A comet exploded over North America about 13,000 years ago, causing a long bout of climate cooling, according to a controversial new theory presented today.

The extraterrestrial impact may help explain massive mammal die-offs and the demise of one of the earliest American cultures.

It would also be the first known extraterrestrial impact to affect modern humans.

Evidence for the impact comes from a thin layer of sediment found throughout North America, said James Kennett, a geologist at the University of California in Santa Barbara.

"There are materials with particular chemistries in that layer that collectively provide very strong evidence that the layer was produced by this extraterrestrial impact," he said in a telephone interview.

Kennett said the layer contains tiny spheres of carbon and metals, bits of diamonds, and extraterrestrial concentrations of helium 3 and the element iridium.

The layer dates to 12,900 years ago, he added.