LIMITS OF LIFE, EARLY EVOLUTION AND THE SEARCH FOR HABITABLE PLANETS

There are very few natural environments on Earth where life is absent. Microbial life on Earth has proliferated into habitats that span nearly every imaginable physico-chemical variable. Only the availability of liquid water and temperature are known to prevent growth and survival of organisms. The other physical and chemical variables that are thought of as extreme conditions, such as pH, pressure, high concentrations of solutes, damaging radiation, and toxic metals, are life-prohibiting factors for most organisms but not for all. Life has adapted to survive and in some cases grow over the entire terrestrial ranges of these variables. Even under conditions where the availability of water and high temperatures render an environment incapable of supporting active life, these conditions do not necessarily render the environment sterile. Many organisms have adapted mechanisms for long-term survival at temperatures more than 100°C above their maximum growth temperature, or in a desiccated state. However, there are some combination of physical and chemical conditions, such as high salt and low temperatures, and high salt and high temperatures, for which no known organisms have been found to grow. The first-order priority in the search for extraterrestrial life is the identification of planets and moons that have measurable characteristics that resemble Earth and meet the habitable conditions that allow the growth of carbon-based life, as we know it. The basic requirements of Earth-life are liquid water, light or chemical energy, and sources of carbon, nitrogen, phosphorus and other nutrients. However, it is possible that an extra-solar Earth-like planet may evolve successful ecosystems and perhaps even highly complex organisms that bare no resemblance to Earth life at the biochemical level or in the way the biosphere modulates atmospheric conditions. The following questions will be discussed in the context of the limits of life and the search for life elsewhere: Do the environmental conditions that limit terrestrial life represent the actual limits of carbon-based life, that is, what are the limitations of evolutionary innovations in carbon-based life? Are there alternate carbon-based biochemistries that would allow organisms to exist under very different environmental conditions than can Earth-life?