Earth may be a (relatively) cozy blue marble today, but it wasn't always so hospitable. For the first billion years, life had to survive on an eruptive, asteroid-pummeled wasteland with no oxygen at all. Worse still, the ancient Earth was awash in UV radiation—the stuff we use to sterilize hospital clean rooms and laboratories.
"Microorganisms are very resistant to a range of harsh conditions—extreme temperatures, vacuum, cosmic radiation," the astrobiologist Gerda Horneck, of the German Aerospace Center, told me over the phone. "The exception is solar UV radiation."
But we know that life did survive—just not how. While we may never be sure (unless we get lucky and dig up our origin story on the moon), research published in the International Journal of Astrobiology offers a promising explanation. Cyanobacteria—the photosynthetic microbes that, long ago, terraformed our atmosphere—might have withstood the sun's deadliest rays by burying themselves inside asteroid-impacted rocks.
Some 2.5 billion years ago, the Earth was irrevocably transformed as cyanobacteria began pouring oxygen into our atmosphere. The Great Oxygenation Event not only spurred the evolution of oxygen-breathing life, it created the stratospheric ozone layer, which has since shielded fragile organics from DNA-shattering UV. Before the Great Oxygenation, life was forced to hide out in refugia—caves, deep sea vents and the like—where it could avoid the scorching sun.
Scientists have long wondered how the upstart cyanos were able, in those early days, to gain the critical mass to terraform an entire planet. Asteroids might have helped. Back when life was just getting started, space rocks were pummeling the Earth with a thousand times the ferocity they are today. Porous, asteroid-shocked rocks could have shielded microbes from the brunt of of the sun's UV while still letting in enough sunlight for photosynthesis to continue.
To test that idea, Casey Bryce, a PhD candidate at the University of Edinburgh, collected samples of asteroid-bashed gneiss from the Haughton Impact Crater and cells of the cyanobacterium Chroococcidiopsis, and flew them up to the International Space Station. There, rocks were inoculated with microbes, and, for twenty two months, left to tough it out in the cold, radiation-filled vacuum of outer space.
Control samples—microbes that weren't buried in a rock—were quickly fried by the fierce radiation of outer space. But after nearly two years of exposure, rock-buried cyanobacteria managed to survive.
"These data provide a demonstration that endolithic [ie, rocky] habitats can provide a refugium from the worst-case UV radiation environments on young planet," Bryce and her co-authors wrote. "This result could extend to other terrestrial-type rocky planets lacking a sufficient atmospheric UV radiation shield."
That is, a means by which life might survive on Mars, or another oxygen-free but potentially-habitable exoplanet. Another reason to think carefully before discounting the possibility of life on any potentially habitable world—even one which appears to be an asteroid-pummeled wasteland. Four billion years from now, that blasted rock could be the next Earth.