Scientists are releasing the results of a decade of research that has uncovered a vast and mysterious world of underground microbes that could help explain how life began on Earth and how it might look on other planets.
"The branches of life we know are not all there is," said Karen Lloyd of the University of Tennessee. "There are indeed deep branches in the tree of life that no one has ever known before and many of them are underground."
Lloyd is one of hundreds of scientists around the world involved with the Deep Carbon Observatory, which studies the action of carbon many kilometers deep. Team members who analyze how carbon makes life underground should present their findings this week at the annual meeting of the American Geophysical Union.
Scientists know that microbes may live far below the surface since they were discovered in oil deposits in the 1920s. But over the past 10 years, it has become clear that the genetic diversity of tiny creatures living in the microscopic pores and veins of rocks under our feet is at least equal to that above the surface.
Nearly three-quarters of Earth's bacteria and archaea (a type of microbe) live underground, scientists say. All types of cell life, including the type that makes up the flora and fauna of the surface, are found there.
These animals colonized up to 2.3 billion cubic kilometers – almost double the volume of the oceans. Some live in temperatures up to 121 C; others gain an existence more than 10 kilometers below the seas.
In total, the amount of carbon the bacteria contain is between 245 and 385 times greater than that found in humans.
It's a strange world, where microbes draw energy from sources like radiation, "eat" sulfur and "breathe" rust.
"This is how life can stand, even if there is no potential to gain the sun's energy," said Barbara Sherwood Lollar of the University of Toronto. It found waterborne microbes of 2.7 billion years deep within the Canadian Shield.
"They breathe other things," said Rick Colwell of Oregon State University. "Many of them are able to use oxidized iron the way we use oxygen."
Sherwood Lollar calls this chemosynthetic life as opposed to life on the surface, which depends on photosynthesis.
Things happen slowly down there. The microbes from the depths can go hundreds or even thousands of years without creating a single new cell. Instead, they repair the damaged molecules and wait for the next earthquake to open a new channel for nutrients.
"These things happen at much longer timescales than we are accustomed to expect," Lloyd said.
Many questions remain. Among the most intriguing is whether life has filtered from the surface or has grown from below.
"Given the fact that there are all the sources of energy and carbon needed to propel life deep into the subsurface, and much of our history included a very inhospitable surface world, it is reasonable to suppose that life originated underground, "Lloyd said.
The same logic applies to other planets, Sherwood Lollar said.
"If we go to other planets, it is possible that the photosynthetic life has never arisen, but maybe the chemosynthetic life has happened," she said.
"It's part of the reason why studying these deep chemosynthetic organisms on this planet is so directly relevant to the pursuit of life on other planets."
It's rare for a scientist to help reveal a whole new outlook on life, Lloyd said.
"We think we have a handle … in general, so all of this is thrown out the window, and every turn we discover that nature has so many surprises in store for us."
– Follow Bob Weber on Twitter at @ row1960
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