Attempts to discern the evolutionary origins of human intelligence often begin with fossils, but, as Byrne points out at the start of his book, it is possible to draw widely differing pictures from the available evidence. Semi-popular accounts of early humans have portrayed them variously as daring hunters, peaceful vegetarians, and scavengers from carnivores' kills. As for the Neanderthals, at one time we are told they were sensitive and religious people who buried their dead with flowers and spoke a human language, and next moment we are given to understand that they were scarcely distinguishable in their behaviour and mentality from gorillas and chimpanzees.
Byrne hopes to present a more balanced and objective view of early human mentality. He does this in three ways. First, he looks for reliable differences in intelligence between living animal species and considers in what ways these differences contribute to their survival. Second, he deduces from this what the intelligence of their extinct ancestors probably was like. Third, he tries to decide which selection pressures would have favoured the evolution of these characteristics.
A fundamental question he confronts in taking this approach is to ask what intelligence actually is and what purpose it serves. Even if we accept that IQ tests measure something objective in humans, it is not possible to give such tests to animals since mostly they depend on language.
Much of the book is taken up with discussion of learning and imitation in chimpanzees and gorillas and what we can conclude about their intelligence. All this is quite contentious material, as Byrne acknowledges. A good feature of the book is that all the chapters end with a short section of suggested further reading, which will be useful for anyone who wants to see what others have said on the matters discussed.
Having established that certain animals can usefully be called 'intelligent', Byrne turns to the question of what intelligence is good for. It certainly allows complex mental mapping of the surrounding territory and the use of tools, both of which are no doubt useful in obtaining food. But a current favourite as an explanation for primate intelligence is its role in complex social interactions. In other words, success in a social group depends more on whom you know than on what you know.
Byrne is not entirely convinced by the case for a strong social component in the development of intelligence. He points out that both apes and monkeys mostly live in groups of comparable size, so there must be some other factor that explains the difference in intellectual capacity. Byrne suggests that there were environmental reasons that led the ancestor of apes and humans to develop intelligence. This ancestor was presumably African.
Humans and chimps diverged about 6 million years ago. The development of language was a major event that occurred at some indeterminate point after the chimp/human split, and it must have been preceded by the acquisition of the ability to imagine other individuals' minds. This is present in apes.
This book provides a stimulating and readable account of how human intelligence may have evolved. It covers a lot of territory and draws together lines of argument that are more often found in separate areas of science.
18 November 2009