A massive asteroid collision 470 million years ago may contain clues as to how to stop global warming.
The crash on the asteroid belt between Mars and Jupiter set off a glacial age on Earth and paved the way for the evolution of new species.
Scientists already knew about the ice age, but they didn't know why.
They examined traces of space dust on 466 million-year-old rocks from a fossil site in Sweden, looking for a type of helium isotope often found in asteroids.
They found that a large amount of debris thrown into the atmosphere by the collision had partially blocked Earth's sunlight.
Study author Philipp Heck, curator of the Field Museum in Chicago and an associate professor at the University of Chicago, said: "Normally, the Earth gets about 40,000 tons of extraterrestrial material each year.
"Imagine multiplying this by a factor of a thousand or 10,000."
Dust floated toward Earth for two million years, gradually cooling the planet and allowing new species to adapt.
It also divided the planet into zones – colder temperatures at the outer ends and warmer conditions at the equator.
Scientists say the discovery, documented in Science Advances, may help them explore different ways in which the planet can be artificially cooled.
Birger Schmitz, professor of geology at Lund University and study leader, said the result was "completely unexpected."
"Over the past 25 years, we have leaned against very different assumptions in terms of what happened," he said.
"It wasn't until the last measurements of helium that everything fell into place."
Finding a way to cool the earth could help avert a major climate crisis.
Previously, scientists used computer simulations to show that asteroids could orbit the earth in a way that allowed them to create fine dust that blocks sunlight.
Schmitz said: "Our results show for the first time that this dust sometimes cools the earth dramatically.
"Our studies can provide a more detailed and detailed empirical understanding of how this works, and this in turn can be used to assess whether model simulations are realistic."
Heck was more cautious, saying that all ideas need to be analyzed "very critically and carefully, because if something goes wrong things could get worse than before."