Points to Zimmer, a senior editor at Discover magazine, for tackling unplowed ground in popular paleontology: no less than the movement of life from sea to land (over 350 million years ago) and the later reverse migration as land mammals returned to the sea. These transitions are dubbed ""macroevolution""--big changes, as opposed to the smaller changes of microevolution. The bare bones of current theory has it that we are descended from lobe-finned fishes. During a wet period when plants were creeping toward the water's edge and swamps abounded with life, these fish developed fins with fingers and toes to maneuver on muddy bottoms and pick at plant life while staying mainly in the water. But one thing led to another, and more land-lubbering species emerged. The one thing Zimmer emphasizes is the role of ""Hox"" genes, which control major events in embryogenesis, such as the shape of the basic body pattern and the formation of limbs from tissue ""buds."" A mutation in timing or patterning of Hox genes can do wonders for changing form and function. The reverse transition from land to sea is an equally complex story and maybe even more controversial. It involves what Zimmer describes as a misfit group of hoofed, long-snouted, carnivorous predators called ""mesonychids"" drawn to the sea for the rich herring and other catches. Subsequent changes over a few million years involved loss of fur, hips, and lower limbs and development of fins and fluke and other essentials of life in the depths. Zimmer uses the latest cladistic diagrams to plot the species splits and changes over time--pointing out that they are at odds with molecular geneticists' DNA analyses, which would have hippos as whales' closest living relatives. Don't hold your breath waiting for resolution on that score. But do credit Zimmer with this scholarly disquisition on two of evolution's most absorbing transformations.