The diversity of the earth's plant and animal life is amazing—especially when one considers the near certainty that all living things can trace their lineage back to a single ancestor—a bacterium—that lived more than three billion years ago. Taking his cue from Chaucer, noted Oxford biologist Dawkins (The Selfish Gene, etc.) works his way narratively backward through time. As the path reaches points where humanity's ancestors converge with those of other species—primates, mammals, amphibians and so on—various creatures have tales that carry an evolutionary lesson. The peacock, for example, offers a familiar opportunity to discuss sexual selection, which is soon freshly applied to the question of why humans started walking upright. These passages maintain an erudite yet conversational voice whether discussing the genetic similarities between hippos and whales (a fact "so shocking that I am still reluctant to believe it") or the existence of prehistoric rhino-sized rodents. The book's accessibility is crucial to its success, helping to convince readers that, given a time span of millions of years, unlikely events, like animals passing from one continent to another, become practically inevitable.