For more than a decade, Tim Crow has contributed to the scientific literature thought-provoking and sometimes provocative ideas on the origins of cerebral asymmetry, language and psychosis. Now, in this edited volume, he has assembled contributions from palaeontology, linguistics and genetics in addressing the question of whether modern humans evolved gradually or quite suddenly. In making a case for the latter, Crow is continuing a long tradition of sometimes heated debate over the exact nature of our descent. Staunch Darwinists, faithful to the principle of gradualist change by natural selection, have, for more than a century, come up against sceptics (from Alfred Russel Wallace – the co-discoverer of the theory – to Stephen Jay Gould), who cite the discontinuities in the fossil and archaeological record, as well as human-specific language, as evidence for saltational or sudden change.

The cornerstones of Crow's hypothesis are: that modern human creativity and reason reflect a recent and sudden departure from the evolutionary trajectory of our hominid ancestors; that syntactic, symbolic language and directional handedness constitute ‘our oldest title-deeds as rational beings’ (Reference MullerMuller, 1996); and that a mutation on the X and Y chromosomes prior to the migration of our ancestors out of Africa 100 000 years ago was subject to sexual selection and represented the speciation event. These three principles are well-reflected in this volume, with authoritative commentaries on the earliest human fossils and symbolic artefacts, on the differences between animal vocalisations and ‘singular’ human language and the uniqueness of directional asymmetry, and finally on the sex-linked protocadherin genes as possible candidates for a speciation event.

The arguments Crow has collated are well constructed and heavily referenced and, to the reader unfamiliar with this terrain, almost convincing. However, despite a consensus that the emergence of Homo sapiens involved something quite drastic, there are hints of some doubters in the camp. For example, Chris Stringer, of the Natural History Museum, explains the difficulties in defining ‘modernity’ and how ‘modern morphology’ and ‘modern behaviour’ are fluid concepts, whose origins are not necessarily coincident. And in a chapter on laterality, Michael Corballis supports the notion of a laterality gene, but argues that it is located solely on the X chromosome.

In an era in which complex cognitive processes are understood in polygenic terms, it is difficult to imagine that a single gene is responsible for cerebral asymmetry and language. Tim Crow is well accustomed to the sometimes lonely role of pioneer and is not deterred by sceptics. His contribution to modern psychiatry is legend, and for this reason alone, The Speciation of Modern Homo sapiens deserves a read. It is different and it is not specifically about psychosis, but it is a challenging and thought-provoking discussion on what it means to be human – and that is of relevance to us all.