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Many starts for life on Earth?

Life on Earth may have been destroyed over and over again, NASA says. Space debris crashing into Earth's surface at up to 40,000 miles per hour may have repeatedly wiped out all life during the first billion years, NASA's Ames Research Center said Tuesday.

The theory is fueled by data gathered during the Apollo lunar flights that showed a pattern of "asteroid-like bodies" smashing the moon billions of years ago.

Although signs of crashes on Earth have disappeared, the lunar patterns allowed scientists to estimate the size and frequency of similar crashes on Earth.

The research concludes that one or more impacts by chunks of debris roughly the size of California may have wiped out life altogether, forcing the life-forming process to start again.

Smaller impacts may have wiped out life on land and at the water's surface. But primitive creatures deep in the ocean survived the smaller hits and continued to evolve, Ames said.

The theory challenges assumptions that life began just once in tide pools or brackish ponds.