The first partial skeleton of a man, found in a cave in Neander's valley in Germany in 1856, was held to be that of a lunatic or perhaps of a Cossack deserter from some unsavoury conflict who had mysteriously abandoned his uniform. Typically, still, the Neanderthal is envisaged as a hulk. Characteristically - almost diagnostically - in textbooks and seaside postcards, the men are shown with three days' growth of unkempt beard. Both in scholarly and in popular iconography, the Neanderthals are nature's down-and-outs.
Yet the one certainty, which surely should have been obvious from the beginning, is that the ill-shaven, run-down image must be false. Whatever adorned the faces of Neanderthal men, it was not designer stubble. Their Mousterian tool-kits included no razors. More to the point, the vision of a crudely shaved Neanderthal offends all principles of modern biology. For although Neanderthals these days are commonly placed in their own species - Homo neanderthalensis - they are very definitely human beings; and one of the outstanding features of human males is their beardedness. Modern biological theory suggests, however, that far from being half- shaven, Neanderthals would have been very bearded indeed.
The beards of human males, like the tails of peacocks and the antlers of stags - or the great round cheek-pads of male orang-utans - are prosaically known as "secondary sex characters". Their growth is generally promoted by the male hormone testosterone, which also underpins fertility.
Human males, like stags, are clearly very different from the females and, in particular, much bigger and stronger. But human males in a state of nature, contrary to the mythology of the seaside postcard, do not use their brute strength to drag off the females of their choice. It is the females who do the choosing. They decide whose genes should be admitted to their precious eggs and they may then try to hold the attention of the male as a powerful and potentially valuable ally and forestall his engenderment of rival children in rival females.
In all this, the male's only active role is to sway the female's choice. To this end, he advertises. Beards, tails and antlers all declare: "I am male and I have survived until maturity." Male sex characters are not therefore the mere epiphenomena of physiology - or not necessarily. Some have clearly evolved to subserve the central purpose of reproduction.
For it is not enough merely to proclaim maleness. The game is competitive. Each male must declare his virility more vigorously than his rivals. So what Charles Darwin called "sexual selection" ensures that the signals become more emphatic as the generations pass: the tails get longer, the antlers grow wider, the beards get bushier. Yet, as Darwin's great collaborator Alfred Russell Wallace pointed out, sexual signals must in the end be real. They must reflect actual strength, or wisdom or maturity. Natural selection would soon punish females who were deceived by colourful wimps and took on board the genes of popinjays. A system of sexual signalling that was founded entirely on lies would bring the lineage to an untimely close.
Modern research has generally confirmed Wallace's insight. One key idea, from Professor Bill Hamilton at Oxford, is that emphatic sexual signals tend to signify freedom from infection. Birds are often riddled with parasites, which chew their feathers and dull their colours; so males with bright, tidy feathers are likely to have the fewest parasites. Field research has tended to support Professor Hamilton's prediction. A more general and largely complementary notion is that of the "handicap principle". This says that sexual signals must in some way be costly to their sender. Thus the peacock's tail and the stag's antlers require tremendous food energy for their growth and, once grown, hinder their owner's movement. To put the matter anthropomorphically, such signals say: "Look what a tremendous burden I can endure! Think what awesome reserves of strength I must have if I can do such things!" The handicap principle sounds fanciful, but theoretical mathematical reasoning has tended to show it is an obvious symbol of virility, linked to testosterone output; yet it is also a handicap for precisely the kind of reason that Professor Hamilton would acknowledge. For, as was pointed out by another distinguished naturalist, Edward Lear, an ebullient beard is a tempting habitat, if not for Lear's two larks and a wren then certainly for intruders of less charming mien. In short, male facial hair says to the females: "Look at me! I am bigger and stronger than all the other males, even though I have this potentially disastrous nest of parasites around my face." But the signal would not have worked - would not have fooled the females for an instant - unless the beard was big and bushy and unless, crucially, it was indeed conspicuously free from parasites: beautifully clean, shining and tidy, a symbol of the cleanliness and orderliness within.
So the last thing a Neanderthal would have done was to shave, even if he had the wherewithal; and his big, flowing beard would certainly not have been unkempt. For many mammals and birds, grooming is an obsession. The whiskers of an alpha Neanderthal male were surely tended individually by those in whose interests it was to curry favour, each nit assiduously crushed in horny fingertips. Thus cleansed, his beard signalled - accurately - his strength, vigour and charisma.
One further thought. Neanderthals are invariably depicted with dark hair and beards, as if to emphasise their swarthy shiftlessness. But creatures like this, who achieved their greatest extremes of bodily form in the highest latitudes, would surely have been blond or ginger so as to soak up as much as possible of the northern sunshine. I see Neanderthal males as sunflowers at the glacier's edge, with great shocks of yellow and red hair on their long, sloping heads and huge, round, burnished beards glowing in the slanting sun. Here was no Caliban. Here, more likely, was the archetype of Wotan and Thor.
Colin Tudge's latest book, `The Day Before Yesterday', will be published by Jonathan Cape early in 1996.