Scientists say they have found the place where an asteroid or comet smashed into Earth 250 million years ago and may have triggered the largest extinction in history, setting the stage for the appearance of dinosaurs and eventually humans.
The object created the Bedout crater in a rise of the ocean floor about 120 miles northwest of Australia, researchers say.
They say their analysis of the glass and minerals found at key sites in Antarctica and Australia and dug out of the crater, at depths of 10,000 feet, show that the object hit at the same time as the planet began the largest-scale extinction in its history.
The Permian-Triassic extinction wiped out about 95 percent of all marine species and up to 70 percent of all terrestrials.
The timing is more than a coincidence, the researchers say.
"I think right now we have a pretty strong story," said Luann Becker, a geologist at the University of California Santa Barbara, who has been studying impact craters since 1991.
The findings, based on an analysis of those core samples, appear today in Science.
Researchers think the object was an asteroid, but say it could have been a comet. They also are unsure whether it smashed into what was then land, the ocean or one of the swampy regions that covered much of the planet at the time.
"It was a very different-looking world back then," Becker said. The planet had only one ocean and one land mass, and many of the animal species that were wiped out then would be unrecognizable today.
The researchers believe the object was about six miles in diameter and sent debris into the air that scattered over the planet, raining down rock-like bits of glass; they found samples thousands of miles away.
Clouds of dust and debris from the impact would have blocked out the sun for an extensive period, with catastrophic effects on plants and animals.
But many paleontologists remain skeptical that an asteroid or comet caused the extinction. Fossil records suggest that many species died out gradually over millions of years and not as the result of a single event.
"I don't think it's a slam-dunk for impact as the cause, but I'd say this certainly makes it a more possible contender," said Douglas E. H. Erwin, a senior paleobiologist at the National Museum of Natural History, who was not a part of the study but reviewed the findings and participated in a news briefing yesterday.