Few researchers deny that a giant space rock slammed into the earth 65 million years ago, at the Cretaceous-Tertiary (K-T) boundary. Whether or not that colossal collision wiped out the dinosaurs¿among the two thirds of the world's species that disappeared at that time¿has remained an open question. A report in today's Science says yes: the mass dying was the swift work of an asteroid or lone comet.

Although a single impactor is the most commonly quoted cause of the K-T mass extinction in the popular press, some scientists still argue that intense volcanism in the Deccan Traps of present-day India was the true killer. Either event would have ejected tons of dust and gases into the atmosphere, essentially choking life out of existence, but their influence on climate would have lasted different lengths of time. The effects of an impact would diminish quickly; dust and gas from the Deccan volcanism would have squelched biological productivity for all of the 500,000 years or more that the eruptions occurred.

According to sedimentation rates outlined in the new study, ocean life rebounded within only 10,000 years of the initial die-off. "The Deccan volcanism might have stressed out the environment, but the impactor dealt the final blow," says Sujoy Mukhopadhyay, a geochemist at the California Institute of Technology and lead author of the research paper.

That conclusion points to an extraterrestrial killer, but what type? Mukhopadhyay says he and his colleagues had originally set out to determine whether the impactor was a member of a comet shower, as some workers have suggested. If that were true, then extra amounts of cosmic dust and associated helium-3 (a rare isotope of helium) would have bombarded the planet just before and after the impact. On the contrary, the researchers found nearly steady abundances of helium-3 in rocks spanning the K-T boundary, suggesting the impactor must have been working alone.