The controversial Gaia Hypothesis of James Lovelock of Coombe Mill, Launceston, Cornwall, U.K., and his colleagues variously contends that throughout Earth history the global biosphere has influenced, even controlled, the physicochemical evolution of Earth's environments (especially oceans and climate) for its own benefit. Since the origin of life, the biosphere has influenced selective pressures on evolution, maintained the Earth in a kind of homeostasis, and thus created an environmental optimum through time, regulated by and for the biosphere. Rarely has a hypothesis immediately sparked such passionate response. There is something in it for everybody, from hard core scientists to philosophers, ultraconservationists, students of world religions, mystics, politicians, and space enthusiasts; they were all there in San Diego, March 7–11, 1988, for the AGU Chapman Conference on Gaia Hypotheses. For 4 days an impressive list of specialists presented and debated the pros and cons of Gaia Hypotheses from diverse perspectives: modern and ancient biology, ecology, biochemistry, the physicochemical systems of the Earth, oceans, and atmosphere, and the evolution of the solar system. Focus was on modern to Pleistocene atmosphere-ocean-Earth systems, case histories of their interaction with the biosphere, and relatively simple models drawn from these observations and projected back through time. Equivalent studies on the geological and paleobiological history of the Earth-life system over the past 3.5 b.y. were underrepresented. Extended debates that followed generally strong presentations were lively, argumentative, and remarkably civil despite widely held views. The grace with which Jim Lovelock moved between his strongest critics and supporters set high standards for the debates. Everybody acknowledged a high learning curve.