¡°Why only us?¡± the linguist Noam Chomsky pondered in a recent book. What makes humans human and separates us from our fellow primates? One giant mutation was Chomsky¡¯s unconvincing answer. He is not, however, alone in considering this conundrum, nor in failing to solve it; evolutionary biologists have wrestled with the problem ever since Charles Darwin published The Descent of Man in 1871. Alfred Russel Wallace, who formulated the theory of evolution simultaneously with Darwin, could never accept that natural selection alone could generate the complexities of our human brains, language and culture. Darwin sadly chided him with ¡°almost murdering our child¡±, but had himself to resort to hand-waving.

A new generation of biologists has risen to the challenge, drawing on the latest findings from primatology, palaeoanthropology and epigenetics to offer a more convincing story. Kevin Laland¡¯s ambitious new book is, to my mind, the best account yet. He draws extensively on his own and his colleagues¡¯ research, ranging widely from the behaviour of nine- and three-spine sticklebacks to the techniques of flint-knapping among our Palaeolithic ancestors, the reasons for the dominance of right-handedness, and the evolution of dance. He binds this diversity together through empirical evidence and mathematical modelling within a revisionist Darwinian synthesis.

For classical Darwinists, evolutionary change occurs through random genetic change, resulting in offspring that differ mildly in their fitness. The fittest survive to reproduce in their turn. The emphasis is on randomness; the environment of the developing organism cannot feed back on to the genome and thus direct change ¨C this would be the ultimate Lamarckian heresy of the ¡°inheritance of acquired characteristics¡±. Thinking has now changed; spurred on by the latest findings in epigenetics and evidence from field studies, it has become clear that rapid, directed evolutionary change is indeed occurring. One of the most striking examples in humans is lactose tolerance ¨C the capacity of adults to digest milk products. In other primates and in human hunter-gatherer societies, the enzymes required for this are present only in infants; the cultural changes introduced by the development of agricultural societies fostered the genetic modifications enabling adults to digest this nutritious food source. This is gene-culture co-evolution, and it is, as Laland convincingly demonstrates, a key to understanding the rapidity of human evolution.

But for such co-evolution to occur, culture needs to build on prior mechanisms present in other social species: social learning ¨C that is, the ability to learn from the example of other conspecifics about, for instance, food and danger; and teaching, by parents of offspring. Both are present in rudimentary form in our close primate relatives, but only in humans, aided by the development of proto-languages, have they developed to the extent that co-evolution becomes a runaway, accelerating process.

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Laland sees such co-evolution as the major driving force in the emergence of modern humans. He eschews the crude evolutionary psychology that gave the field such a bad name back in the 1990s. However, to employ gene-culture interaction, adaptation and natural selection to cover almost all aspects of human activity, from religion and charitable giving to architecture and art, ignoring the higher order accounts of sociology and economics, is, to me, a step too far in what is otherwise a richly rewarding and powerfully argued book.

Steven Rose is emeritus professor of neuroscience, Open University.

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Darwin¡¯s Unfinished Symphony: How Culture Made the Human Mind?
By Kevin N. Laland?
Princeton University Press, 464pp, ?27.95?
ISBN 9780691151182 and 9781400884872 (e-book)?
Published 22 March 2017