This just in:
ancestral humans adopted bipedal posture so that males could fight with the strength of their forelimbs, making their punches more dangerous.
OK. I actually thought I knew the range of arguments for bipedalism. I guess I appreciate having something novel to think about?
The research described in news reports I read was published in PLoS ONE, an open access journal, which means you, dear reader, can review the full paper for yourself. Written by biologist Daniel Carrier, it reports on contemporary experiments he conducted with 15 men with experience in boxing or the martial arts that show that
bipedal posture does provide a performance advantage for striking with the forelimbs.
The “bipedal” posture is, one hopes, obvious.
Here’s the description of the posture the modern test subjects adopted to provide an analogue of our long distant ancestors, pre-bipedalism:
In the ‘‘quadrupedal’’ posture, subjects supported themselves on their knees and one arm (i.e., tripedally), such that their trunk was oriented horizontally (pronograde), while they struck with the other arm.
So far, so good. Standing and punching works better than crouching awkwardly and punching.
How do we get from this biomechanical demonstration that punching is easier if you aren’t using your arms for locomotion to human evolution?
Simple: Fight Club!
I know: the first rule of fight club is that we don’t talk about fight club. But this paper goes into great detail in imagining the primordial pre-human horde with quadrupedal males trying to fight for their right to mate with, one assumes, the prettiest darned girl-things possible.
The Salt Lake Tribune managed to get Harvard biological anthropologist Dan Lieberman, to comment on the article, so I will let Lieberman make the appreciative response:
“He’s making the argument that there is a performance advantage for a biped in hitting other people. It’s good to challenge old ideas and come up with new ones.”
But here’s the thing: the entire argument is based on some pretty old ideas, and while the biomechanical measurements show (and quantify) the advantage standing to punch provides, the argument for this driving the development of bipedalism seems exceptionally shaky.
And exceptionally conservative in its views on gender difference and behavior.
Carrier assumes a world in which females selected their mates based on their success in violent confrontations. Males are imagined competing in a quite literal sense for females.
This inference is based on a generalization about contemporary great apes:
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.
This characterization homogenizes what has become a more complicated (and thus more interesting) picture of sexual competition among males in our closest cousins, as well as in our own species, aptly captured by an article in the British newspaper The Telegraph titled What kind of man are you– chimpanzee or bonobo?
The study discussed in this article, published in the Proceedings of the National Academy of Science, was conducted by biological anthropologists at Harvard. It showed variation in how human males reacted in anticipation of competition. The Telegraph summary is a reasonably accurate description of the findings:
males of both species showed hormonal changes in anticipation of competing for the food, but bonobos and chimpanzees were completely different in which hormones increased.
Male chimpanzees showed an increase in testosterone, which is thought to prepare animals for competition or a fight.
By contrast, male bonobos showed an increase in cortisol, which is associated with stress and more passive social strategies in other animals….
Human males usually experience an increase in cortisol before many types of competition in a similar way as seen in the bonobos.
However, if men have what is called a “high power motive,” or a strong desire to achieve high status, they experience an increase in testosterone before a competition.
Returning to the new proposal that fighting led to the permanent adoption of bipedalism in the human lineage, another weakness is simply that the primates where violent confrontations do take place in anticipation of mating that inspired the theory all manage to fight each other without permanently abandoning their quadrupedal posture. Standing up temporarily to bash at a rival seems not to lead to changing the habitual posture permanently.
The model has (if you will pardon the pun) a missing link. The route it traces from behavioral change to the permanent (and one assumes, genetically based) alterations of our skeletal anatomy that bipedalism entails are never explained. “Selection” for better performance in the all-important fights that would favor greater reproductive success somehow would have gradually led to bipedalism. But standing up on your hind legs does not change your anatomy.
The only hint at a possible mechanism comes from the suggestion that the model would potentially explain modern studies documenting a preference by women for taller men. The Salt Lake Tribune quotes Carrier again:
Taller men could deliver blows downward, thus giving them an edge in competing for mates and defending territory and resources.
“If this were true, females who chose to mate with tall males would have had greater fitness for survival,” he said.
So, the proposed evolutionary process seems to be this: males fight for the favors of females, and in that process adopt temporary bipedal postures. The males that can deliver blows from a higher position defeat those who cannot stand as tall, leading to their greater reproductive success. The ability to stand taller temporarily must have reflected some variability in anatomy based on underlying genetic variation that consequently was reproduced more, leading to hominid ancestors who could… what? the logical conclusion is, who had longer bodies so that their arms were raised higher during temporary bipedalism.
But of course, the article is not really, primarily, about how these postures became permanent. It is instead an eloquent polemic for the importance of “specialization for aggression” in human evolution. It is an explicit call to reconsider the wide dismissal of an “aggressive ape hypothesis”.
I think I agree with Dan Lieberman on what it actually shows:
“there is a performance advantage for a biped in hitting other people.”
But beyond that, most of the rest seems way to wedded to gendered assumptions about male and female roles and behaviors, and too free of evolutionary argument to convince me, at any rate.