At the core of the "Social Brain" hypothesis is the suggestion that social group size in primates is limited by the relative size of the neocortex. This posed a challenge for the early hominins who moved beyond the forest habitats occupied by their ancestors, as in open habitats, they would be more vulnerable to predators and their main defence would be that of a belonging to a larger group. At the same time, increased group size also makes direct contact between individuals more difficult. Physical grooming - a primary means of maintaining relationships in primate groups - must be replaced by other, more symbolic connections (such as language) if the group is to remain cohesive. In meeting these challenges, our ancestors embarked on a process of brain evolution which facilitated larger effective group sizes and the transmission of innovations over long distances and between widely scattered groups. This capacity to handle large, dispersed, social networks is the key to human evolution.

It's a persuasive thesis. However, it is significant that this book does not represent a balanced synthesis of all the disciplines which have contributed to the "Social Brain" hypothesis. Rather, it emphasises the work of the 7 year long "Lucy to Language" project which focused on the archaeological evidence. and the book reflects the strengths and weaknesses of this emphasis.

I tend to look for strong evidence and assess it objectively. I found the evidence for the rule of three and Dunbar's number, as presented, a little weak, given that humans tend to find patterns even in random numbers. More particularly, the aspect that I found most disconcerting is that, in places, the book asserted the social brain hypothesis as though it were established fact, thereby displaying a rather worrying bias, given that many, if not most, people do not accept that the hypothesis is well established. In other parts, the book was much more cautious about the speculation that group size drove brain size and hence human evolution. With the addition of more objective skepticism, the account made for more pleasant and indeed, for me as an outsider, very interesting reading.

It is relevant to add that, great apes aside, there is much less evidence for the social patterns of our hominid ancestors and one can well take the view that even wild speculation is better than nothing (provided that it is not taken too seriously). In Thinking Big, the speculation is mostly carefully explained. All in all, it seemed to me to be a valiant attempt to peer through the mists surrounding our prehistoric past, albeit that I did not always find the assertions about the glimpsed apparition convincing. ( )