Some traits get passed down because they help animals stay alive. Some traits get passed down because they enhance fertility. And some traits get passed down because they just make creatures more sexually attractive.
Which aspects of the human race only exist because at some point, enough of our ancestors decided they were too sexy to do without?
Anyone who has seen a peacock has to have wondered how a bird like that could possibly have survived in the wild. They have huge tails that they tow around, like a tugboat towing a barge. The only difference is, this barge is a luminescent signal to any and all predators in the area, and the tugboat is made of meat. It doesn't take much imagination to know that, in the wild, a lot of peacocks get killed off for having a tail like this. And they don't have it because it's useful. They don't have it because it keeps them safe. They have it because it's too damn sexy to lose. We see this a lot, in both function and behavior. Animals fight, risking death, for the chance to mate. Others create elaborate displays, or make loud sounds that can draw predators.
But what characteristics do humans have that serve no conceivable purpose other than attracting mates?
We hear a lot about things that can help us to find a boyfriend or girlfriend. An overall healthy look, not oozing with sores, is considered an attractive quality, for example. Enough body fat to develop secondary sex characteristics, or enough muscle to look bulky, is also considered a major plus. But those qualities also help with survival — intact skin, enough muscle to protect yourself, and a little extra weight, are all useful for staying alive in the wild.
Perhaps, though, we are a civilization of peacocks, consistently over the millennia selecting traits in mates that slow them down, make them conspicuous, and get them eaten by lions.
In terms of sexual selection, in humans as well as animals, there are two main schools of thought; Fisherian and non-Fisherian. Ronald Fisher was a biologist in the 1930s, who took a look at the extreme and bizarre plumage displayed by birds during mating season, and thought, "There is no reason for this whatsoever." Some juggling of genes and chattering of neurons, and female birds prefer their males to be bright. From then on, the brightest survive. It's the scatter-shot view of evolution. When we see certain characteristics, we assume, since they have survived so many years of life-or-death struggle, that they must give a species some advantage. The Fisherian model, though, takes the attitude that sexually attractive characteristics are just as likely to be weird quirks. If females prefer, for example, bright red tail feathers or a certain motion in the mating dance, then only the males with bright red feathers survive. There's no reason for it other than finnicky mating.
On the other side of the ideological fence are the non-Fisherians. They believe that, while sexual selection might be arbitrary to a certain extent — there are plenty of ways to be impractical in nature — their display must have a purpose. A peacock has to be fast, strong, or cunning to live out its life with that kind of tail. Birds of Paradise, in their display, prove that they are well fed, physically coordinated, and strong. Carrying any trait to an extreme, and doing it well, shows genetic fitness.
So what characteristics do humans have that might only exist for mating purposes?
For starters, there's our hair. It's one of the stranger characteristics of the human animal. Although there are a few bald mammals in the world, and some of them live in the same hot areas where humans evolved, none of them have the weird patch of long hair that humans carry on their heads. Not too many have beards or mustaches, either. Long hair can get tangled, it houses fleas and other parasites, it catches on things, and it gets dirty. But people all over the world have it. Various explanations have been put forward. Infants cling to it, although human infants don't have the ability to cling to things while their parents move around. It's warm, although an all-over coat of fur would be warmer. Contrary to popular belief, most of your bodyheat is not lost through the top of your head. A head loses no more heat than any other body part. And humans everywhere use hair as personal decoration. It very well might always have been simply a way to catch another person's eye, going back millenia.
Body odor might be another Fisherian trait. It wasn't always as off-putting as it is considered now. In medieval times lovers used to put a peeled apple under an armpit for a while, wrap a ribbon around it, and present it to their sweetheart. (At least it was better than the plague.) And remember Napolean's famous note to Josephine, given to her while he was still weeks away from their estate, "Don't bathe. I'm coming home." Given it kicks in a puberty, it might be a sexual signal. It's certainly not doing weak-nosed humans any favors in a world full of predators and prey that navigate the world by smell. Even now, we rely on scent. We just prefer it to be artificially generated.
There are even theories, speculative at best, that different racial groups evolved to have different traits, because isolated groups of people developed arbitrary concepts of sexiness. Most evolutionary biologists, when pressed on this idea, will bring up the epicanthic fold — the eye-fold often seen in people of east Asian descent. It may have started as a genetic characteristic that protected against the cold and glare from snow, but plenty of people around the globe were exposed to extreme cold. It's perfectly possible that people in Asia just found it hot. Historically, there is evidence of the arbitrary nature of beauty. The robust, pale, beauties of the Renaissance era are extremely different the thin and tan ideal women of the modern Western era. There just isn't any evidence that these arbitrary beauty standards lasted long enough to have a real effect on the population.
This brings us to the secondary sex characteristic that has spawned so many publishing empires: boobs. The fact that human women compete for male attention raises a few eyebrows in the biology world. It's common for males to bring food, prepare mating areas, do displays, or even fight for female attention in the animal world. Human women, on the other hand, are known to do a little displaying and fighting as well. There's a case to be made that female competition for mates is solely a cultural expression — but when evolutionary biologists consider the problem, they turn to boobs.
Breasts definitely don't do anything but get in the way when they're not actively being used, and yet they're right there, on women in every culture. So, for that matter, are fuller thighs, and rounder hips. If they're not there, women will try to get them added surgically. And often, when they're not there, body fat isn't there either. Since body fat is needed to get through a pregnancy, but boobs work equally well to feed children, whether they're big or small, they're prime examples of sexually selected characteristics that might seem arbitrary, but convey an evolutionary advantage.
Or, people argue, maybe boobs do have another purpose: They poof out in the front to keep our feeble infants from suffocating when they breastfeed. Like most theories about the whys of evolution, ideas about sexual selection remain unproved. There's no way of being entirely sure that certain things don't convey an evolutionary advantage, or that they both convey an advantage or a disadvantage, or that they are arbitrary. This generation didn't invent picking certain people for their sexiness, but how much difference past generations made in this area can only be speculated on, never known.
If we ever develop peacock's tails, though, I imagine the speculation would be brief.
Top Image: Arpingstone via Think Quest