By Robin McKie | observer.guardian.co.uk

Fireballs set half the planet ablaze, wiping out the mammoth and America's Stone Age hunters

Scientists will outline dramatic evidence this week that suggests 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.

Primitive Stone Age cultures were destroyed and populations of mammoths and other large land animals, such as the mastodon, were wiped out. The blast also caused a major bout of climatic cooling that lasted 1,000 years and seriously disrupted the development of the early human civilisations that were 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 that include West will report that they have found a layer of microscopic diamonds at 26 different sites in Europe, Canada and America. These are the remains of a giant carbon-rich comet that crashed in pieces on our planet 12,900 years ago, they say. 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 the clothing worn by men and women. 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 later have died 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 around this time.

These people were some of the fiercest hunters on Earth, men and women who made magnificent stone spearheads which they used to hunt animals including the mammoth. Their disappearance at this time has been a cause of intense debate, with climate change being put forward as a key explanation. Now there is a new idea: the first Americans were killed by a comet.

It was not just America that bore the brunt of the comet crash. At this time, the Earth was emerging from the last Ice Age. The climate was slowly warming, though extensive ice fields still covered higher latitudes. The disintegrating comet would have plunged into these ice sheets, causing widespread melting. These waters would have poured into the Atlantic, disrupting its currents, including the Gulf stream. The long-term effect was a 1,000-year cold spell that hit Europe and Asia.

The comet theory, backed by observational evidence collected by the team, has excited considerable attention from other researchers, following publication of an outline report of the work in Nature

'The magnitude of this discovery is so important,' team member James Kennett, of the University of California, Santa Barbara, told the journal. 'It explains three of the highest-debated controversies of recent decades.'

These are the sudden disappearance of the first Stone Age people of America, the disappearance of mammoths throughout much of Europe and America and the sudden cooling of the planet, an event known as the Younger-Dryas period. Various theories have been put forward to explain these occurrences, but now scientists believe they have found a common cause in a comet crash. However, the idea is still controversial and the theory is bedevilled by problems in obtaining accurate dates for the different events.

'We still have a long way to go,' admitted West. 'But we have a great deal of evidence, from many sites, so this is quite a powerful case that we are making.'

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