This book explores the possibility that life exists on Mars. It provides an interdisciplinary overview of the early evolution of life in hydrothermal ecosystems on Earth, focusing on the problem of remote sensing and incorporating geological work relevant to the search for evidence of early life on Earth and Mars. It discusses the belief that studying thermal spring deposits as part of this search may be the best opportunity to test whether life on earth is a "unique experiment," or whether there is life elsewhere in the solar system.