Ancient "space rock" impact caused mass extinction

There is now evidence that an ancient object which exploded over North America was composed of strange materials like

"tiny spheres of glass and carbon, ultra-small specks of diamond - called nanodiamond - and amounts of the rare element iridium"

The explosion occurred about 12,900 years ago at the end of the last Ice Age, and probably caused the extinction of species like the mastodon, sabre tooth tiger, and American camel. It also forced one of the oldest known North American cultures, the Clovis people, to abruptly shift from a centralized agricultural society into dozens of fragmented, hunter-gatherer communities.

Why a rock from space would be composed of nanospheres, nanodiamond and iridium remains unexplained.