One of the greatest intellectual triumphs of the 20th century has been the fusion, in the middle decades, of Charles Darwin's theory of evolution by natural selection with Gregor Mendel's genetics. Out of this, in the 1940s, came the "modern synthesis", or "neo-Darwinism"; and thence in the 1960s came George Williams's observation that natural selection acts most forcefully not on individuals (as Darwin supposed) and still less on groups or species (as it had become fashionable to believe) but on individual genes - the idea summarised by Richard Dawkins as "the selfish gene". Then, in the 1970s, Bill Hamilton, at Oxford, showed how "selfish genes" can give rise to the most unselfish behaviour - co-operative and even "altruistic" - and from such insights were created first the science of sociobiology and then its scion, conscientiously stripped of disturbing political overtones, known as evolutionary psychology.
Evolutionary psychology is truly changing the course of human introspection. The subject is new and the data sparse - relative to the task in hand - but the theory is rich and already bearing fruit. Marek Kohn's wistful and highly personal account of his own attempts to grapple with its ideas beautifully captures the state of play.
Kohn illustrates his arguments with what may seem, at first sight, like a diversion - for several chapters he asks why it is that for a million years, up until about 250,000-300,000 years ago, our ancestors made, without appreciable improvement or variation, the flattened, tear-shaped, sharp-edged hand-axes that are now the stock in trade of every local museum. At least they are called "hand-axes", but what were they really for? You can certainly skin a horse with a hand-axe, but so you can with any splinter of flint if you knock off a bit with a suitably sharp edge. So why be more elaborate? Many "hand-axes", too, show no signs of wear - and why make a tool and then not use it? There is also evidence (for example, from the half-million-year-old Boxgrove site in West Sussex) that early hunters or scavengers made these elaborate axes on site and then abandoned them. Why? Were they too stupid or forgetful to carry them from one place to another? And why didn't they ever change the design? Why make them in the first place if simpler devices would do?
Nobody knows. Most scientists tend to favour minimalist notions and conventionally speculate that once our ancestors had learnt how to produce something that worked, after a fashion, they just kept on doing it, not yet having realised the joys and advantages of innovation - and still less of R&D. But Kohn, and also Steve Mithen, at Reading University, offer a quite different explanation, incorporating two of the prime showpiece notions of modern evolutionary psychology. The first is sexual selection - Darwin's own idea that our behaviour and appearance, and those of all other creatures, are shaped at least as much by our need to find mates as they are by the need to survive; and the second is the extraordinary "handicap principle", first put forward by Amotz and Avishag Zahavi and given extra rigour by Alan Grafen.
The Zahavis proposed, uncontentiously enough, that much of evolution is concerned with an animal's need to send signals, to tell competitors of its own or other species that it is not to be tangled with; or to tell potential partners that it would make a good mate. To do this - and here is the strange bit - the animal "deliberately" adopts a handicap. Thus, when gazelles see lions, they do not creep away unobserved but instead they "stot" - leaping high in the air off all four feet, conspicuous as can be. The wanton squandering of energy shows the lion that they are much too frisky to be worth chasing. The peacock's otherwise inexplicable tail tells the females what a fine chap he must be to survive with all that superfluous plumage.
Now suppose, says Kohn (and Mithen, independently), our ancestors made fancy axes not to cut meat but to show potential mates how clever they were - like rolling a cigarette with one hand. That it takes time to make an axe is precisely the point: this is the self-imposed handicap, the behavioural equivalent of the peacock's tail. To chip an axe on site, while lesser mortals hack at the battered flesh with their prosaic shards of flint, and swagger in only when it's good and ready is the ultimate cool. Of course the design of the axe should not change. If it varied, there would be no standards by which to judge particular brilliance. By the same token, Kohn argues, all executives wear grey suits so that their peers and minions can spot the minute advantages that sort the well-off from the seriously rich.
A tale such as this has testable elements which, by definition, are science; but in the end it is speculation, which puts it beyond science but also shows that science cannot tell us all we want to know. After all, the tale may be true. The story may seem absurd - many have found the handicap principle ridiculous - but absurdity has a way of proving correct (as the critics found who rejected gravity or continental drift). Rejection on such grounds is merely what Dawkins has called the "argument from personal incredulity".
Why should such stuff matter? Because we are our ancestors' descendants and have inherited their psychology. Some insights may seem minor and merely amusing (like the executives' grey suits) but others are deadly serious and can be acted on, as in the extraordinarily rigorous studies by Martin Daly and Margo Wilson into homicide, which show why young men worldwide kill each other for the smallest slight and that a child's greatest risk of violent assault comes from step-parents. Similarly David Haig's analysis of the struggle between mother and foetus could lead to changes in obstetric practice, and Simon Baron-Cohen's analysis of autism is already influencing therapy.
There is strife at present, often bitter, between evolutionary psychology and traditional sociology - which Kohn, unlike many, would like to heal. The two disciplines, he suggests, have much to learn from each other. Kohn has proved once more that he is a fine commentator on science.
Colin Tudge is a research fellow of the Centre for Philosophy at the London School of Economics