There is a hypothesis that the Ordovician–Silurian mass extinction 450 million years ago was caused by a Gamma-ray burst! http://en.wikipedia.org/wiki/Ordovici

There is a hypothesis that the Ordovician–Silurian mass extinction 450 million years ago was caused by a Gamma-ray burst! http://en.wikipedia.org/wiki/Ordovician%E2%80%93Silurian_extinction_events#Gamma_ray_burst_hypothesis http://en.wikipedia.org/wiki/Ordovician%E2%80%93Silurian_ext... It stripped the Earth's ozone and killed all the surface-dwelling organisms.