are not actually known. There are suggestions that complex life could arise in sub-surface conditions which may be similar to those where life may have arisen on Earth, such as the tidally heated subsurfaces of Europa or Enceladus. Ancient circumvental ecosystems such as these support complex life on Earth such as Riftia pachyptila that exist completely independent of the surface biosphere. Notes References Further reading A defense of the Rare Earth Hypothesis by a UK geologist. Henderson, Lawrence Joseph (1913). The Fitness of the Environment. The Macmillan Company Gonzales, Guillermo; Richards, Jay W (2004). The Privileged Planet. Regnery Publishing, Inc. External links Home page of Rare Earth (archival) Reviews of Rare Earth: Athena Andreadis, PhD in molecular biology. Kendrick Frazier , editor, Skeptical Inquirer. Gregg Easterbrook, "Are We Alone?" The Atlantic Monthly, August 1988. Article that anticipates REH in some respects. Solstation.com: "Stars and Habitable Planets." Earth Astrobiology Origin of life Fermi paradox Hypotheses Astronomical hypotheses