Fatal gene that leads to wilt on the Y-front

The core of Steve Jones's new book for me is the chapter on James James's skull. In 19, James, a distant relative of Jones, who lived on the slopes of Plymlymon in central Wales, sold his head to the anthropologist H. J. Fleure for the sum of £70. (It is not clear whether James was alive or dead when the transaction took place.) What interested Fleure was the example that James represented of the "little dark people", the apparent remnants of the Celtic peoples who once inhabited these isles until they were driven to the mountainous fringes by more aggressive populations from mainland Europe. Those who are interested can visit the skull in the National Museum of Wales in Cardiff.

Here we have a typical Jones moment: an arresting story that is part of his family history (as well perhaps of mine as a fellow Welshman) from which particular account a general history can be unfolded. And the general history is gripping, taking us on various apparent digressions through the evolution of Welsh surnames, the myths about the Welsh prince Madog's landing in what is now Alabama in 1170, to the settlement of Patagonia by Welsh-speaking people in the 19th century, via the migration patterns of Native Americans and tribal life in central Siberia.

But the journey ends in a jolt to our fondest ideas about those Celtic myths (and mists), and a complete undermining of Fleure's anthropological dreams in favour of an even more dramatic history. Far from the Welsh dwellers on the slopes of Plymlymon being descendants of the Celts who once dominated large tracts of central and southern Europe, the DNA suggests that they are survivors of yet more ancient peoples, whose closest relations in Europe are the Basques. The male chromosomes of Basques are almost identical to those of Welshmen. But there is more: via certain people of central Siberia, Welsh people are also linked by their Y chromosomes to the original settlers of North and South America who crossed the land bridge from Siberia to Alaska thousands of years before Caesar fought the Celts in ancient Gaul. Madog may never have landed in Alabama, but his genetic cousins might have been all around him in the form of Native American tribespeople.

Perhaps I found this account especially enthralling because of a common identification with those "little dark people" (I was similarly fascinated by an account of baldness). But Jones offers more than an opportunity for romantic reflection. What makes this book valuable is that it demonstrates how and why the new insights of genetics can illuminate the past and the present simultaneously, without falling into the new mythologies offered by sociobiology and evolutionary psychology.

Jones insists that we cannot derive the sexual behaviour of modern humans from animals, for the obvious but often ignored reasons that animal sexual behaviour is too diverse and that we are human: "The main lesson from nature is that, for ourselves alone, sex does not make the world go round."

Similarly, there is no more reason to believe that contemporary gender and sexual patterns have their origin in a postulated history of our ancestors on the African savannah 40,000 years ago than to believe in the Celtic histories elaborated by Tudor scholars.

On the other hand, the evidence we now have about our genetic story does allow us to understand a complex history of human migration, settlement, intermarriage, birth and death across millennia in which biology and the body are modified by environment and human agency. And the story of the Y chromosome, Jones argues, provides a particular insight into our past - and future. Manhood, he suggests, "tells a social tale as much as one written in nucleic acids".

In his earlier book, Almost like a Whale , Jones paid homage to Darwin's On the Origin of Species by rewriting it in the light of contemporary knowledge. In this book, he updates Darwin's The Descent of Man , but the distance from the founding father is perhaps greater because the final message of Y is that Darwin's fundamental assumption of male superiority is not borne out by the biology. This is in part a melancholic account of male vulnerability. The Y chromosome, far from being the marker of male dominance, is a virtually redundant part of the human genome. "Males are wilting away," Jones writes. On every measure, from sperm counts to social status, from fertilisation to death, men are in decline. From middle age onwards, we live in a woman's world. The Queen sends nine times as many congratulatory telegrams to her female as to her male subjects. And peering into the future, with genetic technology advancing apace, Jones sees a story of ever-increasing redundancy. Men carry a "fatal gene".

All this is nicely subversive of those deterministic biological accounts that portray male dominance as being written in our genes. But I cannot help thinking that this inverted teleology is more the book's massage than its message. The joy of the book is in the detail of male vicissitudes, from conception onwards. It is stacked full of wonderful anecdotes and vignettes. Of the Dutch microscopist Antoni van Leeuwenhoek tentatively writing to the Royal Society in 1677 offering a first account of the movement of sperm. Of the Urinette Company going bust, because its design of a receptacle into which women could urinate while standing never caught on - but not before the male legislators of Texas voted to outlaw the contraption. Of the trichological cycles (patterns of baldness) of the American and Russian leadership. Of the fate of the Bounty mutineers, of Harry Potter, of the gay gene and, of course, of James's skull, and much more. And flowing through is not a pessimistic message of decline, but rather a story of greater equality between men and women - because "genes must often defer to social reality". Then science, and biology, falls silent, and human agency, male and female, takes over.

Jeffrey Weeks is professor of sociology, South Bank University.