Evolutionary psychologist Robin Dunbar knows that size matters - it's why we're humans and not apes. Geoff Watts explains.

Robin Dunbar's latest book underwent a transformation at the writing stage. Nothing unusual about that; many authors use the process of putting ideas on paper to explore and refine them. But in Dunbar's case the change was particularly appropriate - his new book, The Human Story , is a history of the unpredictable course of our species' cognitive and social evolution.

Dunbar says the book "began as an exploration of whether apes should have human rights", but "it became apparent in the course of writing that, interesting as this may be, the real issue was why humans are not apes. They're our nearest and dearest, but there is this huge gap between us. If you look through the bits of evidence - genetics, phylogenetic history, anatomy and so on - the area where we really come out as starkly different is in imagination.

"This seems to underpin all the superficial differences that you see between us and apes. Only we can build cityscapes, send men to the moon, have literature and science and so on. And all the really big changes come down to aspects of cognition. Increasingly, it seems to me that these are largely a function of brain volume: the size of the computer." The Human Story charts the historical development of that computer.

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Equally appropriate (though of no significance, he claims) is that Dunbar grew up where the human story began: in Africa. His family was part of the Celtic diaspora that kept the Empire going, and his father ended up in West Africa as a power engineer. Dunbar junior, aged 13, returned to Britain to attend secondary school and, in the late 1960s, to read philosophy at Magdalen College, Oxford.

At that time, evolutionary psychology - the subject that was to become his chosen discipline - had yet to be invented. But if you'd wanted to prepare a man with no particular interest in science for a career as one of Britain's leading authorities on the subject, the grounding he received as an undergraduate could hardly be bettered.

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Philosophy at Oxford had to be read in conjunction with some other topic.

"I chose psychology as the least bad option," he recalls. "First-year psychology was taught by the ethologist (and Nobel prizewinner) Niko Tinbergen. If I'd gone anywhere else, I probably would not have had that introduction to animal behaviour."

His career path was also influenced by a student expedition he joined. "We did a project on primate behaviour in Ethiopia. It was a subject I knew little about, despite having grown up in Africa."

Dunbar responded enthusiastically to the experience - and also to the general expansion of fieldwork on primates that was then taking place, a burst of activity that sparked the careers of researchers such as Jane Goodall. Parallel interests in animal and human behaviour have been a central feature of Dunbar's career.

In 1997 he was appointed to a chair in evolutionary psychology at Liverpool University. There are several definitions of his discipline, but all share one central idea: that the behaviour of every organism, human and otherwise, is influenced not only by its present circumstances but by its evolutionary history. Put like that, it sounds pretty innocuous. But to some it is not, even though the controversy it generates is nothing compared with the rows that have dogged its lineal ancestor, sociobiology.

Much of the argument has focused on sociobiology's much-denied determinism: the notion that we are inescapably products of our genes. Evolutionary psychology has faced similar charges, though more mildly put and less often.

"What happened in the late 1970s," Dunbar says, "is that most of the people who were involved in the so-called 'sociobiological revolution' decided, largely for a quiet time, to change the name (of their subject). The term sociobiology ceased to be used. This gave its critics the false impression that it had died the natural death they believed it deserved."

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Dunbar believes it is useful to understand why certain patterns of behaviour have come to be. "For one thing you understand better how to change them," he says.

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When designing incentives to encourage humans to behave well, for example, it might help to know what mix of costs and benefits is prompting them to behave badly. Obvious? Perhaps. But social policy-makers, driven by political and other prejudices, are prone to neglect the issue.

In addition to research and teaching, Dunbar takes a close and rather despairing interest in the public understanding of science - or rather the lack of it. Part of the problem, he believes, stems from the sheer breadth of our accumulated knowledge.

"Science has become so technical and esoteric that it's difficult even for scientists to understand all that's going on. If you look at the history of science from the middle of the 19th century onwards, you find more and more fragmentation, with an increasing number of disciplines."

Now, he says, we've reached the stage where scientists need to know more about what peers in other disciplines are up to. Nowhere is this more the case than in the study of behaviour. The mind's machinery affects what is possible: how flexibly an organism can behave, how long its memory is, what information it can use and so on. And, of course, everything an organism does takes place in a social context. So you also need to know about its life history, its demography, its ethology, its ecology - to say nothing of the developments in genetics.

Dunbar says: "If you go back 30 years, you could write an article on, say, animal behaviour that everyone could understand because it was about robins in the back garden. Nowadays you'd have to preface it with a disquisition on evolution, genetics, physiology, community ecology or whatever. It becomes a much bigger task. The man in the street doesn't have the luxury of the time needed. Nor do the media have the space to provide that information."

For a man who's done more than his fair share of popular science writing, Dunbar takes a surprisingly bleak view of the difficulties of communicating science. "Because it's very media-friendly - it's about things we know and care about in our immediate experience - the complexity behind it is simply lost in the public telling."

Does this matter? Yes. "Most of the debate about genetically modified crops, for example, would not have happened if people had had a better understanding of the science underpinning it."

The Human Story is published this week by Faber and Faber, ?12.99.

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