An hour-long hailstorm from space bombarded the Earth 13,000 years ago - plunging the planet into a mini-ice age, scientists claimed today.
The catastrophe was caused by a disintegrating comet and saw the planet sprayed by thousands of frozen boulders made of ice and dust.
The collisions wiped out huge numbers of animal species all over the world, disrupted the lives of our stone age ancestors and triggered a freeze that lasted more than 1,000 years.
The theory is the brainchild of Professor Bill Napier, from Cardiff University, who says it explains the mysterious period of extinction around 11,000 BC.
Scientists have long been puzzled by what caused a sudden cooling of up to 8C (14F) just as the Earth was warming up at the end of the last ice age.
The change in climate caused retreating glaciers to advance once again, and coincided with the extinction of 35 families of North American mammals.
Some geologists have argued that the world was hit by a giant asteroid - a smaller version of one which wiped out the dinosaurs 65million years ago.
The collision left behind tell tale traces in the rocks - including a black 'mat' of soot an inch thick thought to have been created by continental wide wildfires.
Microscopic 'nanodiamonds' created in massive shocks and only found in meteorites or impact craters have also been discovered dating back to the disaster.
These findings have led to claims that a 2.5mile long comet or asteroid smashed into the ice sheet covering what is now Canada and the northern US.
But other scientists say the chances of the Earth being struck by such a large object only 13,000 years ago are one thousand to one against. And they say a single impact cannot explain such widespread fires.
Professor Napier's theory suggests the devastation took place when the Earth strayed into a dense trail of fragments shed by a large comet.
Thousands of chunks of material from the comet would have rained down on Earth, each one releasing the energy of a one megaton nuclear bomb.
The impacts would have filled the atmosphere with smoke and soot and blotting out the Sun.
Prof Napier says a comet swooped into the inner solar system between 20,000 and 30,000 years ago and has been breaking apart ever since.
'A large comet has been disintegrating in the near-Earth environment for the past 20,000 to 30,000 years and running into thousands of fragments from this comet is a much more likely event than a single collision,' said Professor Napier.
His model, published in the journal Monthly Notices of the Royal Astronomical Society, suggests that the 'hailstorm' would have only lasted about an hour.
It would have caused thousands of impacts, generating global fires and depositing nanodiamonds at the 'extinction boundary' marking the point in time when many species died out.
One recent impact that may have come from the comet is known as the Tagish Lake meteorite, said Professor Napier.
The object fell on Yukon Territory in Canada in January 2000. It contained the largest amount of nanodiamonds of any meteorite studied so far.
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