a few million years, from the original quarry and other sites in Sweden and Russia, the team found a surge in both small extraterrestrial grains and in chemical isotopes that trace even finer extraterrestrial dust. "All this dust floated in, just when the sea level fall started," Schmitz says. "Suddenly it clicked."

Rebecca Freeman, a paleontologist at the University of Kentucky in Lexington, says the timing is "perfect." "It isn't necessarily the answer to every question, but it certainly ties together a lot of observations," she says.

Peter Reiners, a geochemist at the University of Arizona in Tucson, argues that the falling asteroid dust would have also delivered iron to the world's oceans, chilling the climate a different way. The dust would have nourished photosynthetic microbes at the sea surface,NORTHERN SYRIA -- Syrian forces backed by the U.S. military are beginning what's being called the final push to drive the Islamic State of Iraq and Syria (ISIS) out of Raqqa. The militant group has controlled the city for more than three years.

The U.S.-backed Syrian militia group that's leading the fight on the ground in Raqqa says victory is just days away.

ISIS is thought to have around 250 fighters left in the city -- and they are holed up in a handful of buildings including the main hospital and the sports stadium.

CBS News spent two days in Raqqa with those American-backed fighters and we saw the skeleton of a city -- block after block has been devastated by fighting, and by U.S. coalition airstrikes.

A U.S. special operations commander