Space rock caused 'great dying'

Many scientists believe an asteroid collision killed off the dinosaurs

Scientists have found new evidence that the greatest extinction in the Earth's history was triggered by an asteroid.

About 250 million years ago, something unknown wiped out most of the life on the planet.

It was far more devastating than the impact that ended the rule of the dinosaurs 65 million years ago.

New geological evidence suggests that the "great dying" was caused by a space rock slamming into the Earth.

Dozens of rare mineral grains found in ancient rocks in Antarctica could be the "smoking gun", according to scientists in the US.

The tiny bits of chondritic (stony) meteorite bear all the hallmarks of coming from outer space.

Asish Basu, a geochemist at the University of Rochester, New York, believes the fragments date back to the Permian-Triassic period, 251 million years ago, when about 90% of marine life and 70% of land species vanished.

If this is the case, it would be the second time an asteroid or comet has wiped out life on Earth.

"It appears to us that the two largest mass extinctions in Earth history ...were both caused by catastrophic collisions with chondritic meteroids," Basu, and four colleagues from other US universities, write in the journal Science.

Cosmic gas

This is the latest piece of evidence in support of the theory that a celestial body hit the Earth at the time of the "great dying".

In 2001, US researchers discovered tiny capsules of cosmic gas trapped inside rocks from the Permian-Triassic.

Isotopes of helium and argon gases commonly found in space were found within a cage of carbon atoms.

These unusual ball-shaped molecules, known as buckyballs or fullerenes, are thought to have been deposited by a space rock colliding with the planet.

However, not all scientists are convinced by the theory that the extinction was caused by an asteroid.

The fossil record shows that some species, particularly on land, were able to survive whatever happened.

Four out of six lines of lizard-like reptiles, called procolophonoids, made it through to the age of the dinosaurs.

How some vertebrates could have escaped such devastation remains a mystery.