Silvio Berlusconi, the Italian Prime Minister, has had a hair transplant. Tiny plugs of skin from the back of his scalp (where the hair still grows) have been transplanted to the front to correct his baldness. If Berlusconi has done this to “consort with minors” in the words of his wife, he has made a mistake. Such minors have already discounted his baldness and may, indeed, have been attracted to it.

The biology of human hair has engaged many great scientists. We are rare among mammals in being hairless, and Charles Darwin was only one of many researchers to note that our hairlessness is a huge disadvantage, physically. The fur of other animals protects them against sunburn, it protects them against trauma, and it warms them in winter and, even, by trapping sweat, can cool them in summer. So why have we lost our fur?

Although many scholars have tried to identify a useful function for human hairlessness, they have failed. Indeed Alfred Wallace, the biologist who jointly described evolution with Darwin, concluded that our hairlessness proved the existence of God. Only a supernatural being, unconcerned with natural selection, could have designed it.

But Darwin showed that hairlessness was proof of a different type of evolution, not by natural selection but by sexual selection. Under natural selection, individuals survive if they are adapted to their environments: a brown bear, being conspicuous, would not last long in the Arctic, so it evolves into a polar bear. Sexual selection is not concerned with the environment but with sex: individuals breed only if they find a mate, so animals have to attract one. Consider the peacock.

The peacock whose tail is dull and lifeless will not attract peahens. This is not because peahens are frivolous but, rather, because they understand instinctively that a male with a beautiful tail is a healthy mate. So evolution rewards peacocks for growing ever-more elaborate tails - they get more sex.

We human beings, too, are highly selected sexually, but in our case it is women who are the peacocks: the more beautiful they are, the greater the number and quality of the men who court them. This is why, some 75,000 years ago, we made our last two evolutionary advances: we lost our body hair and we invented art.

We know when we lost our body hair because the molecular biologist Mark Stoneking has dated it. One of the problems with being a mammal is that your fur gets infested with lice. When we lost our body hair the lice evolved into three distinct species to adapt to new environments. We now have head lice (the type our children get at school) pubic lice (the type they get soon after leaving school) and body lice (the type soldiers got in the trenches). Using standard DNA techniques, Mr Stoneking showed that the three species diverged about 75,000 years ago.

At the same time, as archaeologists have found in the Blombos Cave in South Africa, the first art appeared, in the shape of engraved chunks of ochre and shells made up as necklaces and bracelets.

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.

Which brings us back to Mr Berlusconi. Hair plays a social signalling role in many older mammals. It goes grey - which can be a good thing. It is only the silverback gorilla(so-called named for obvious reasons) who can corral a harem of females, in part because gorillas of both sexes revere older males. We have retained our head hair so enabling that social signalling: grey hair on men can reinforce an alpha message of chiefdom. As can baldness.

Men have evolved to attract women. Because only some men go bald, we must assume that different women are attracted differently. Some women will be attracted to young men, but young men are untried and therefore risky, so some women will seek sugar daddies instead. Mating with sugar daddies invokes a different set of risks but the trophy wife is nonetheless making a rational choice - one that may well have been rewarded preferentially in the Stone Age - to which she is in part guided by baldness in her man.

Now, what sort of girl will fancy Mr Berlusconi? Clearly the sugar daddy type. But such a girl will subconsciously be looking for baldness in her beau and she may be put off by the mixed messages Silvio's head is transmitting.

The biology of baldness is complex. Some theorists believe that it renders older men so unattractive that - rather than sowing additional wild oats - they are forced to spend more time with their families and so help their children to survive. But the myriad Becky Sharps in literature and history help to disprove that theory.

By having his hair transplant Mr Berlusconi has confused his potential partners. If they are gold-diggers, not romantics or libertines, the last thing they will desire is a hint of relentless wrinkly coition.

Terence Kealey is vice-chancellor of Buckingham University

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