Book Reviews

At the close of the decade of the Brain, we are faced with the inevitable question: what have we learned? Do we understand the brain better 10 years on than we did amidst all the hype and excitement of the chase a decade ago? The answer must, I suppose, be a quali®ed ``yes’’, tinged with an awful lot of ``no’’. We now know a great deal more about the mechanisms of brain development and quite a lot more about certain components of the brain (vision is something that springs to mind). But there remains a great deal we don’t know about the higher social functions of the brainÐthose diffuse and hard-to-pin-down functions that are really the core to what we do. In this lavishly illustrated book, John Allman does not seek to ®ll in that missing piece of the jigsaw, though the social brain does make a brief appearance in the later stages. Rather, his main concern is with giving us a broad overview of everything. The opening chapters detail the basic physiological mechanisms and the genetic processes that underpin brain evolution (notably the importance of homeotic genes and their role in segment duplication). The story of brain evolution is sketched in broad taxonomic scope, and there is a quick guide to the history of neuroscience from phrenology onwards. There are some excellent sections on visual maps in the cortex and the evolution of chordate visual systems (not forgetting those honorary chordates, the cephalopods, who have contributed so much to our understanding of the brain), as well as a fascinating chapter on the evolution of mammalian brains. Setting brain evolution into its ecological context (brains are, after all, one of themost energetically expensive organs in the body) provides an important reminder of the fact that evolution is driven at one end by the functional demands of more ef®cient sensory and executive systems and at the other by the mundane constraints of wiring ef®ciency and the costs of homeothermy (without which large brains could not be supported). As biologists never tire of pointing out, there is no such thing as a free lunch: every evolutionary advance creates additional problems that have to be surmounted before the bene®ts can be gained. Primates mark the extreme in this respect, for they have larger brains for body size than any other group of animals. Because primate brains are so similar to our own (we all share these disproportionately enlarged neocortices), Allman devotes two whole chapters to them. One of the most signi®cant early developments in primate brain evolution was binocular vision. Conventional wisdom supposes that this arose because primates needed to be able to judge distances while scampering about in the treesÐa claim that has never looked too convincing, given that squirrels manage the same without the bene®t of full binocular vision. Allman suggests, perhaps more plausibly, that primates have binocular vision because they, unlike squirrels and their ilk, are predators, and that places much higher demands on accurate distance judgement. And indeed, most of the early primates (and many of the contemporary prosimian primates) are insectivores. The story of the visual system’s remarkable architecture from Hubel and Wiesel’s pioneering work onwards forms a major component of the penultimate chapter. In primates, of course, this story extends into colour vision, for this marks a second major innovation in primate brain THE QUARTERLY JOURNAL OF EXPERIMENTAL PSYCHOLOGY, 2000, 53B (4), 373±379