for National Geographic News
Over the past 20 years scientists have warmed up to the idea that the majority of life on our planet lives not on Earth's surface but beneath its crust. The theory has spurred new ideas about life's origins on Earth and where to look for life on other planets.
Earth's crust gets warmer the closer it is to the molten iron-nickel believed to be at our planet's core. One question that scientists who study life beneath Earth's crust face is, at what temperature is it too hot for life to survive?
Since scientists believe Earth at one point was mostly molten, the answer to the question may shed light on how early life could have first evolved on our planet.
"If Earth had to cool to a certain temperature at which life was possible, maybe the high-temperature life could have existed that much sooner," said Derek Lovley, a microbiologist at the University of Massachusetts, Amherst.
Much of this life beneath the crust, which scientists refer to as biomass, are microbes that use hydrogen and minerals like iron to get energy from food sources in the same way that humans use oxygen to obtain energy from our food.
Lovley is at the forefront of research into such microbes. He has discovered dozens of different species, including Strain 121, a microbe that grows at 121° Celsius (250° Fahrenheit)the highest temperature currently known for life.
The ability to grow at 121° Celsius is significant because for over a century it has been the temperature used to sterilize medical equipment. Scientists thought that such temperatures would kill all life-forms.
"It's kind of a benchmark," Lovley said. "This is like breaking the four-minute mile."
Strain 121, which goes dormant at temperatures below 80° Celsius (176° Fahrenheit), lives in environments known as hydrothermal vents on the ocean floor. The vents spew hydrogen- and mineral-rich hot water from deep in the Earth's crust to the surface.
For several years scientists have known that other microbes survive in and around hydrothermal vents at temperatures above 100° Celsius (212° Fahrenheit). Strain 121 just "opens that window where life can exist a little bit wider," Lovley said.
Jack Farmer, an astrobiologist at Arizona State University in Tempe, said that opening this window for life on Earth expands the potential for life to develop and persist elsewhere in the solar system and beyond.
"As the upper temperature limit for life has increased, new opportunities for habitable environments have opened up, and subsurface hydrothermal environments are among the most important," Farmer said.
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