifetime and continues vigorously to this day, commonly dividing into two competing models, the researchers explained.

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

Supporters of the knuckle-walking origin think we and African apes evolved from a common knuckle walking ancestor. That connection, they contend, is still evident in wrist and hand bone features shared by African apes and by fossil and living humans.

But Kivell found otherwise when she began comparing juvenile and adult wrist bones of more than 100 chimps and bonobos, our closest living primate kin, with those of gorillas.

Significantly, two key features associated with knuckle walking were present in only 6 percent of the gorilla specimens she studied. But she found them in 96 percent of adult chimpanzees and 76 percent of bonobos. In all, she looked at specimens from 91 gorillas, 104 chimps and 43 bonobos.

Kivell and Schmitt suggested that 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. By contrast, 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."

As a result, chimp and bonobo wrists have special features that gorillas lack -- little ridges and concavities that serve as "bony stops" to keep their wrists from over-bending. Gorillas don't need those, she added.

"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 added stability 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 and Schmitt think this suggests independent evolution of knuckle-walking behavior in the two African ape lineages.

Some scientists point to features in the human anatomy as our own vestiges of a knuckle-walking ancestry. One notable example is the fusion a two wrist bones that could provide us extra stability, a feature we share with gorillas, chimps and bonobos.

But some lemurs have that feature too, and they do a variety of different movements in the trees but do not knuckle-walk, Kivell said.

Altogether, the evidence leans against the idea that our own bipedalism evolved from a knuckle-walking ancestor, the pair wrote. "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 species that spent its time in the trees moved to the ground and began walking upright.

There are no fossils from the time of this transition, which likely occurred about seven million years ago, Kivell and Schmitt said. But none of the later fossils considered to be on the direct human line were knuckle-walkers.

Source: Duke University (news : web)


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Citation: Bipedal humans came down from the trees, not up from the ground (w/ Video) (2009, August 10) retrieved 25 June 2019 from https://phys.org/news/2009-08-bipedal-humans-trees-ground-video.html
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Aug 10, 2009
This all sounds very interesting but since no one has ever been able to prove that any 'less evolved' species has EVER evolved into a 'more evolved/advanced' species, why are we still discussing any of this in the context of humans evolving from apes or anything else. We've created lots of interesting graphics and charts but none of that has anything to do with provable reality.



Why don't we actually analyse the data & the facts we have to develop a realistic Theory of how Humans developed here on Earth based on actual evidence instead of Theory and Mythology?

Maybe it is finally time to apply Scientific Method to this branch of Science.

Thanks!

Aug 11, 2009
Gee, somehow I'm reminded of the patent office head that resigned because there could not possibly be anything of significance left to be invented.

Apparently evolution, speciation, etc is finalized knowledge like, for example, the firmly established understanding of human sensory receptors... oops, the itch receptor was just 'discovered'.

"Certainty of understanding inevitably demonstrates ignorance of reality."

RFC
Aug 11, 2009
What is it with these stupid aphorisms people end their posts with? And they invariably hide some passive aggressive slam against another poster.

To those of you who do this, seriously, if you did that in real life conversation, you'd get your ass kicked. There's a reason for that. Oh, and if you DON'T put in the stupid aphorism at the end, I'm a lot more likely to pay attention to what you written in the first place.


Aug 11, 2009
I would like to hear an elaboration of the comparative biology/morphological explanation that leads them to believe the ancestral and modern wrist/hand features are evolved from arboreal features...

Why do these fused carpals have to have come from an arboreal ancestor? Could tool use or swimming or other adaptive activities have lead to this? And wouldn't hip or feet physiology be a better indicator of where upright walking came from anyway?

Aug 12, 2009
I kinda agree. I checked my family tree. However, I suspect that many Dems descend from knuckle-draggers.

Aug 14, 2009
RFC I suspect in real life you've lost a lot of fights - both intellectual and physical. I certainly doubt that you'd pay more attention in person. Probably one of the kids that slept through my class.

Aug 19, 2009
SDMike2, you're berating of others in this forum is tedious and if you ARE actually a teacher at a college you're probably one of the worst teachers to ever exist. professors like you that berate and belittle students who question your "almighty and superior" knowledge don't get ahead in class because you single them out for not agreeing. i thought the first thing in an intellectual conversation about sciences was to be respectful of everyone's ideas brought forth in the discussion. you however, tear down others ideas to make your already inflated ego even more bloated and i feel it's about ready to rupture.

now. as for the debate at hand, i agree with defunctdiety in the assertion that possibly the fused carpals did not necessarily have to come from arboreal ancestry. there could have been numerous activities that lead to the modern wrist/hand configuration we have. hip or foot physiology might be a very viable way to indicate where upright walking came from. but i will say however that despite all of these wonderful theories from defunctdiety it seems (for the time being) that it would have come from an arboreal ancestry. hopefully we'll find out soon enough!

again, SDMike2, grow up. being a professor, if that's even true in your case means you should show some sort of maturity. and you clearly do not possess ANY maturity. whatever "learning institution" you supposedly work for should drag you out back and put you on exhibit. it would be an interesting one. the never evolving mind of a ego maniac.

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