I read this twice and still couldn't figure out if this was satire or serious darwinian thinking.
Satire, or "science" -- what do you think?
Thursday, April 15, 2010
Darwinian Fact of the Day: Why you are bald and your wife is no longer furry
"Art and hairlessness co-evolved because they fed off each other. The girl whose skin was least hairy could paint it, tattoo it, decorate it and clothe it more adventurously than could her furry sisters. So she got more and better men. And in consequence her children - even the males, though to a lesser degree - lost their hair too. We had become the naked ape."
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Answer: Neither. It is an interesting hypothesis, but a difficult one to test. Without a testing protocol, it couldn’t be considered “science”. It is speculation.
If this was a scientific thesis, I would challenge it at several fronts:
Humans are far from the only hairless mammals. Many porcine species, some rodents, all modern pachyderms. Occam’s razor would suggest the same forces the led to their hairlessness also lead to ours and surely they do not have much of an artistic pedigree. Then there are the universally hairless cetaceans, but of course the forces of change in their habitats are pretty clear, and very different from ours.
He makes the case for correlation (timing of the rise of art and adornment coincides with the loss if significant hair), then hypothesizes causation. But he might have it backwards, it may go the other way: adornment may be a result of the loss of hair as a sexual signaling device.
He also makes the bold case that a lack of hair is a distinct disadvantage, then a few paragraphs later points our one of the major disadvantages of hair: fleas. Fleas only afflict animals that are domestic: they sleep the same place all the time. The flea life cycle relies on this. If you sleep in a different place every night you don’t get fleas. Deer don’t get fleas. Wild dogs don’t get fleas. Once the human lifestyle became more domestic, fleas became a significant health hazard: loss of hair may provide a survival advantage in this case.
Of course, when it comes to selection, being a slight disadvantage may be OK if it offsets a larger disadvantage. Growing hair is energy-intensive. Grooming hair is energy intensive. When food is scarce, this energy loss may be prohibitive, especially if you live in a place (say, central Africa, where Homo Erectus and Homo Sapiens evolved) where the insulation advantages of hair are limited. Interesting that a different, hairier species, the Neanderthals, evolved in places where the insulation offered by hair was of greater advantage…
What is the evolutionary advantage of men having beards? What is the evolutionary advantage of women not having beards?
Why wouldn't they both either have, or not have, beards?
Simple answer is, I don’t know. However, I could speculate. But before I do, let’s analyse your conditions.
First off, the beard is far from a ubiquitous primary feature of human males. They are several groups of modern humans, mostly in central Africa and Asia, where male beards (indeed, all body hair) are sparse or non-existent. There are also wide variations in the amount of body hair women have (and yes, I am talking about naturally, not shaved, waxed or plucked). This tells us that there are regional differences in the “advantages” offered. Therefore, they are either climatic or cultural. (I guess they could be diet-related, but it is hard to imagine a functional path there…)
Second, a beard is only one of several traits that humans have (along with many other animals and plants) that are collectively called “sexual dimorphism”. You might notice mallard ducks vary somewhat on body colour. Only one gender of hemp plants have psychotropic effects, etc. In all cases that I am aware of, this dimorphism is related to either sexual signalling or requirements for reproduction.
Let us assume beards are not “required” for reproduction, as both bearded and non-bearded men have children. That leaves sexual signalling.
I would suspect the sexual dimorphism of human beards is similar to that of the mane on the lion or the brilliant plumage of a peacock. The females of neither of those species have those structures. In the case of a lion, it is pretty much accepted that the mane is a cheap way for the male lion to look bigger and therefore more dangerous to potential rivals (Same reason your housecat’s hair stands on end when it is scared/angry). In lion society, the males battle for a brood of females. The mane is advantageous. The females, not involved in those battles, need not that advantage. The peacock wastes a lot of energy building this completely non-functional tail to attract peahens. How that signalling got so out of scale is the subject of much study (summed up really well in Dawkin’s The Blind Watchmaker).
What I don’t know is when the "beard" form of sexual dimorphism started in hominids, nor what the societal structures were that favoured them. Were beards advantageous for men, but smooth skin preferred by men when making mate selection? Or was smooth skin advantageous, but women preferred bearded men when they made mate selection? I would suspect it is some combination of both at different times in early hominid development.
It is ideas like this that make evolution such a fascinating subject of study.
Thank you for your thoughtful responses.
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