There are lots of possible explanations for why humans became bipedal, including changing ecosystems and food resources. But what if it was just so it was easier for early humans to beat the crap out of each other?
That's the theory put forward by researchers at the University of Utah, who also believe their experiments into the link between bipedalism and fighting can explain why women prefer tall men. Oh yes - as evolutionary psychology papers go, this one is a real doozy...and yet if you can get past some of the more fanciful conclusions, it still might have some very useful things to tell us about why we walk upright.
Let's start with the basic science before we jump to some of the more colorful ideas. The fundamental premise is that men can hit harder when they're standing on two legs than when they're on all fours (I know, I was shocked too), and that they have more power hitting downward than upward. This means that taller men have a fighting advantage over shorter men, which seems reasonable enough. But it also means bipedal humans had an advantage over their quadrupedal counterparts, and that's where things start to get just a little weird.
Biologist David Carrier, who led the study, explains what this could mean:
"The results of this study are consistent with the hypothesis that our ancestors adopted bipedal posture so that males would be better at beating and killing each other when competing for females. Standing up on their hind legs allowed our ancestors to fight with the strength of their forelimbs, making punching much more dangerous."
That's all well and good, but what about how this explains why women are attracted to tall men? Carrier has that covered too:
"It also provides a functional explanation for why women find tall men attractive. Early in human evolution, an enhanced capacity to strike downward on an opponent may have given tall males a greater capacity to compete for mates and to defend their resources and offspring. If this were true, females who chose to mate with tall males would have had greater fitness for survival."
Again, the basic finding of all this - that a human (or indeed any primate) has more power when fighting upright - is quite a reasonable one, and it's important to provide experimental support for that idea. After all, we can see support for that in the behavior of our primate cousins, which often shift off their normal quadrupedal posture to a more upright stance whenever they are preparing to fight.
And, to be fair, there are some issues with other leading explanations for why humans are bipedal. One popular explanation is that, as the African forests receded and turned into savannas, our ancestors needed to travel far on foot and no longer needed to be able to climb trees, which would explain a shift towards being bipedal. The problem with that, says Carrier, is that it's actually harder to walk upright than on all fours, no matter how you slice it:
"If you're a chimpanzee- or gorilla-type ancestor that is moving on the ground, walking bipedally has a cost. It's energetically more expensive, it's harder to speed up and slow down, and there are costs in terms of agility. In every way, going from four legs to two is a disadvantage for locomotion. So the selective advantage for becoming bipedal, whatever it was, must have been important."
This is where fighting comes into the picture. It isn't just primates that stand upright when it comes time to fight - tons of mammals, from anteaters and rabbits to lions and bears, will stand up on their hind legs and use their front legs in combat.
Of course, humans are the only ones who actually became fully bipedal. If fighting really was what fueled the shift towards walking upright, then our ancestors must have been extraordinarily violent, far more so than pretty much any other mammal, as all other species apparently evolved to favor locomotion over fighting. If Carrier is correct, then humans (and our evolutionary ancestors) have been choosing to fight for a long, long time.
Honestly, none of that seems too crazy. It's when we get into the idea that all this can explain why women prefer tall men that I start to feel skeptical. But here's the argument - multiple studies have shown that women prefer tall men, and there are any number of correlations between height and positive traits in both men and women. And that, Carrier says, is the problem - if women are attracted to tall men because of all their positive traits, than why aren't men attracted to tall women, which has been proven repeatedly to not be the case?
As far as Carrier is concerned, it's because tall males had a fighting advantage, and women have evolved to prize that in potential mates:
"From the perspective of sexual selection theory, women are attracted to powerful males, not because powerful males can beat them up, but because powerful males can protect them and their children from other males. In a world of automatic weapons and guided missiles, male physical strength has little relevance to most conflicts between males. But guns have been common weapons for less than 15 human generations. So maybe we shouldn't be surprised that modern females are still attracted to physical traits that predict how their mates would fare in a fight."
Carrier stresses that he is in no way suggesting that women prefer violent or even physically abusive men - rather, they might tend to be attracted to males that, from an evolutionary perspective, would likely be more able to protect their resources. I'm not sure I'd go even that far, but there is a distinct possibility that all of human evolutionary history runs on beating the crap out of each other.