Terrence Deacon
Penguin Press, £8.99, pp 527 
ISBN 0 14 026405 1
———————
Rating: ★★★
Terrence Deacon, fresh from mingling pig and rat brains in his xenotransplantation lab, shows how humans are unique in the use of language and how, through “symbolic reference,” complex networks in the highly developed frontal cortex allow us to think using symbols. This book is in three parts: “Language” focuses on the nature of language and why it is confined to humans, “Brain” shows how the brain produces language, and “Co-evolution” allows a synthesis of conclusions about how both have evolved.
My 8 year old son recently tried a cloning experiment with toenail clippings (human), leaves (plant), and ants (insect) suspended in a jar of water with a battery and some wire. He enjoyed chapter titles like “Using fly genes to make human brains” and “An alien brain transplant experiment.” This book looks like a science fiction thriller, but, in a paperback disguise, this is a classic and heavyweight text on how humans have co-evolved language and higher brain function.
Deacon pinpoints language as starting two million years ago, crediting Homo habilis and Homo erectus with language capability. He describes how quirks of language use such as jokes are fascinating because they provide an insidiously logical punchline that we are unable to anticipate or predict. Mice have a larger brain proportionately than humans, but the human brain is not better because it is big. It is because humans have evolved a smaller, streamlined body, thus focusing neural connections on the important higher cerebral functions. Apoptosis allows highly efficient designing and sculpting of neural connections, through homeobox genes, causing reduction of underused connections and upregulation of useful structures.
Deacon’s theories may be correct. Language research has no animal models but is analogous to recent research into vision and the concept of “plasticity.” Reduced vision to one eye in kittens early in life stops lateral geniculate nucleus fibres competing for synaptic discharges and transmissions to cortical cells. This dynamic process can be reversed, but only when the system is plastic—when early connections have not irreversibly formed. Lesions in adult cats are permanent. Children with Williams syndrome have hyperlexia because of underdevelopment of much of the brain with sparing of the frontal cortex, whereas in autism lack of communication may be due to late maturation of the cerebellum.
Transplanting rat brains into pig brains doesn’t “mess up the neural switchboard,” suggesting that cerebral architecture is preprogrammed in both species. This we would expect. Humans and monkeys have a difference of only 1.5% of their DNA. The human genome project will soon allow us to define the 1.5%, and inserting a gene for neural speech development in humans into monkeys may allow growth of the frontal cortex and larynx at the expense of, say, the sense of smell.
The future looks exciting. Maybe my son’s children will be using xenotransplantation and cloning kits for their science homework.