Known as "the Great Dying" among paleontologists, more than 90 percent of the Earth's marine species perished, along with 70 percent of the land's reptile, amphibian, insect, and plant species. This mass extinction preceded the rise of the dinosaurs.

What caused the extinction event has been a matter of heated debate among scientists—until now. Researchers from the University of Washington and the University of Rochester have found extraterrestrial clusters of carbon atoms known as buckyballs in sediment samples from China and Japan. Their presence suggests that a massive comet or asteroid collided with the Earth, triggering the catastrophic collapse of life.

The study, which also involved researchers from New York University and NASA, is featured in the February 23 issue of the journal Science.

More than Just Volcanoes

"We've come full circle on the old argument about whether a collision or volcanism caused these extinctions," says lead researcher Luann Becker, assistant professor of Earth and space sciences at the University of Washington. "We can say with a good degree of certainty that there's a coupling of the two events."

Where exactly the comet or asteroid hit is unknown. It was probably somewhere between 3.7 and 7.4 miles (6 to 12 kilometers) wide, similar in size to the asteroid that crashed into Mexico's Yucatan Peninsula about 65 million years ago—the collision thought by many scientists that lead to the extinction of the dinosaurs.

The impact 250 million years ago occurred during a time of extreme volcanism on Earth. Volcanic eruptions can change climates in several ways. Massive amounts of sediment and ash expelled during an explosion can block sunlight from reaching the Earth, causing a significant drop in temperature, inhibiting the photosynthesis that allows plants to grow, and causing food chains to collapse.

Alternatively, carbon dioxide and other greenhouse gases ejected into the Earth's atmosphere by eruptions trap the sun's heat, sending temperatures soaring, and causing glaciers to melt and sea levels to rise.

When these changes in temperature or sea level occur faster than a species can adjust to them, extinction results.

"If the species cannot adjust, they perish. It's a survival-of-the-fittest sort of thing," says Becker.

But volcanism alone couldn't explain the rapid rate of extinction.

"As paleontologists have gotten better and better resolution on how quickly the extinction took place, it became more and more clear that more than one cause was needed to explain it," says Becker.

"The volcanism went on for millions of years, and the extinctions took place much, much faster." Fossil evidence shows that the extinction took place in 8,000 to 100,000 years—a heartbeat, in geological time.

Buckyballs: Cosmic Stowaways

The collision, which caused the rapid extincitions, left its signature in the sediment layers of the earth in the form of buckminsterfullerenes—buckyballs or fullerenes for short.

Buckyballs are microscopic clusters of carbon atoms that bond together to form a hollow geodesic cage, most frequently described as resembling a soccer ball. The cage can trap gases and is robust enough to survive interplanetary travel.

Becker and her colleagues were able to demonstrate that this form of carbon has an extraterrestrial origin.

The researchers studied the gases trapped within fullerenes found in high concentrations in the 250 million-year-old sedimentary layer, which divides the Permian and Triassic periods. The trapped gases contain forms of helium and argon structurally different from those found on Earth and could only have been formed in the extreme temperatures and gas pressures of outer space.

"These gas-laden fullerenes were formed outside the Solar System, and their concentration at the Permian-Triassic boundary means they were delivered by a comet or asteroid," says Becker.