Julia Hinde at the Geoscience 98 conference at Keele University
At least 16 asteroids - each more than 250 km in diameter - may have collided with the early Earth, each wiping out all but the most basic life, says a leading US geophysicist.
Norman Sleep of Stanford University, told the conference that though there are no early rocks remaining on earth, evidence of asteroid collisions with the moon and Mars leads him to conclude that Earth must have been a victim of many devastating asteroid collisions which would have sent vast quantities of debris into the atmosphere.
"I believe the major danger to life on Earth in early times was asteroids hitting the earth," Professor Sleep told the conference. "At times of collision the only safe place to be would have been in the deep subsurface, at least one km down.
"The only life which therefore survived such impacts was high temperature organisms living in the subsurface."
But he suggests that between each of the major asteroid collisions with Earth, there were millions of years in which there was adequate time for sophisticated organisms to develop.
These would then be wiped out in each of the collisions, leaving just the most basic micro-organisms living deep in the earth at high temperatures.
"Between the impacts life may have developed. The impacts may have left some survivors but life was probably wiped back to micro-organisms. This could have happened up to 16 times," said Professor Sleep.
At the time of the early Earth, there was considerable debris in space as a result of the original formation of the planets. These asteroids occasionally collided with each other and with planets, gradually reducing in quantity.
The likely devastation caused by a 250 km asteroid can be compared with the asteroid which is thought to have led to the eradication of dinosaurs. This was thought to be about 10 to 20 km in diameter.
Professor Sleep also suggested that during the early life of the earth, atmospheric temperatures could have been surprisingly cold, even with layers of ice covering the oceans.
He says that though the Earth would have been hot, with molten lava erupting to the surface, and there would have been considerable associated degassing with high levels of carbon dioxide being released into the atmosphere, any sort of greenhouse effect was likely to have been countered by the effects of asteroidal collisions.
Rock fragments thrown up as a result of these collisions with the earth's surface may have combined in the oceans with the excess carbon dioxide to form limestone, said Professor Sleep, thus preventing a build up of carbon dioxide and greenhouse warming.