Back in the 1980s, scientists proposed that the Younger Dryas cold period started when a meteorite or comet struck North America. The idea stuck for decades but is being refuted now by a new study claiming that the droplets of rock soil used to support the theory were just from something much more ordinary in the Stone Age.

In a study published in the Journal of Archaeological Science, researchers from the University of California, Davis; the University of Aarhus, Denmark; University of Lyon, France; and University College London analyzed droplets of siliceous scoria, granules of porous material associated with melting, from four northern Syrian sites. The samples date back to 10,000 to 13,000 years ago and were compared to similar droplets that were supposedly the result of the cosmic impact responsible for starting Younger Dryas. The cold period lasted about a thousand years, coinciding with the extinction of great beasts like the mammoth and the disappearance of the Paleo-Indian Clovis people.

"For the Syria side, the impact theory is out. There's no way that can be done," said Peter Thy, a project scientist from the Department of Earth and Planetary Sciences at UC Davis and the lead author for the study.

According to the researchers' findings, scoria droplets are made of local soil, not from other continents as would be expected of an intercontinental impact. Additionally, the droplets' texture and thermodynamic modeling showed them to be the result of short heating events at moderate temperatures, not the intense, high heat that should have come from a large impact event.

So if the scoria droplets were not made by a meteorite or comet impact, where did they come from? Researchers say they were the result of house fires. The Syrian sites were early agricultural settlements set along the Euphrates River. Majority of locations utilized structures made of mud-bricks, some of which showed signs of melting due to intense fire. With this, researchers conclude that the scoria were formed when fires ripped buildings apart, dropping to the ground as burning mixtures of straw and local soil.

While the study points out that cosmic impact is not to be blamed for wiping out mammoths from the face of the earth, it offers no clue as to what actually started the Younger Dryas and allowed it to persist for so long.

Other authors for the study include: Gry Barfod, George Willcox and Dorian Fuller. The Danish National Research Foundation provided funding support for Barfod's work.