Laelaps

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A male western gorilla (Gorilla gorilla), photographed at the Bronx Zoo.

ResearchBlogging.org The origin of human bipedalism has long been a hot topic among paleoanthropologists. At the very least it is seen as something of a marker for the emergence of the first hominin, yet it remains unclear whether the earliest hominins evolved from a terrestrial, knuckle-walking ancestor or a more arboreal ape. A common interpretation is that since our closest living relatives, gorillas and chimpanzees, are both knuckle-walkers then the first hominins, too, evolved from a knuckle-walking ancestor. As Tracy Kivell and Daniel Schmitt explain in a new PNAS paper, though, this idea may overlook subtle differences between chimpanzees and gorillas that may help us understand the evolution of the earliest hominins.

Gorillas are physically larger than chimpanzees, so it might be expected that they would have more rigid wrists that would help stabilize them as they walked around on their knuckles. This is not what Kivell and Schmitt found. Not only did gorillas have a much greater range of wrist motion than chimpanzees, but it was the chimpanzees that had adaptations in their wrists to increase stability. Despite being more terrestrial and knuckle-walking on the ground more often gorillas actually showed fewer “classic” knuckle-walking adaptations in their wrist bones than chimpanzees did!

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The wrist postures of a chimpanzee (left, extended posture) and a gorilla (right, columnar posture) compared. From Kivell and Schmitt (2009).

As Kivell and Schmitt note, gorillas and chimpanzees are not the same when it comes to knuckle-walking. It seems that gorillas have more wrist flexibility because they hold their lower arms and wrists in a columnar fashion which gives them the support they need. Chimpanzees, on the other hand, bend their wrists backwards in an “extended” position when supporting themselves on their knuckles, and their range of motion is restricted by the shape of the wrist bones. Why do gorillas and chimpanzees differ in this way?

Surprisingly, some of the “knuckle-walking features” of the chimpanzee wrist are seen in arboreal, quadrupedal primates. It may be, then, that the traits of the chimpanzee wrist commonly associated with knuckle-walking are actually adaptations to climbing in the trees. Perhaps, the authors hypothesize, this is because the primates that exhibit this wrist-limiting morphology extend their wrists to grasp with their hands while moving on all fours in trees. If they are correct the wrist morphology of chimpanzees may have more to do with what the apes do in the trees than what they do on the ground.

There are other traits of the wrist and lower arm associated with knuckle-walking the authors did not investigate, but it is significant that 1) not all knuckle-walking apes had all the “knuckle-walking traits”, and 2) at least some of those traits are probably more associated with life in the trees than on the ground. The authors state;

The results of this study show that researchers need to reevaluate all posited knuckle-walking features and reconsider their efficacy as indicators of knuckle-walking behavior in extant and extinct primates. In this context, the absence of several posited knuckle-walking features in extant knuckle-walkers (and the presence of some of these features in nonknuckle-walkers) makes it difficult to argue that there is unambiguous evidence that bipedalism evolved from a terrestrial 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 knucklewalking behavior are in fact evidence of arboreality and not terrestriality.

It cannot be assumed that the earliest hominins evolved from an ancestor that knuckle-walked on the ground, and the differences in the knuckle-walking traits in gorillas and chimpanzees suggests that it could have independently evolved in each lineage. At the very least the authors have indicated that knuckle-walking in chimpanzees and gorillas cannot be considered to be identical, and it is very interesting that some traits thought to indicate life on the ground may in fact be adaptations to life in the trees. More fossil evidence and comparative studies will be required to test the hypotheses the authors propose, but this paper provides a good starting point to reconsider how the earliest hominins evolved.

See Afarensis for another take.

Kivell TL, & Schmitt D (2009). Independent evolution of knuckle-walking in African apes shows that humans did not evolve from a knuckle-walking ancestor. Proceedings of the National Academy of Sciences of the United States of America PMID: 19667206

Comments

  1. #1 John
    August 14, 2009

    For some reason, perhaps misreading something, I had thought that bonobos had recently been determined to be our closest relative. IIRC, they stand erect substantially more often than chimpanzees as well. It would be interesting to see this study done with them.

