Paleoarchean World

Scientists say they have found evidence that a gigantic meteorite, twice as big as the one which is believed to have wiped out the dinosaurs, collided with Earth billions of years ago.

Deposits of the rock were found in South Africa and Australia, said a report in the Science journal.

The twenty-kilometre (twelve-mile) wide asteroid is believed to have hit the planet with such force that it would have caused tidal waves kilometres high and torn up the bottom of the ocean.

Researchers from Stanford University in California and Louisiana State University say the cataclysmic event happened about 3.4 billion years ago, before continents were formed and when only bacteria existed.

It is not known exactly where the giant meteorite hit as the scientists have not yet located a crater which would have been left by the impact.

Evolution altered

The report said the rock probably came from an asteroid belt between Mars and Jupiter.

Researcher Gary Byerly said the object was likely to have been part of a shower of meteorites, some as wide as 50km (30 miles).

"These impacts were very large. They really changed the course of the evolution on Earth," he said.

The report does not say what changes the impact might have affected.

"There isn't a big extinction event you can identify [that is] as cut-and-dried as the extinction of the dinosaurs," said co-author, Donald Lowe.

'Incredible [tidal waves]'

Mr Lowe said it would have taken the rock less than two seconds to pass through the ocean and slam into the sea bed.

"That would generate enormous waves kilometres high that would spread out from the impact site, sweep across the ocean and produce just incredible tidal waves - causing a tremendous amount of erosion on the microcontinents and tearing up the bottom of the ocean," he said.

Geologists found traces of the meteorite in South Africa's Barberton greenstone belt and Pilbara block in western Australia.

The sites contain rocks formed more than three billion years ago and which contain information dating back to the beginning of the solar system.

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