Steve Jones

Y: THE DESCENT OF MAN

Jones is always an entertaining writer, with a wicked sense of humour. The slightly jokey title sets the tone for the book. It is about the Y chromosome, possession of which determines maleness in humans (things are different in many other species). 'Descent' here refers to genealogy but also to decline, because one of the themes in the book is the progressive reduction in the biological and social importance of men.

In earlier times writers (nearly always men, of course) took it as given that masculinity was the ideal form of humanity and women were an inferior approximation at best. Even Darwin seems to have acquiesced in this assumption. In reality, however, the exact opposite is the truth. The default human form is the female and males are a deviation from this. Being male is always a struggle and the struggle entails a cost for the individual. In many ways men are less healthy than women and have shorter lives.

Jones is very conscious of age and its infirmities, both as they affect the individual and as they influence his progeny. He points out that the sperm of a twenty-year-old is separated by about four hundred cell divisions from the sperm that made him, whereas the spermatozoa of a man of fifty have gone through more than a thousand extra rounds of replication. At each cycle there is a chance of a mistake occurring—a mutation. This can increase the chances of some abnormalities in the offspring by as much as twenty times.

As in some of his other books, Jones is concerned throughout to emphasise the complicated interdependence of nature and nurture. Genetics, he says, is a science of the extremes. It can tell us a lot about the inheritance of some diseases but it is less useful in most ordinary circumstances. 'For the norms, rather than the extremes, the importance of genes fades in the face of the environment, and biology has rather little to say.'

It is easy to think up possible genetic explanations for deviations from the norm but the validity of such suggestions is usually unclear. Attempts to identify a 'gay gene' have largely failed. One of the great merits of this book is that Jones repeatedly shows how simplistic assumptions about the role of biology in determining behaviour are nearly always wrong.

Thoughtful books like this are a valuable corrective to the 'understanding' of science that many of us acquire through the media today.

19 November 2009