A BIG book about BIG questions--""Who are we? Where do we come from? Why are we this way and not some other? What does it mean to be human?""--with all the Sagan/Druyan trademarks: crystal-clear scientific exposition, a dash of pseudotheology, and lots of big numbers. Sagan and wife Druyan begin with a standard recital of the origins of sun, planets, and life on earth, but soon move on to their central theme: the triumph of evolutionary theory and the truths unveiled through study of our animal kin. An enjoyable brief life of Darwin sets the tone: enraptured regard for scientific orthodoxy, couched in lucid prose with playful asides (an imagined Hollywood version of the classic Wilberforce-Huxley debates on the origin of species). When the authors turn to the role of DNA in speciation, those big numbers start cropping up: ""ten trillion or so cells of your body""; ""perhaps a billion AGCT nucleotide pairs."" And so does the amateur theology, in which Sagan and Druyan plump hard for deism, the notion of a God who created the universe and then absconded. Things rocket forward when the authors focus on the gaudy canvas of sexuality, consciousness, language, and so on in the animal kingdom. A barrage of eye-popping anecdotes leaves the impression that animal social life is a perpetual power play, with dominant males on top and submissive females on the bottom. Many parallels are drawn with humans--e.g., teenage boys playing ""chicken"" to establish a pecking order. Since the authors contend that no sharp line divides animals and humans, this leads to troubling conclusions about human nature. But Sagan and Druyan think we can overcome our animalistic impulses, although they never quite explain how: The ""shadows of forgotten ancestors"" weigh heavily on us all. Too long and preachy, but, still, crack science-writing for the masses, and the Sagan name will vault it onto the charts.