Careful study showed that this was not the result of contamination:
In a surprise to scientists, cyanobacteria have been found thriving nearly 2,000 feet below the strange landscape, where sunlight, water, and nutrients are scarce. Researchers previously thought these microbes could survive only while basking in the sun’s rays, although they are otherwise a versatile bunch; researchers have found them alive nearly everywhere on Earth.
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Control samples helped the team determine that the microbes did not come from contamination due to the drilling fluid nor from processing in the lab. And the cyanobacteria were not found in random locations, as you might expect if the samples had been doused in contaminated liquid. Instead, they were congregating along the fractures in the rock, eking out an existence in the tiny pockets of air.
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The cyanobacteria don’t appear to differ greatly from the same kinds of microbes that thrive at the surface. Metagenomic analysis suggests that they are descendants of rock-dwelling lineages who make their living in tough environments, such as in the desert or within shadowy caves.Maya Wei-Haas, “Surprise Life Found Thriving 2,000 Feet Underground” at National Geographic
The microbes, which live on hydrogen gas, demonstrate the adaptability of hardier micro-organisms, leading researchers to wonder about their possible survival in space or on other planets: “Two rovers are set to launch in 2020 to search for signs of life on Mars: the European Space Agency’s ExoMars and NASA’s Mars 2020. Both are equipped with drills to collect rock core in search of ancient microbial life—but perhaps they may dig up something more recent.”
See also: Researchers: Horizontal gene transfer may have helped early microbes move out of hot springs
and
Rob Sheldon: What if the “building blocks from space” are really degraded life? “The data is equally consistent, if not more consistent, with UV degradation of biological molecules made by in situ life. Like Behe’s loss of function or Sanford’s genetic entropy, degradation is far, far easier than synthesis it is only the resolute rejection of the 2nd law of thermodynamics that makes these stories plausible.”