A massive asteroid bigger than Mount Everest slammed into the earth 250 million years ago causing the greatest mass extinction on record, say the scientists who believe they have found the "smoking gun" of the collision.
Geologists located the huge undersea crater off the Australian coast where they think the asteroid hit with the force of 1 million nuclear bombs, an impact that almost snuffed out life on earth. About 90 per cent of marine organisms and 80 per cent of land animals and plants died out at the end of the Permian and the beginning of the Triassic periods, for reasons that had not been explained.
If the asteroid impact is confirmed as the cause of the "great dying", it will be the second example of an extraterrestrial object being linked with a mass extinction. The other was the demise of the dinosaurs some 65 million years ago.
Luann Becker, of the University of California at Santa Barbara, and colleagues at the Australian National University in Canberra said that they have gathered extensive evidence of a 125-mile wide crater off the north-west coast of Australia.
Dr Becker said that her team had found fragments of a meteor in a geological layer that corresponded to the date of the Permian mass extinction 250 million years ago.
Analysis of geological cores drilled by oil companies prospecting in the region had also revealed convincing data to suggest that the crater was created by a massive object from outer space.
The scientists have found melted rock and shocked-quartz crystals that contain the tell-tale fractures that they believe are the result of a cataclysmic collision involving a huge explosive force. "Few earthly circumstances have the power to disfigure quartz, even high temperatures and pressures deep inside the earth's crust," Dr Becker said.
The study, published in Science journal, relied on two oil-company cores drilled in the Seventies and Eighties through a geological feature called "The Bedout High", which had not been previously analysed.
When the scientists started to investigate the cores, which had been stored untouched by the Geological Survey of Australia, they soon realised that they had probably been drilled through an impact crater.
The great dying at the end of the Permian period is the greatest of the five known mass extinctions. No type of life was spared: plants, insects, reptiles, fish, molluscs and microbes were all affected.
Some scientist have suggested that severe volcanic eruptions at the time may have sent soot and ash into the atmosphere and shut out the sunlight for years. Others have suggested that climate change, brought about by the formation of a giant supercontinent, was the cause.
Originally it was thought that the mass extinction took place over millions of years but more recent studies suggest that could have occurred in less than 10,000 years - a very short period in geological history.
"I think palaeontologists are now coming full circle and leading the way in saying that the extinction was extremely abrupt," Dr Becker said.
A similar impact crater has been found at Chicxulub in Mexico, which scientists have dated to about 65 million years ago - the time when the dinosaurs became extinct - which was also marked by a period of intense volcanic activity.
Dr Becker added: "We think that mass extinctions may be defined by catastrophes like impact and volcanism occurring synchronously. With the discovery of the Bedout I don't think we can call such catastrophes occurring together a coincidence any more."