As this quote from Mr Spock of the original Star Trek TV series highlights, science fiction writers (and scientists) have imagined many different forms of extraterrestrial life. These range from life consisting of 'pure energy', to life in the thermonuclear furnace of a star, to life within a few degrees of absolute zero which uses superconductivity to provide its energy. It may be rather hard to gain concrete evidence for the existence of such life, at least given our present abilities, even if such things do exist. Perhaps we would be better sticking to life as we know it. In this chapter, I will focus on what our knowledge of organisms in extreme environments might tell us about what is perhaps the greatest unanswered question in science: is there life elsewhere in the universe?
Organisms are found in a wide range of conditions on Earth. There are those that can live at temperatures above 100 °C, at high or low pH, or at high salinities, those that can survive temperatures below 0°C, those that can dry out completely but survive, those that live at high pressures in the ocean's depths and those that can tolerate high levels of radiation exposure. The discovery of the communities of organisms associated with deep-sea hydrothermal vents in the 1970s (see Chapter 2) came as a complete surprise. Here was an ecosystem that did not derive its energy from sunlight but, instead, its primary producers, which produced the organic material, were chemoautotrophic bacteria that obtained their energy from chemical oxidations.
The discovery of extremophiles and cryptobiotes that can live in and/or survive a variety of extreme conditions, and the realisation that some utilise a range of chemical energy sources, means that we now know that life is possible under a much greater range of conditions than was ever dreamed possible. This realisation has helped fuel the present
revival in interest in the possibility of life elsewhere in our solar system and beyond. Life on other planets is not only possible where there are 'normal' Earth-like conditions (moderate temperatures, pressures, pH and radiation exposure, and liquid water) but also where the prevailing conditions are similar to the extreme conditions that some organisms on Earth can survive. Life evolved under conditions on Earth that were more extreme than those existing today and hence may evolve under similar conditions on other planets.
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