Bipedal humans came down from the trees, not up from the ground (w/ Video)
August 10, 2009A 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 gorillas 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 evolutionary anthropology. "This model should cause everyone to re-evaluate what they've said before."
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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 Proceedings of the National Academy of Sciences.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 ancestor 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.
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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!
Is solid H20 anymore "advanced" than liquid H20? Is night more "advanced" than day? Is an atom with a positive charge more "advanced" than one with a negative charge? Is lead more "advanced" than hydrogen?
Every form of life, which is adapted to any niche is perfectly "advanced" for that niche. If a variation of that form of life or an entirely different form of life appears which is more efficient at reproducing in that environment, then the first form may disappear, but the second form will not be more "advanced"-- it will simply be better adapted to an enviroment, or just plain "lucky".
Humans may be more "complex" than single-celled forms, although that too is debatable since single-celled forms have been around for so much longer. But in no way does having five fingers instead of a hoof, or walking on two legs instead of four confer any degree of "advance-ness".
The only thing that can be said to be "advanced" in evolution is time. It "advances" relentlessly, apparently in one direction, over eons, and forms of life live, reproduce, pass away, and are replaced with other forms of life, not because they were not "advanced", but because conditions have changed-- continents have moved, objects have hit the earth, large-scale volcanism has occurred, or some other slow or rapid process has tipped the physical parameters of nature's scales in favor of one equally evolved species over another.
There are none so blind as he who will not see.
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."
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.
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?
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.