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75 Living things on Earth, including humans, alter their environments and thereby influence their own evolution in a process known as “niche con- struction.” Organisms on other worlds also might modify their environ- ments and the selection forces they experience.** We encountered our first exotic biosphere in 1977, when researchers found strange forms of life clustered around vents on one of Earth’s mid-ocean ridges. There, beneath thousands of feet of water, far out of the reach of sunlight, was an entire ecosystem living off heat and chemicals from the Earth’s interior. Some researchers speculate that we might find similar forms of life in the global ocean believed to underlie the icy surface of Jupiter’s moon Europa. Extrasolar planets may evolve unexpected biospheres that are even more exotic. Life can exist in a much wider range of conditions than we once believed. Research has shown that terrestrial microorganisms can survive in boiling water or freezing cold, even in the interstices of rocks far below the Earth’s surface. Certain types of heat-loving bacteria (hyperthermophiles) are able to replicate at temperatures above that of boiling water. Cold-loving (psycrophilic) organisms are plentiful at temperatures below freezing in Antarctica. Others function in highly acid or base solutions (acidophiles and alkaliphiles), in intensely salty brines (halophiles), under crushing high pressures (piezophiles or barophiles), or in extremely dry conditions (xerophiles). Living things have evolved to deal with both metabolic and external sources of DNA-damaging agents by developing elegant mechanisms that repair the damage. Some bacterial types thrive in the interiors of working nuclear reactors.” Our definition of the Earth’s biosphere has greatly expanded. Simpler organisms (prokaryotes) have been found 3 kilometers below the surface, with some evidence suggesting that this can be extended to at least 4 kilo- meters (2.5 miles). Researchers have discovered prokaryotic populations as deep as 800 meters below the sea floor; they occur in sediments through- out the world ocean and may constitute as much as one-third of Earth’s total living biomass.” Astronomer Thomas Gold proposed that the “deep, hot biosphere” could be more extensive than the surface biosphere we know best. The bulk of terrestrial life may dwell in the rocks, down to a depth of 10 kilo- eentaen ae nen We may have two separate biospheres on Earth. Planetary scientist Von Eshleman suggested that near-surface and subterranean life-forms are essentially independent; either could exist without the other. Life might Extremophiles and Dual Biospheres Extremophiles and Dual Biospheres meters or more.