New Scientist 15 Dec 2001, page 10

"...life on Earth may have been repeatedly disrupted by bursts of gamma rays from space. The bursts would have disturbed the Earth's atmosphere, exposing organisms to high levels of mutagenic radiation"

The scientists believe the bursts only reach Earth once every 5 million years (it was previously thought never) and degrade the atmosphere, exposing organisms to UV radiation (and cosmic rays, I presume, which the atmosphere also protects us from).

"The impact of gamma-ray bursts has not been widely discussed, nor do biologists, including myself, have much information about their frequency or exposure levels" - admits scientist Mitch Sogin.

Similar research reported at Space Daily