Fashionable bipedalism

Gene ExpressionBy Razib KhanMay 19, 2011 10:12 PM


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There's a new story blowing up in the media about the origins of bipedalism through male-male competition. The hook is good enough that the headlines write themselves. For example, io9 has a sober and skeptical review of the paper, but the title is naturally going to be more excitement inducing: Did early humans start standing upright so they could beat each other up? And I'm not going to get into what British tabloids are saying about this. This is a sexy story, I can't really blame anyone. The original research is in PLoS ONE, so read it yourself, The Advantage of Standing Up to Fight and the Evolution of Habitual Bipedalism in Hominins. The conclusion is short and sweet: These results indicate that bipedal posture does provide a performance advantage for striking with the forelimbs. The mating systems of great apes are characterized by intense male-male competition in which conflict is resolved through force or the threat of force. Great apes often fight from bipedal posture, striking with both the fore- and hindlimbs. These observations, plus the findings of this study, suggest that sexual selection contributed to the evolution of habitual bipedalism in hominins. This is sexy because it involves fighting and sexual selection. War & love. This is a great recipe for a model which appeals to our narrative sense of a "good story." The guts of the results seem mostly to deal with biomechanics. Yes, a bipedal posture does allow one to strike with more force. As a shorter guy than the mean (5 feet 8 inches) who has gotten into physical fights I can attest to the importance of height differentials, it's oh so much better to swing at someone at your height or lower (I grew earlier than my peers, so I didn't get below-the-mean until 16). If you go up against someone who is taller it always feels like you have to be in a defensive dodge crouch (though thankfully if you come at a taller guy fast and furious and they get surprised they can go into a cringe pose which ameliorates the height differential). But naturally some large questions come to mine in regards to the hypothesis posited in the paper above. All the pieces fit plausibly. But most plausible models really don't end up to be correct. The problem when you have a domain like paleoanthropology with a lot of models which are plausible, and have some support, is that there tends to be a bias toward attention and selection of models which align well with culture-specific or human universal cognitive biases. By the former I mean that after the 1960s it seems pretty straightforward that there's going to be a more natural constituency for the "sexy ape" thesis proposed by those who argue that our past is most well modeled by bonobo chimpanzees. In contrast in the early 20th century the type of competition heavy on a "red in tooth and claw" interpretation of Darwinism would lead to attention to what later emerged as "man the hunter." So you have a set of models, x, y, z, .... and you just pick out the one which suits you. This gets instantiated in "expert" shopping. Just pick out the expert who gives the appropriate support to what your own preferred hypothesis is. This gets trivially easy when you construct a Google query to validate your opinion. It is often pretty obvious who is doing this in the comments of this weblog, I just go and construct the Google query which I think they are likely to have performed, and boom, it turns out that the exact same citations are at the top of the stack. The second issue are topics of interest which transcend a time. Sex and war are going to be of human interest, period. Even if the models are "politically incorrect," people are going to be attracted to some theories because of deep rooted cognitive biases. Like the evolution of language the topic of bipedalism is very old, and also still active. Hypotheses bubble up like a thousand flowers. But for various reasons it doesn't seem like the natural iterative process of science has been working to make the set of possibilities tractable. I don't want to seem like a disciplinary imperialist, but I think both language and bipedialism as universal competencies need the insight of genetics to prune the set of plausible possibilities. By this, I mean that we need to understand which substitutions at which loci occurred over evolutionary time to get a sense of the arc of the trait's genetic architecture. I think that the glimmers from genetics has at least pushed the needle of likelihood toward the proposition that basic language competency predates the emergence of behaviorally modern humans (Neandertals seem to have had the same variants which we had presumed to be necessary preconditions for language faculty). Back to this specific hypothesis, that a few issues that do jump out at me. This paper has sexual selection as the dynamic driving the male-male competition. Sexual selection occurs, and it has been woefully understudied. But it also gives you a one-size-fits-all explanation for all sorts of phenomena. It is a cheap theory which can offer itself up whenever other possibilities are closed off. Unfortunately from what I have seen in the comments on this weblog too often sexual selection hypotheses get taken as definitive as Newtonian mechanics by the public. That is, readers will often assert that "sexual selection explains X," instead of "sexual selection has been posited as an explanation for X." In some cases sexual selection is a pretty obvious explanation. Elephant seals and gorillas have enormous inter-sex differences in size, and, a huge reproductive skew (a few males overwhelm the next generation in terms of ancestry). So you need a major reproductive advantage for some males, as well as a good indication of long term persistent trait differences as signals of the dynamic. I honestly don't see much evidence of this in human beings. We exhibit some sex difference in size, unlike gibbons, but not nearly as much as gorillas, or even common chimpanzees. Additionally, the evolutionary genetics of sex differences need to be considered. Because males and females share most of their genome content, with only the sex chromosomes differentiating the two classes, the evolution of differential traits requires a lot more time and persistent selection than non-sex differential traits (i.e., you need modifier genes to come and selectively mask the tendency for a change on one gene to impact both sexes in the same direction). Extremely rapid and punctuated instances of sexual selection such as "Fisherian runaway" are likely to be epiphenomenal (that is, they would lack staying power). A second issue which jumps out is the reliance are a lot of moving parts in terms of the correlations. In particular, the sequence of logic whereby taller men can bring more force to conflicts, taller men are preferred sexually, taller men tend to be endowed with higher I.Q., etc. The key is always to remember that a set of correlations can all be true, but that doesn't mean that they cohere into a whole integrated system. I'm basically saying that correlations aren't transitive. Concretely, if A correlates with B and B correlates with C, that does not entail that A correlates with C, though it may. That means that I get somewhat cautious when seeing interlocking sets of correlations; they may cohere together, and so are possible, but they also may not. Additionally, there are some details which need to be examined closely. In the paper the author correctly notes that tall individuals tend to have higher I.Q.s. This has lead to speculation on this weblog from commenters about pleiotropy between genes for height and intelligence, or height and I.Q. as independent signals for mutational load. There's a problem with this: the height-I.Q. correlation disappears within families. In other words, it is likely that the correlation is driven by other factors such as assortative mating across these two favored traits. I don't think this detail necessarily knocks down the logic in the paper, but my point is that with so many moving pieces it is easy to have weak or even misleading premises. I think the "problem" of bipedalism will be solved within the next two generations. But I probably wouldn't bet it will be solved with the next 10 years. And I think I tend to have an optimistic bias, so who knows?

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