LONDON - Evidence suggesting a comet exploded over the Earth nearly 13,000 years ago, creating a hail of fireballs that set fire to most of the Northern Hemisphere, will be outlined by scientists this week.
Stone Age cultures were destroyed and populations of mammoths and other large land animals were wiped out. The blast also caused a major bout of climatic cooling that lasted 1000 years and seriously disrupted the development of the early human civilisations emerging in Europe and Asia.
"This comet set off a shock wave that changed Earth profoundly," said Arizona geophysicist Allen West.
"It was about 2km-3km in diameter and broke up just before impact, setting off a series of explosions, each the equivalent of an atomic bomb blast.
"The result would have been hell on Earth. Most of the Northern Hemisphere would have been left on fire."
The theory is to be outlined at the American Geophysical Union meeting in Acapulco, Mexico.
A group of US scientists including West will report that they have found a layer of microscopic diamonds at 26 different sites in Europe, Canada and North America.
They say these are the remains of the giant carbon-rich comet that crashed 12,900 years ago.
The huge pressures and heat triggered by the fragments crashing to Earth turned the comet's carbon into diamond dust.
"The shock waves and the heat would have been tremendous," said West. "It would have set fire to animals' fur and to human clothing. The searing heat would have also set fire to the grasslands of the Northern Hemisphere.
"Great grazing animals like the mammoth that had survived the original blast would have died later in their thousands from starvation. Only animals, including humans, that had a wide range of food would have survived the aftermath."
The scientists point out that archaeological evidence shows that early Stone Age cultures clearly suffered serious setbacks at this time.
In particular, American Stone Age hunters, descendants of the hunter-gatherers who had migrated to the continent from Asia, vanished.
Their disappearance has been a cause of intense debate, with climate change being put forward as an explanation.
Now there is a new idea: the first Americans were killed by a comet, as were countless others.