Misshapen but not that abnormal

Although we know from everyday experience that no two human beings are ever absolutely alike and that many are born with "defects" such as misshapen limbs or extra fingers, that is probably the limit of our knowledge of human diversity. In this beautifully written book, developmental biologist Armand Marie Leroi tells us how the human embryo is formed into a boy or a girl, details the processes that could go wrong, and introduces us to a large cast of "abnormal" characters in history. Although primarily about the genetics of development, some of its most eloquent pages are devoted to history, anthropology and the arts, and Leroi's prose, even when loaded with the acronyms that modern biologists seem unable to do without, remains supple and lucid.

The range of developmental anomalies explored in Mutants is immense. Apart from conjoined twins, hairy women, giants and dwarfs, we encounter infants with mermaid syndrome (legs fused together), those with single eyes, hermaphrodites, albinos and piebalds, and a dizzying array of bone deformities and malformations. In each case, Leroi presents us with historical examples drawn from sources ranging from Pliny to tabloid newspapers and explains what we now know about their biological causation.

Much is still shrouded in mystery, but one could not fail to be impressed by the depth of knowledge that developmental biologists have already acquired.

The chief value of the book does not really lie in its biological expositions, but in its accounts of individuals. Stories such as that of the Chinese man with a parasitic head embedded in the right side of his own lead us to the heart of the mysteries of human identity, and the book manages to talk about "freaks" without turning into a freak show. It explains a galaxy of infirmities and anomalies with the aid of genetics but stops short of reducing the experience of human beings wholly to the machinations of genes, selfish or otherwise.

One reason for this success is undoubtedly Leroi's consciousness of the absolute ubiquity of anomalies. The human body, he reminds us, is always "replete with variety and error". Many of those are virtually invisible, others merely distinctive or attractive (red hair, for example), and a relatively small proportion disabling or downright lethal. His point is that there is no sharp and clear difference between the "normal" and the "pathological" - if there are any dividing lines at all between the two, they are fuzzy and indistinct. The people we find immediately "abnormal" are simply those with more anomalies than the people we accept as "normal".

Moreover, Leroi emphasises, anomalies are indispensable for the scientific study of normality. Given the complexity of biological processes, it makes a lot of sense to analyse biological mishaps and, by identifying what went wrong, deduce how the mechanism works under normal circumstances. The great 19th-century German embryologist Wilhelm Roux enjoined his peers to produce anomalies artificially and to use them to understand normality. Leroi says: "The monstrous, the strange, the deviant, or merely the different reveal the laws of nature." Rather dispiritingly, however, he then adds that "once we know those laws, we can reconstruct the world as we wish". For all his appreciation of the poetry and the mysteries of existence, then, Leroi may well be a scientist after all, and his compassionate book a mere manifesto for genetic engineering.

Chandak Sengoopta is senior lecturer in the history of medicine, Birkbeck College, University of London.