Robin Dunbar's The Human Story: a New History of Mankind's Evolution is a popular account of anthropological research on what it means in evolutionary and behavioural terms to be human. If this sounds trite or hackneyed consider that poor old homo sapiens has had to endure two quite profoundly humbling scientific findings over the past 15 years or so. One is the realisation that we humans share a common ancestor with chimpanzees more recently than chimpanzees do with the other apes. Some have even suggested that we are just a third chimpanzee species; the other two being the common chimpanzee and the bonobos. Being lumped with these two shouldn't be taken lightly. Common chimpanzees can be aggressive and warlike. Bonobos are small, and the females are hyper–sexually active, normally with each other. The second realisation began to emerge in the middle 1990's and hinted at an extraordinary degree of genetic overlap between humans and chimpanzees. Now that most or all of the genes of both species have been mapped, we must accept that we are at least 98 per cent similar in our genetic make–up to chimpanzees. In fact, it is even more extreme than that figure suggests. So far, there are only two or three genes present in one of the species that are not found in the other and they do not stand out as having unusual functions. One, for example, acts to attach sugar molecules to a protein. The upshot is that there is no obvious genetic holy grail defining what it means to be human.

So what is left to distinguish us? Anthropologists like Dunbar are not content with phenomenological accounts of differences between the apes and humans. Yes, of course, we have art and literature, religion and politics, science, philosophy and mathematics. But what evolutionary features underpin these manifestations? Happily, three stand out.

One is that while other ape lineages slowed to an evolutionary crawl with respect to changes in their morphology, the lineage that would lead to humans rocketed forward producing a tall, gracile, upright and intelligent ape with a brain about three times the size of a chimpanzee's. Why us? Dunbar has been undeterred in his pursuit of the idea that our large brains evolved principally to manage our social lives. Small brains, in Dunbar's view limit our ape cousins' abilities to plan, negotiate, cajole and inveigle our family, friends, and enemies. But these things come naturally to us.

Possibly as a consequence of evolving a 'social brain', Dunbar believes we acquired something called a 'theory of mind'. Descartes, as everyone knows, searched our brain for the seat of our souls, regarding souls as the quintessentially human trait, even if a God–given one. He found them in our pineal glands, but modern evolutionary thinking has abandoned souls in favour of the unmatched human capacity to think in terms of 'I know, that you know, that I know' relations. The capacity probably resides in our neo–cortex, which, as the name implies, is a modern evolutionary innovation. Some animals seem to have a sense of what another animal might know, but humans almost seem to be able to read minds. This makes us empathic and kind, and devious and sly. A theory of mind is the calling card of a truly insightful social animal.

Our very large brains and capacities for extreme sociality have been evolving for perhaps the last 500,000 years, and at some point along this trajectory language appeared. Dunbar adopts an intriguing position on the origin of language, seeing it as taking over many of the social functions of physical contact and grooming that for many monkey species take up a considerable part of each day. Some may find these views of the evolution of language surprising, but they bear careful study. Much research agrees that the majority of our communication is gossip of one form or another. Just think of all those people on the train texting on their mobile phones. They are not writing books.

Dunbar has been active in research on many of the topics in this new book and so brings to it an easy and confident style that will appeal to a popular audience. The book is pleasingly illustrated with anecdotes and vignettes, and provides, in one place, an interesting and up to date account of why we are not just a third chimpanzee.

The Human Story is available from Amazon (UK)