How humans came to have such large brains is a bit of an evolutionary puzzle. They are very useful now, but many of our talents would seem of little use in the environment of 100000 years ago. In The Runaway Brain
Christopher Wills puts forward the argument that once our intelligence exceeded a certain threshold, continued growth was pretty much inevitable, inependent of the external environment. At least that's what I think he is claiming - my one problem with the book is that although Wills gives us an excellent tour of many areas of evolution and genetics, he never seems to collect up the claims he is making into a form in which they can be judged by the reader.
The book is in four parts. The first looks at 'Mitochondrial Eve', and discusses when she might have lived. The second examines human evolution in terms of the fossils which have been found. The third part deals with genetics an looks at how certain kinds of behaviour are genetically determined. The final part moves on to brain evolution, and it is here that I hoped that Wills would bring all of the threads together to reach a conclusion, but I don't feel that he did. The book is not difficult to read, but goes into a lot of detail, and some readers might find it slow going. It's best suited to the reader who enjoys a book with plenty of challenging ideas, rather than one who wants to get to the end as quickly as possible.