Most other mammals think that humans are excessively ugly. They probably see us in the same way we see naked mole rats. We (mostly) have no hair.
What happened to our hair? There are many explanations for human hairlessness but they all share one common characteristic—they are adaptationist just-so stories.1
If you're looking for the best example of an adaptationist then you need look no further than Elaine Morgan, author of the Aquatic Ape speculation [see Elaine Morgan and Aquatic Apes]. She has written an article for last week's issue of New Scientist: Why are we the naked ape?. It won't come as a big surprise to learn that she dismisses all of the speculations about the evolution of hairlessness, except one: we lost our hair because our ancestors lived in the water.
That's not the point I want to make. Here's what Elaine Morgan says in the first few sentences.
RIGHT from the start of modern evolutionary science, why humans are hairless has been controversial. "No one supposes," wrote Charles Darwin in The Descent of Man, "that the nakedness of the skin is any direct advantage to man: his body, therefore, cannot have been divested of hair through natural selection."The idea that our lack of hair might just be an accident is completely foreign to someone like Elaine Morgan. She's probably being deadly serious when she asks the question, "If not natural selection, then what?" For adaptationists, natural selection is the only game in town and no other sorts of explanation are possible.
If not natural selection, then what?
If it's genetic and visible, then it must be an adaptation. If one just-so story is refuted then make up another one to take it's place. That's what the article is all about. One by one, she dismisses sexual selection, overheating on the savannah, neoteny, avoiding parasites, evaporating sweat, leaving only aquatic ape speculation that hasn't been refuted, or so she claims.
One of the problems with the adaptationist program was described by Gould and Lewontin (1979), "If one adaptationist argument fails, assume that another must exist ...." Why not start thinking about other, non-adaptationist explanations?
Why is that so hard?
1. The one exception is the idea that our nakedness is an epiphenomenon resulting from neotony.
Gould, S.J. and Lewontin, R.C. (1979) The Spandrels of San Marco and the Panglossian paradigm: a critique of the adaptationist programme. Proc. R. Soc. Lond. B 205:581-598.