A Smack From Space Linked to Earth's Worst Extinctions

By RICHARD MONASTERSKY

Geological sleuths now think they have nabbed the culprit in the planet's greatest murder mystery. New evidence implicates a comet or asteroid in the extinctions that wiped out the vast majority of species living 250 million years ago -- a cataclysm so great it has been called the closest life has come to extermination.

The clues are...