RIVER OUT OF EDEN

Dawkins (Zoology/Oxford Univ.) returns to the concerns of his The Blind Watchmaker (1986), presenting the case for Darwinian natural selection as the only reasonable explanation for biological diversity. The book's initial premise is that the "purpose" of life is the transmission of DNA down through the generations. Dawkins offers the metaphor of a river branching into myriad substreams to explain the central phenomenon of evolution: Each species has ancestors in common with other species but is in the present day separate and distinct; traced far enough back, each can be related to all the others. Thus, the study of the DNA in human cells (transmitted only from female ancestors), combined with fairly straightforward mathematics, leads to the conclusion that an "African Eve" — one woman who lived some 200,000 years ago — is ancestral to all living humans. (Dawkins hastens to add that she is not the only such common ancestor, nor even, probably, the most recent.) He looks at the roles of predation, cooperation, varying sex ratios, and other "strategies" that organisms develop to promote survival of their DNA. And he disposes, quietly but firmly, with arguments that certain structures in modern organisms — wings, eyes, orchid blossoms — appear so perfectly adapted that no cruder version could accomplish the tasks they perform so well. These structures, in fact, improved in slow increments, states Dawkins. The length of time for natural selection to evolve a complex eye, starting with a light-sensitive spot on the skin and incorporating minuscule changes with each generation, was less than half a million years, and the trick has been done independently at least 15 times. Finally, he considers the question of whether life on Earth is unique, or whether other planets might have evolved intelligent species. Clear and lively, with concrete examples throughout, this account addresses the major issues in modern evolutionary theory without dodging or pulling punches. An excellent overview of the subject.