The phrase mass extinction often calls to mind such potential culprits as asteroid impacts and volcanism. But new research suggests that in the case of a die-off of marine creatures that occurred two million years ago, at the interface of the Pleistocene and Pliocene epochs, a different phenomenon was to blame. According to a report appearing in the February 25 issue of the journal Physical Review Letters, cosmic rays from the explosion of a nearby supernova may have done these animals in.

Conventional explanations for the extinction, which claimed the lives of numerous molluscs, look to the emergence of the Panama isthmus or climate cooling from Northern Hemisphere glaciations. But analysis of a cluster of stars in our galactic neighborhood known as the Scorpius-Centaurus association led Narciso Bentez of Johns Hopkins University and colleagues to conclude otherwise. They determined that Scorpius-Centaurus has produced 20 supernovae explosionsthe swan songs of dying starsduring the last 11 million years. One of these outbursts, the team proposes, occurred around two million years ago, and took place close enough to Earth for its cosmic rays to catalyze large-scale destruction of the ozone layer. The resulting increase in harmful ultraviolet rays from the sun reaching Earth, they say, could have killed the marine plankton that molluscs depend on for food.

The supernova explosion hypothesis for mass extinctions has floated around in academic circles for more than four decades, but the new study may make the strongest case yet. The next step, Bentez and his co-authors note, will be to determine more precisely the time and distance at which each supernova explosion took place. "A coincidence in time between the [supernova] expected to have the strongest effects on the biosphere and the Pleistocene-Pliocene extinction," they write, "would strongly support the existence of a link between both events."