And despite a thorough search, no increased levels of iridium have been associated with that time. So researchers have been looking for other evidence of an impact.

And now they seem to have found it. The new study, announced today, uncovered extraterrestrial gases trapped inside special molecules, known as Buckyballs, in ancient soil layers. Scientists say the evidence points to a colossal whack from a comet or asteroid roughly 3.7 to 7.5 miles (6 to 12 kilometers) wide -- about the same size as the one that ultimately destroyed most dinosaurs 186 million years later.

"These two extinctions are like bookends for the age of the dinosaurs," said Robert Poreda, a University of Rochester scientist who worked on the study. Full results are in the Feb. 23 issue of the journal Science.

In the past 500 million years, there have been about five giant extinction events, researchers say. The new finding means that at least two of them were caused by impacts. The others also may have been.

"This suggests that the evolution of life on Earth is strongly coupled with our cosmic environment," said Christopher Chyba, a SETI Institute researcher who was not directly involved in the study. The new work even has implications for the origin of intelligence on Earth, and possibly elsewhere, Chyba said. It hints at the possibility that Earth's biosphere is regularly disrupted, every 100 million years or so, by giant impacts that would render human life impossible.

"We ourselves would truly not have been here [if not for] the extinction that eliminated the dinosaurs," Chyba said at a news conference at NASA Headquarters. "A biosphere only has a certain amount of time to develop a technical intelligence which is capable of realizing that there is an impact hazard, cataloging that hazard and...avoiding the next impact."

Bottom line: Only a civilization smart enough to spot the hazard and mitigate it can survive.

Chyba said that because other planets around other stars would likely face the same risks, the next logical thought is that any potential intelligent ET would also have to be quick-witted to avoid going the way of the dinosaurs. He said this might "increase the likelihood of technical civilizations."

Because Earth's crust has stretched and folded so dramatically in the past 251 million years, the researchers say there is no way to pinpoint where the space rock hit. But they can guess at the wild events that followed.

Earthquakes and volcanoes would have rattled the planet, explains the University of Washington's Luann Becker, lead author of a study. Lava poured out in volume -- enough to cover the planet 10 feet (3 meters) deep. The oceans dropped 820 feet (250 meters).

Worse, the combined effects of the object vaporizing on impact, along with all the volcanism, poisoned what was left of the seas and choked the air with ash and deadly gases. Sunlight may have disappeared for months. Or, Becker and her colleagues say, carbon dioxide may have trapped the Sun's energy and sent temperatures soaring.

Either way, it was "not a pretty picture for life, which is why it's the greatest of all mass extinctions recorded on Earth," Becker told SPACE.com.

Some 90 percent of all sea life perished, along with 70 percent of land animals and most terrestrial plants.

The researchers say the volcanic activity was likely going on before the impact, but was then fueled into a frenzy. The one-two punch, it seems, may be what's needed to precipitate the worst extinctions. Poreda called the whole scenario a "blast from the double-barreled shotgun."

The discovery, and the new technique used to make it, could lead scientists to find that some of the other 20 or so mass extinctions in the last billion years were also caused, or at least helped along, by cosmic collisions.