The Extremophiles Handbook defines extremophiles such that, “Extremophiles are organisms that are adapted to grow optimally at or near to the extreme ranges of environmental variables,” and Holger Jannasch has noted, “The term ‘extreme’ is based on an average range of conditions for all known forms of life, especially of the eukaryote domain with a slight anthropocentric tinge.”

Extremotolerant organisms can tolerate extreme environmental conditions; extremophiles require extreme environmental conditions in order to live. What are extreme environmental conditions? Any departure from the mesophilic norms of environmental variables. These mesophilic norms can represent any of the parameters of habitability such as temperature, atmospheric pressure, pH, salinity, radiation, toxicity, and the chemical environment.  

Before extremophiles were identified as a distinct class of organisms, there was already an existing scientific literature on halophiles, psychrophiles, and thermophiles, that is, organisms that live in highly saline environments, extremely cold environments, and extremely hot environments. The first paper to introduce the term “extremophile” was R. D. Macelroy’s 1974 “Some Comments on the Evolution of Extremophiles,” which cites prior studies of halophiles and thermophiles.   

Macelroy’s paper concludes with the reflection that no common mechanism is known for all extremophiles. Since the extremophilic conditions to which organisms can adapt include the entire range of environmental variables, adaptation to extreme environmental conditions may be opportunistic, involving whatever mechanism is immediately impacted by the extreme conditions. However, if the evolutionary order is reversed, and life originated with extremophiles, mesophilic organisms are an adaptive radiation from extremophilic ancestors, and may retain, in a modified form, biological mechanisms originally adapted to extreme environmental conditions.