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Planet Earth

#77: Did an Early Pummeling of Asteroids Lead to Life on Earth?

Early organisms apparently survived the Late Heavy Bombardment—which may have made our planet a much comfier place to live.

By Michael D LemonickDecember 22, 2009 6:00 AM


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The “late heavy bombardment” of asteroids that clobbered Earth and the rest of the inner solar system for 20 million to 100 million years, ending 3.85 billion years ago, is generally regarded as one of the most hostile eras in our planet’s history. Collision after collision would have blasted and heated the surface, wiping out any primordial organisms trying to eke out an existence. New studies are turning this view on its head, however, hinting that the ancient rain of asteroids may actually have established a more congenial environment for biology to take hold.

There already were clues that the late heavy bombardment was not the full-on killer it was once thought to be, says planetary scientist Oleg Abramov of the University of Colorado. The earliest evidence for living organisms dates back almost exactly to the time when the rain of asteroids ended. Unless life appeared nearly instantly, it must have survived the onslaught. Abramov’s study, published in the May 21 issue of Nature (and covered in The Loom), shows how that could have happened. Certain modern bacteria thrive deep underground; their ancestors may have done the same, he argues.

In fact, some scientists think life might have originated in subsurface hydrothermal systems. “Nobody had calculated how far sterilization would have extended below the surface,” Abramov says. He and his coauthor, geochemist Stephen Mojzsis, also of the University of Colorado, found that if early bacteria were living more than 4 kilometers (2.5 miles) down, the impacts could have helped life by creating more hot-water-filled cracks for microbes to inhabit.

Soon after, the late heavy bombardment might have aided life on the surface, too. Back then the sun was so dim that Earth should have frozen solid, and yet geochemical evidence indicates the presence of oceans 4 billion years ago. One proposed explanation is that heat-trapping greenhouse gases kept things balmy, but it was not clear where those gases could have come from. Geochemist Richard Court of Imperial College London published a report (covered in 80beats) showing that impacting rocks would have shed tremendous quantities of carbon dioxide and water vapor, both of which effectively trap heat. “People had always known that the bombardment would have changed the atmosphere’s chemistry,” he says, “but nobody had really done the experimental work.”

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