Nearly a century and a half after Charles Darwin formulated it, the theory of evolution is still the subject of considerable debate. Oxford scientist Richard Dawkins is among Darwin's chief defenders, and an able one indeed-- witty, literate, capable of turning a beautiful phrase. In River Out of Eden he introduces general readers to some fairly abstract problems in evolutionary biology, gently guiding us through the tangles of mitochondrial DNA and the survival-of-the- fittest ethos. (Superheroes need not apply: Dawkins writes, "The genes that survive . . . will be the ones that are good at surviving in the average environment of the species.") Dawkins argues for the essential unity of humanity, noting that "we are much closer cousins of one another than we normally realize, and we have many fewer ancestors than simple calculations suggest."