From the BBC News:
A gamma ray burst could have caused the Ordovician extinction, killing 60% of marine invertebrates at a time when life was largely confined to the sea.
These cosmic blasts are the most powerful explosions in the Universe.
The scientists think a 10-second burst near Earth could deplete up to half of the planet’s ozone layer.
With the ozone layer devastated, the Sun’s ultraviolet radiation could have killed off much of the life on land and near the surface of oceans and lakes.
More here. And there’s more about gamma ray bursts at PBS’s Nova here. [Thanks Robin.]