Robert Sapolsky lives in San Francisco and is Professor of Biology and Neurology at Stanford University. He is a research associate with the Institute of Primate Research, National Museums of Kenya. He is the author of The Trouble with Testosterone and Why Zebras Get Ulcers. This extract is taken from his newly published book, A Primate's Memoir: Love, Death and Baboons in East Africa (Jonathan Cape, £17.99 in UK)