    Then again, maybe the premise of this post is totally wrong.

  2. #2 mo
    August 14, 2009

    No, chimpanzees and bonobos are each others closest relative, this means our closests relatives are both the chimpanzees and bonobos, which makes us a side branch in the present chmimpanzee/bonobo/human clade ;-)

    bonobos probably have wrists like chimpanzees.

  3. #3 Sto
    August 14, 2009

    I wonder how this study relates to the “aquatic ape” hypothesis, and whether researchers take that hypothesis seriously at all.
    http://www.ted.com/talks/elaine_morgan_says_we_evolved_from_aquatic_apes.html

  4. #4 Will Baird
    August 14, 2009

    sto #3:

    The Soggy Monkey Theory (or AAH) is not taken seriously at all. Elaine Morgan has been very good at pitching to the public, but not made the case at all for the scientific community.

    The conventional wisdom of the mid to late nineties – we were brachiators that adapted to life on the ground – is just confirmed by this study. This study /does/ make me wonder why knuckle walking developed twice from bipedalism though.

  5. #5 james beasley
    August 14, 2009

    or it may show that there isn’t very much at all except DNA that places us in the same family !

  6. #6 afarensis, FCD
    August 14, 2009

    Sto, most paleoanthropologists pay no attention whatsoever to the aquatic ape hypothesis some of the reasons why can be found here.

    Brian, considered in light of DeSilva’s recent paper (Hawks mentioned it here) this is a really interesting paper. I was going to go into more detail but I was having some trouble pulling pictures from the article (seems to have something to do with the version of adobe I was using). IMHO, there has been some typological thinking on knuckle-walking and more research on the biomechanics and morphology of knuckle-walking in chimps and gorillas, as well as fist-walking in orangs, is much needed.

  7. #7 BdN
    August 14, 2009

    @Sto

    There have been recent discussions about this on scienceblogs. See here and here.

    @John

    For some reason, perhaps misreading something, I had thought that bonobos had recently been determined to be our closest relative. IIRC, they stand erect substantially more often than chimpanzees as well. It would be interesting to see this study done with them.

    Then again, maybe the premise of this post is totally wrong.

    To add to mo’s answer, bonobos ARE chimpanzees but in daily language chimpanzee tends to be interpreted as meaning “the common chimpanzee”. They both belong to Pan and, accordingly, bonobos are indeed part of the study.

    @Will Baird

    This study /does/ make me wonder why knuckle walking developed twice from bipedalism though.

    I don’t know where you find that knuckle-walking developed from bipedalism. And surely not twice!

    @james

    or it may show that there isn’t very much at all except DNA that places us in the same family !

    What is this even supposed to mean ?

  8. #8 José
    August 16, 2009

    I could be wrong on this, but aren’t gibbons a fully bipedal ape? I can’t seem to find anywhere on the net where non-swinging gibbon locomotion is described, so I’m basing this on a few nature shows I’ve seen and a YouTube video of a little punk annoying a dog – http://www.youtube.com/watch?v=91GA8YyGp5I, so my observations are highly scientific. Anyway, it would add weight to the idea that tree swinging is a perfectly good way to evolve bipedalism.

  9. #9 José
    August 17, 2009

    Darn it. My comma was included in the link. I’ll try again.
    http://www.youtube.com/watch?v=91GA8YyGp5I

  10. #11 orjin krem
    April 7, 2011

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  11. #12 ertyufg
    April 7, 2011

    Orjin Krem 100 ML ve 25 ML set olarak satılmaktadır.
    Orjin Krem iki kutu bir arada bir kutu içerisindedir holagram bulunmaktadır.
    Orjin Krem alındıktan sonra bandrollü sayesinde s

  12. #13 nhaye
    April 7, 2011

    This study /does/ make me wonder why knuckle walking developed twice from bipedalism though.

  13. #14 canlı maç izle
    March 14, 2012

    he exaggeraion among atheistic media and pagan scientists were so much that the gullibles started celebrating the percieved defeat of the Holy Bible. But now the world of science is even questioning some fundamental aspects of the theory of Darwinism in view o

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