Fashionable bipedalism

By Razib Khan | May 19, 2011 1:12 pm

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?

CATEGORIZED UNDER: Genetics, Genomics, Human Evolution
  • Hmm

    Why are you so obsessed with height? You write about it so much. I think height is a weak advantage in a fight, if it is an advantage at all – in the same weight category, the taller guy tends to be the slimmer and weaker guy and thus looses to the shorter fellow. Fights are not resolved upright but in the ground. Another thing is, that there may be much stronger forces of selection than height in modern men, but there seems to be somehow a bias when publishing the results. Many studies have shown a zero correlation between height and modern reproductive success (or that average height guys have most offspring), but that doesn’t seem to interest anyone.

    Another thing is very strange – although the case for IQ and ethnicity and IQ and sex is much stronger, it doesn’t appear that way in wikipedia. Much more controversial height – IQ correlation is there. FIRST thing to do would be to correlate the IQ to skull/brain size, and see if after that there would be any remaining effect for the height. Actually IQ and skull and brain size DO correlate positively. One would imagine that also skull size and other body measurements would correlate positively.

    Razib, you and all the Americans seem to be obsessed with height, but remember that it may not be as universal as you seem to think…

  • Razib Khan

    Why are you so obsessed with height?

    don’t ever say i’m obsessed with anything again. it’s not your place to infer or intuit what i’m “obsessed” with. this comment is not an invitation to discuss whether it is appropriate for you to mind-read or impute. i’m giving you some slack because i know english isn’t your first language (i looked u up) and you may not be aware of the presumptive tone of your comment.

  • Robert


    Just don’t confuse a cringe with a twist and bending of the knees prior to the uncoiling of an explosive punch with the full power of the legs and torso behind it.

  • Hmm

    Another thing is, that the male gorilla (and orangutan) is adopted to fight huge males but is is actually _shorter_ than many/most humans. Thus, in species which have been battling the most, not height, but horizontal size and weight has been selected for. The reason, why so many humans are so tall and skinny really needs an explanation – they just look like evolutionary dead ends if you consider fighting alone. Maybe projectile weapons or walking more cost-efficiently have been positive sides of being tall. (And even in boxing, which doesn’t really simulate a fight, it is more important to have long arms to hit your opponent, than to have long legs to be higher.)

  • Razib Khan

    robert, oh shut up. the tall and large are not the ones who pull subtle surprises usually.

  • Hmm

    And sorry, didn’t mean to be/sound hostile. Although “looking me up” sounds a bit scary… 😀

  • Razib Khan


    finns often have impeccable english in lexicon and syntax, but you still sound weird sometimes. this generalizes to a lot of people with excellent command of english, but who are technically non-native speakers (and also, there are different regional variants of english, but my experience is that brit commenters have no issue with delivery, as opposed to weird lexicon or idom).

    anyway, knowing your background made me understand why you came off the way you did. i think your intent of style wasn’t quite what i received it as.

    carry on….

    (i agree with most of your critiques fyi)

  • juan

    @Hmmm Well, the advantage or disadvantage of height in modern combat sports is irrelevant since tribal dominance fights probably didn’t adhere to rigid weight classes. In the real world if you fight a taller man (or ape), he’s probably also heavier and stronger. Our modern image of the tall and lean fighter up against a short, stocky fighter is an artifact of our fairly rigid weight class systems.

  • Robert

    “Maybe projectile weapons or walking more cost-efficiently have been positive sides of being tall.”

    I’ve always kind of taken that as a given. Aren’t Upper Paleolithic skeletons on the whole taller and longer limbed than contemporary humans? Modern baseball pitchers and fast bowlers in cricket are tall. The extra leverage had to have come in handy on a daily basis when hurling an atlatl or taking small game with rocks. This seems like a far more reasonable explanation for natural selection favoring height than the infrequent occurrences of bashing other people about the head and shoulders in combat.

  • Dwight E. Howell


    Don’t write off those events to quickly because they could and did prevent a lot of people from having descendants.

  • RafeK

    I have trained in one or another f0rm of martial arts for most of my life, and my former sparring partners include a guy who is contender in the strike force lightweight division and several local champs. Overall size is an advantage in a fight there can be no doubt, a good big man beats a good small man, its not everything but it means allot, 40 pounds is worth a belt in a jujitsu match for instance in my experience.

    Smaller fighters do have some advantages a lower center of gravity makes it easier to employ and resist take downs and easier to land hard strikes at close range. On the other hand, longer limbed taller fighter have an advantage in controlling range, in landing strikes in general and in securing joint locks and strangulations. Weight is bigger advantage then height when controlling for body composition but they two are strong correlated its why there are weight classes in combat sports.

    The idea fights generally end on the ground is not true in my experience and over exaggerates the importance of grappling in street fights. I worked as bouncer for 3 years, I never once was taken down and when I went to the ground it was to secure control of someone I already had an advantage on with the knowledge I had back up.

    If I had been looking to maim or kill instead of control I would have used stomps and soccer kicks going to the ground is bad strategic decision one can not run away if things go sour or the guy’s buddies show up.

    In the street fights I have seen striking is always the primary strategy and while people do hit the ground fairly regularly grappling its not the primary tactic by which fights are decided. Grappling is a defensive strategy employed when your getting overwhelmed with strikes or used to hold a guy down or in compromised position while you hit him. The strategies of high level wrestlers and jujitsuka seen in MMA are not seen on the street.

