Lynn Margulis, Distinguished Professor in the Department of Geosciences at the University of Massachusetts at Amherst, has been a member of the National Academy of Sciences since 1983. She is best known for her pathbreaking work on the bacterial origins of cell organelles and for her collaboration with James Lovelock on Gaia theory. Her previous books include Symbiosis in Cell Evolution Five Kingdoms (with K. V. Schwartz) and (with Dorion Sagan) Origins of Sex, Garden of Microbial Delights, What Is Life?, What Is Sex?, and Slanted Truths: Essays on Gaia, Symbiosis and Evolution.Lynn Margulis, a member of the National Academy of Sciences and a recipient of the 1999 Presidential Medal of Science, is Distinguished University Professor in the Department of Geosciences, University of Massachusetts, Amherst. Dorion Sagan is the author of Biospheres and the co-author of Up from Dragons: The Evolution of Human Intelligence. He lives in New York City.