Book excerpt: How great is the evolutionary distance between humans and apes, and what is it that creates that gulf? Philosophers and scientists have debated the question for centuries, but Michael Corballis finds the mystery revealed in our right hands. For humans are the only primates who are predominantly right handed, a sign of the specialization of the left hemisphere of the brain for language. And that specialization, he tells us, makes a massive distance indeed, as he describes what exactly it means to be the lopsided ape. In The Lopsided Ape, Corballis takes us on a fascinating tour of the origins and implications of the specialization of the two halves of the brain--known as laterality--in human evolution. He begins by surveying current views of evolution, ranging from the molecular level--the role of viruses, for instance, in transporting genes between species--to the tremendous implications of such physical changes as walking on two feet. Walking upright freed our ancestors' arms for such things as tool-making and gesturing (a critical part of early language). Corballis argues that the evolution of the brain--and language--was intimately tied up with these changes: The proliferation of objects made by early hominids, in an increasingly artificial environment marked by social cooperation, demanded greater flexibility in communication and even in thinking itself. These evolutionary pressures spurred the development of laterality in the brain. He goes on to look at the structure of language, following the work of Noam Chomsky and others, showing how grammar allows us to create an infinite variety of messages. In examining communication between animals and attempts to teach apes and dolphins language, he demonstrates that only humans have this unlimited ability for expression--an ability that he traces back through hominid evolution. After this engrossing account of what we know about evolution, language, and the human brain, Corballis suggests that the left hemisphere has evolved a Generative Assembling Device, a biological mechanism that allows us to manipulate open-ended forms of representation and provides the basis for mathematics, reasoning, music, art, and play as well as language and manufacture. It is this device, he writes, that truly sets us off from the apes. Both a detailed account of human language and evolution and a convincing argument for a new view of the brain, The Lopsided Ape provides fascinating insight into our origins and the nature of human thought itself.