Like squids, scientists protect themselves with clouds of impenetrable ink. Not Carl Sagan. His jargon-free book The Cosmic Connection … involved thousands of readers in the search for life beyond earth. Last year, during the Mars probe, he became a TV celebrity with plausible descriptions of the creatures that might be populating outer space. The Dragons of Eden should involve thousands more in the exploration of inner space—the human brain.

Sagan, 42, occupant of a chair in astronomy at Cornell University, is not a neuroscientist. But he writes about the brain with uncommon sense and even humor….

The Dragons of Eden begins with a summary of how and when intelligence developed in various terrestrial species. In detail, Sagan describes the process of natural selection working toward the emergence of the creature Shakespeare called "the paragon of animals." Sagan also explains differences in the structure of the paragon's brain and...