Our Knuckle-Walking Ancestor?

One of the, more many, interesting debates in paleoanthropology concerns the question of whether our human ancestors went through a knuckle-walking stage at some point in our history. In the recent past, the idea that human ancestors went through a knuckle-walking stage is most closely associated with the work of Richmond and Strait.

In a number of papers Richmond and Strait have argued that knuckle-walking is a derived feature of the African ape/human clade and that humans evolved from a knuckle-walking ancestor. There are a number of morphological traits that have been associated with knuckle-walking (you can find an overview of this morphology here) and a recent paper in PNAS takes a new look at the subject. The new paper, by Kivell and Schmitt, looks at traits in the capitate, scaphoid, and hamate in a wide variety of primate species – both terrestrial and arboreal. The included both males and females, as well as adult and juvenile specimens. The researchers looked specifically at the scaphoid dorsal concavity and scaphoid beak, the capitate distal concavity and dorsal ridge, the waisting of the capitate, the hamate dorsal ridge and the hamate distal concavity. The distribution of these traits across species proved to be interesting. Gorillas, the most terrestrial of the primates, had the fewest traits specifically for knuckle-walking:

This research shows that none of the carpal features discussed here that have traditionally been used to diagnose knuckle-walking can be considered clear functional adaptations to knuckle-walking behavior in all African apes. Contrary to functional predictions, the pattern of development, expression, and frequency of the putative knuckle-walking features listed in Table 1 are not the same in all African apes; they are not more frequent or accentuated in gorillas and are not correlated with increased knuckle-walking behavior or body size. The difference between Pan and Gorilla in the presence or absence of carpal bone morphology thought to limit wrist motion is confirmed by reported patterns of joint flexibility in the 2 genera. Gorilla have a much larger range of wrist extension (58°) (40) compared to that of P. troglodytes (30–42°) (19, 40). It remains challenging to explain this difference if it is assumed that knuckle-walking in Pan and Gorilla is biomechanically similar. The morphological and range of motion data demand a new perspective on knuckle-walking that leads to a reevaluation of long established models of human evolution.

Kivell and Schmitt argue that instead, what we are seeing reflects habitual loading of the wrist in an extended posture. They argue that gorillas and chimps display two biomechanically different types of knuckle-walking – something I find intriguing. This leads Kivell and Schmitt to propose that knuckle-walking evolved independently in gorillas and chimps.

For me, two things stand out. First, the authors looked at variation of the traits within species as well as across species – although Kivell and Schmitt overstate their case on this (Richmond and Strait’s 2000 Nature article, for example, looked at within and among species variation). Second, the morphology was examined in juveniles as well as adults.

Am I convinced? Not really. Kivell and Schmitt examined only a few of the traits that have been used to support the hypothesis that humans had a knuckle walking ancestor. All in all, an interesting and suggestive paper.

8 Responses

  1. Personally, I’ve looked over the arguments favoring the knuckle-walking origin and the arboreal origin, and would reject them both in favor of Filler’s Upright Ape Hypothesis. My arguments are detailed in my discussion of this paper here.

    They expand on, and modify (based on the Thorpe/Crompton team’s arguments) some comments I made in an earlier post at your old blog at SB (but my link points to the copy here).

    Please feel free to respond, in either comment section or a new post.

  2. […] Also check out Laelaps and Afarensis for […]

  3. You may also be interested to know that Dr. Filler has been kind enough to provide a guest post on the subject:

    Diagonal Postures & The Descent from Human to Ape

  4. I checked out Filler’s post on AK’s blog… as far as

    To get from the invertebrate body plan to the vertebrate body plan, you have to flip dorsal for ventral and anterior for posterior. An adaptationist argues that there were selective adaptive pressures that gradually turned the proto-vertebrates a few degrees at a time over millions of years. The morphogentic mutationist realizes that this is completely ridiculous…

    And no further. With lame strawman arguments used to attack ‘adaptationism’, it’s not going anywhere good.

  5. So what, exactly, would an adaptationist use to explain this “inversion”, assuming that’s what happened? I can see all sorts of explanations involving homeotic mutations, but if ‘adaptationism’ rules out such mutations as contributing to evolution, which as far as I can tell the “purest” form(s) of ‘adaptationism’ do, how do they explain it?

    I’m working on a post on the subject of “homeotic mutationism”, though it’ll probably be a while (I’m still mostly focused on neurology for now. My research so far suggests (but doesn’t prove, pending more reading) that at least some proponents of “mutationism” think there are “pure adaptationists” who take this sort of position.

    Rather than dismiss Dr. Filler’s post because of what you see as a “straw man”, why not write a comment with your arguments laid out in the comments section there. I’ve certainly seen arguments (not in anything peer-reviewed, yet) against the contribution of homeotic mutations to evolution. So I’m skeptical that Dr. Filler’s reactions to “adaptationism” are really as much setting up straw men as all that. Just because your own position is somewhat flexible (if it is) doesn’t mean there aren’t “adaptationists” who are inflexible on the subject.

  6. AK – I have to agree with Johhn on this one. That is a pretty simplistic portrayal of the adaptionist program.

  7. So what is the “adaptionist program“?

    Can somebody point me to some recent peer-reviewed discussion(s) written by (one/some of) the adaptationists you’re referring to that provides a non-simplistic” portrayal?

    I’d assume (to start with) that most adaptationists accept the possibility of homeotic mutations. This includes, clearly the subsequent possibiity of a mutation that increases fitness (although the probability is very low).

    But I’ll point out that while Mayr and Gould, for instance, each recognized the validity of the mechanisms proposed by the other (even if they disagreed over how often they occured), many of their followers have gone overboard by denying even the possibility that the mechanisms of their opposition could occur (or have any effect, or whatever).

    As for dismissing the “portrayal” as “simplistic“, I would interpret this portrayal as an invitation to debate, given the existence of a comments section, or the possibility of anybody disagreeing making their own statement on their own blog.

    Personally, having done some research, I (tentatively) find the “adaptionist program itself rather simplistic, regardless of how correct the accusations of “employing adaptationist fallacies in supporting socially regressive views of biological determinism. [Wiki]” OTOH, I’m a big fan of Steven Pinker’s, who is described (in the same Wiki page) as an adaptationist. I’ve read most, if not all, of Pinker’s published books, and I’ve never seen anything that could suggest he limits the evolution of a “Language Instinct” to the gradualist mechanism caricatured in Dr. Filler’s post. There does seem, IMO, to be a tacit assumption built into his language that the mechanism was gradualist. Perhps he makes it stronger than that, and I simply ignored such language because it didn’t make sense to me.

    Certainly, language, like the backbone modifications Dr. Filler discusses, could have arisen through a homeotic mutation to several neural development mechanisms (at once in one mutation). I suppose a “touchstone” for a simplistic adaptationist would be if they denied the very possibility of such an origin for language. A more reasonable position would be that such a mutation could occur, but it would die out unless it was adaptive. Thus, the argument between adaptationists and mutationists ends up looking like the one between two men in a dark room with an elephant, arguing whether it’s like a snake or a pillar.

    But, are you sure (either of you or anybody else) that there isn’t a large body of “adaptationists” out there who would/do take the sort of simplistic position caricatured in Dr. Filler’s post?

  8. I would start here

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