Christopher Wills is Professor of Biology at the University of California at San Diego and winner of the 1999 AAAS Award for Public Understanding of Science and Technology. His books include Yellow Fever, Black Goddess and Children of Prometheus. Jeffrey Bada is Professor of Marine Chemistry and Director of the NASA Specialized Center of Research and Training in Exobiology at the Scripps Institution of Oceanography in La Jolla, California. An instrument he designed to detect traces of amino acids on Mars was recently selected for a mission to the red planet sometime later this decade.