Life on Earth may have first emerged 4.4 billion years ago: study

LONDON, May 21 -- Life on Earth may have begun about 4.4 billion years ago, several hundred million years earlier than previously thought, researchers who had studied asteroid bombardments, reported on Thursday.

Our planet formed 4.6 billion years ago, and many scientists believe it was sterilized by numerous asteroid impacts during the so-called Hadean Eon, 4.5 billion to 3.8 billion years ago, particularly through a cataclysmic event known as the Late Heavy Bombardment about 3.9 million years ago, making it impossible for life to survive.

But Oleg Abramov and Stephen Mojzsis of the University of Colorado at Boulder, found that the bombardments would have melted only a fraction of Earth's crust, and that microbes could well have survived in subsurface habitats, insulated from the destruction.

"These new results push back the possible beginnings of life on Earth to well before the bombardment period 3.9 billion years ago," Abramov said. His paper was published in the British journal Nature.

"It opens up the possibility that life emerged as far back as 4.4 billion years ago, about the time the first oceans are thought to have formed," he said.

The researchers used data from Moon rocks, impact records from the Moon, Mars and Mercury, and previous theoretical studies to build three-dimensional computer models to replicate the bombardments.

The models indicated that at the worst time only 37 percent of the Earth's crust melted and less than 10 percent experienced temperatures above 500 degrees Celsius.

"Even under the most extreme conditions we imposed, Earth would not have been completely sterilized by the bombardments," Abramov said.

Instead, hydrothermal vents may have provided sanctuaries for extreme, heat-loving microbes known as hyperthermophilic bacteria following the bombardments, the researchers said.

Even if life had not emerged by 3.9 billion years ago, such underground havens could still have provided a "crucible" for life's origin on Earth, they said.

The researchers concluded that subterranean microbes living at temperatures ranging from 80 degrees to 110 degrees Celsius would have flourished during the Late Heavy Bombardment, which is thought to have lasted for 20 million to 200 million years.

The models indicated that underground habitats for such microbes increased in volume and duration as a result of the massive impacts.

"Our results strongly suggest that no events since the Moon formation (4.5 billion years ago) were capable of destroying Earth's crust and wiping out any biosphere that was present," Mojzsis said. "Instead of chopping down the tree of life, our view is that the bombardments pruned it."

Physical evidence of Earth's early bombardments has been erased by weathering and plate tectonics over the eons, and the oldest isotopic evidence of life comes from rocks that formed 3.83 billion years ago.