Over the last 530 million years there have been five mass extinctions of species-the last,65 million years ago,when the dinosaurs disappeared.The biodiversity of our planet may now be on the verge,Leakey and Lewin believe,of a sixth extinction,caused this time by the relentless expansion and limitless appetites of human beings. The new science of 'biodiversity',presented clearly and cogently by Leakey and Lewin,combines insights from palaeontology,biology,ecology and even economics.It integrates the role of Darwenian evolution with the increasingly recognised importance of external and unpredictable forces.