  • Kosmatka

    Razib, just let me say that I find the contrast between your masterfully erudite blog posts and your brash—even antagonistic—comment posts to be highly entertaining. That’s at least half of why I come here. First, you tell it how it is. Then you tell it how it is. Please never stop yelling at people.

  • Andrew Planet

    From experience, my very toughened from use thumb fingernails excel for pruning soft tissue plant ends, say herbs or tinder and protrude in a clenched fist as an extension in a punch aiming to pierce someone’s eyes in self defense. Another attribute is the abrasive grip nails in general have on rock and other rough surfaces, useful with large tools. From personal discovery fingernails become bothersome in the creation of ceramics not permitting seamless smoothing out when the clay is still wet. With the advent of ceramics people must have had to file them down to produce clay derived objects. Nails file down excellent on rock, preferably flattish or granular sandy cement. There exists a slightly elongated snail species, whose name I cannot remember, that intentionally grinds down the outer exiting edge of it’s carapace on rough surfaces progressively throughout it’s life.

  • Hmm

    I remember reading something about lower body-upper body comparisons. At the time when “humans” were more polygamous, the leg length was lower in males. Thus, the males had a big upper body in the times they needed to fight. When matings changed to more monogamous ones, the sexual dimorphism got lower and upper body size of males got smaller.

    Street fight is not comparable to a fight in the nature. The surface is solid, hard, no one wants to go to the ground, there are many people watching, forming a ring around the fight etc. In the ancestral environment, there are no “borders” of a ring. Still almost all the drunken fights I have seen go to the ground almost immediately.

    Of course, much more important than the fighting skills would have been social skills, the number of friends etc. I doubt there would have been a lot of male-male fights, but more likely 5 vs. 1 fights etc.

    A perfect fighter would be something like a gorilla, short legs, strong upper body, big canine teeth etc. short neck. I think there is little any human fighter could do against a gorilla, even if they would be the same “weight class”.

  • dave chamberlin

    I think our sports culture has us a bit obsessed with the importance of size. Survival among combatants in our long and very violent past was more a matter of social intelligence and weapons technology than it ever was of size, strength, or height. We have traded considerable strength for hand/muscle dexterity when you compare humans to chimpanzees.
    I am distrustful of new and old theories of why for us two legs good, four legs bad. They all have a point but rarely are they good science, what I mean by that is they all smack of some guy with a big ego trying to get his name attached to a new theory.

  • Hmm

    One more thing in which people’s intuitions go wrong is the relation between bone length and power.

    Long bones -> speed.

    Short bones -> strength.

    This is actually very simple physics if you have ever used a leverage to do something.

    Bone length compares to the short end of the leverage; if you put too much of it, the strength decreases.

    Most people somehow think that having longer legs is somehow related to strength although in reality the relationship is inverse. (Strength vs. speed comparison is bit more complex as many short-distance runners use their shorter legs to create more speed by increasing the stepping interval and thus speed and strength are somewhat mixed in many contexts.) I just read a report about ultra-triathletes body proportions, and it was surprising to find that there was no relation between their performance and any body measurements, including height. The possible answer could be that it balances between power and strength. It appears that there is very big variance when you look at the marathon runners (some look like 165 cm and some around 190 cm), and perhaps the most successful long-distance runner of all time was Finnish Paavo Nurmi, 174 cm (which is neither that short or tall but still). Weight lifters are short for every weight class, and the maximum weight classes the height doesn’t go beyond 180 cm. The reason for this is that the power doesn’t increase as much as the bone length.

    Modern sports in the USA is something strange as everyone uses as much steroids as they can. Thus even the basket ball players in Finland are much skinnier than in the USA. Perhaps the use of steroid hormones gives more height advantage as they can develop bigger muscles that are not proportionate to their body type/natural hormone production. Before in most sports the most important thing was to have the right type muscles and natural hormone production whereas now the sportsmen could be castrated without much effect to their performance… :)

  • dave chamberlin

    @16 You won’t get much argument from me that the USA is a relatively stupid country compared to Finland, but I beg to differ that increased muscle size in american athletes is due to rampant steriod use. Virtually every sport over here is now accompanied by the encouragement of weight lifting in the off season and that is why our athletes are so much beefier than they used to be, or are in Finland today.

  • Hmm

    I didn’t want to bash the USA but to state the obvious; you cannot succeed in the very top level of sports without doping. There has been a wide discussion in Finland about the topic after a doping scandal (in skiing) and it is 100 % clear that none of the world’s top athletes could succeed without some forms of doping. The Finns, the Norwegians and the Russians all dope at the top level of skiing – and this is a sport where there are no large sums of money.

    Steroid use is less clear, but is is very clear you can see steroid use from the athlete’s body. 2 m tall guy never looks muscular with gym training, and most NBA players look like beefcakes. It is a common story that after a Finnish player goes to play in NHL, his body weight increases something like >10 kg in less than half a year (despite he has trained the whole of his life). There is no vitamin which makes such a big difference and it is clear that this is a sign of steroid use. Steroid use is the reason why so many athletes are having testicular cancer etc.

  • Justin Giancolate

    I agree with Hmm about steroid use. In the US I knew many people who did roids in high school; a few may have started in junior high. I don’t think this is uncommon as I’m from a relatively average place.

    This thread should have “Momma said knock you out” playing in it.


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