People have long speculated about the possibility of life in settings other …

• Introduction
• What Is Life?
• Why Organic Chemistry?
• The Universal Nature of Biochemistry
• The Physical Limits of Life: The Habitable Zone
• A Genetic Signature of Terrestrial Life
• Genetic Signature of Terrestrial Life
• References
• Figure

Fig. 1. Universal phylogenetic tree based on small-subunit rRNA sequences. Sixty-four rRNA sequences representative of all known phylogenetic domains were aligned, and a tree was constructed by using the program FASTDNAML, correcting for multiple and back mutations. That tree was modified to the composite one shown by trimming lineages and adjusting branch points to incorporate results of other analyses. Evolutionary distance (sequence difference) between the species shown is read along line segments. The scale bar corresponds to 0.1 changes per nucleotide. [Reproduced with permission from ref. 9 (Copyright 1997, American Association for the Advancement of Science).]