Leroi lifts us from an instinctive horror at the bizarre to a more profound sense of wonder.

Sunday Times, London

Out of the ordinary

What makes a monster? Is it a thing with a hairy face, extra eyes or missing limbs? Or is it simply anything that we don't understand?

From two-headed infants and conjoined twins to supernumerary breasts and fragile bones, human mutants have a long history. In tracing ideas about the "form, nature and varieties of the human body", the biologist Armand Marie Leroi reveals the age-old conflict between the views of deformity as divine design and deformity as accident. But it was, he points out, Francis Bacon, the Renaissance founder of the experimental method in science, who made one of the earliest attempts to "naturalise" monsters - to bring them within the intellectual compass of philosophical natural history: "For once a nature has been observed in its variations, and the reasons for it has been made clear, it will be an easy matter to bring that nature by art to the point that it reached by chance." Or in the words of Leroi: "The monstrous, the strange, the deviant, or merely the different...reveal the laws of nature. And once we know those laws, we can reconstruct the world as we wish."

This book is intended as an interim report on Bacon's project. In a series of erudite, gracefully crafted essays, Leroi guides us through a wealth of medical phenomena - both the normal and the shockingly abnormal. Enriching his insights with examples drawn from science, medicine, history, philosophy and the arts, he lifts us from an instinctive horror at the bizarre to a more profound sense of wonder. He shows us that too tall, too short, too many, not enough and in the wrong place are not simply traits that compel us to marvel but the key to understanding much of modern biology, genetics and evolution.

Among the beguiling cases we meet are a convent girl who found herself changing sex on puberty; infants who, echoing Cyclops, are born with a single eye in the middle of their foreheads; a child discovered to have the remains fo 21 foetuses in it his brain; a soldier whose heart was on the right hand side of his body rather than the left; a village of long-lived Croatian dwarfs; a hairy family who were kept at the Burmese royal court for generations; and a famous juggler who suffered from phocomelia or "seal limb" - a condition in which the limbs look like flippers.

But Leroi's book is more than a cabinet of curiosities. With chapters covering deformities in embryos, limbs, skeletons, growth, gender, skin and ageing, he uncovers the intricate web of genes and molecules that controls both human and animal development. Perhaps the most remarkable of these are the so-called homeotic genes which are common to humans, mice and fruit flies and can shed light on the furthest recesses of human development and evolution. Mutations in these genes can cause flies to grow legs instead of antennae on their heads and mice to grow polydactylous fingers - thus helping biologists trace the 500m-year evolutionary odyssey that reaches from fish with no fingers to humans with our familiar five.

For those not used to thinking in four dimensions, embryology can be a notoriously difficult subject to visualise, but the explanations laid out here are remarkably clear. Adorned with photographs of specimens from medical museums and images from Renaissance texts on freaks and oddities of nature, the book is also beautiful.

But ultimately this is a survey of what we do and do not know about mutation. Why is there so much natural wastage? For most of the more severe physical defects embryos are aborted spontaneously - the spectrum of brain defects that includes cyclopia, for example, afflicts 1 in 200 miscarried foetuses. What's more, although we might know a lot about the mutations underlying severe conditions such as albinism, the genesis of more mundane traits such as brown hair and blonde hair are poorly understood. And what about the genetics of race? For decades geneticists have been at pains to emphasise that there is more genetic variation within individual populations than between different ones, and to dismiss the physical diversity that does exist as "uninteresting" or too sensitive too handle.

But Leroi wants to know about those differences; and his book is a celebration of the intrinsic interest of human diversity. After all, it turns out that on average each of us contains 300 potentially harmful mutations - and one inn 10 of us has an extra rib. As he says, we are all mutants, but some of us are more mutant than others.

Peter Tallack
Sunday Times, London (May 9 2004).