They have also found one type of archaea in about a third of their samples from all over the world, and in all the archaeal communities sequenced. Like the krill that feed a plethora of other animals in the oceans, it may be a keystone species that needs to be present for such primitive organisms to thrive, Colwell said.
Very similar life forms have also been found in communities in wildly differing environments. So either evolution has forced them to evolve to the subsurface in similar ways, or these organisms share an ancient root close to the origin of life.
But interpreting the results takes caution, Colwell said.
Because there are so few of the deep-dwellers and they reproduce so slowly, any whiff of contamination from quickly-growing, plentiful surface microbes can drown out the faint genetic signal from these bacteria.
The dark-dwellers reproduce only every few months or years and have glacially slow metabolisms, with some organisms moving the equivalent of just a few electrons per second, said Jens Kallmeyer, a geochemist at the University of Potsdam in Germany.
"We cannot understand how an organism can possibly survive on that little energy," Kallmeyer told LiveScience.
The findings have broader implications for life on Earth. For one, deep bacteria, like their aboveground brethren, play a role in the breakdown and cycling of carbon in the environment. That, in turn, affects how much carbon dioxide reaches the atmosphere and alters the climate.
But perhaps the greatest insights these groundlings can tell us is about life on other planets.
These creatures are living at the very edges of hospitable environments — with strange and scarce energy sources, little to no water and scorching heat. Many spend their lives bound to minerals, and move only with the random motion of molecules in the sediments. As a result, they can tell us a lot about the limits of life in the harsh environments of other planets, Amend said. [5 Bold Claims of Alien Life]
Many scientists think life on Earth may have emerged from hydrothermal vents in the seafloor, so these primitive subsurface dwellers could reveal insights into the first primitive cells that started life on Earth, Amend said.
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This story originally appeared on LiveScience's OurAmazingPlanet.