There is new evidence that cosmic rays may have caused past
mass extinctions. These mass extinctions occured 55 million years ago and 115 million years ago. In both cases, the biodiversity declined by 10%. Every 62 million years an unknown event occurs that causes mass extinction.

The cosmic ray hypothesis for mass extinctions has been suggested before, but there was no known mechanism. A more commonly believed hypothesis blamed an asteroid hit for the extinction of the dinosaurs.

So far, evolutionary biologists have only been able to establish that the phenomenon seems cyclical, but they haven’t isolated a cause.

ScienceNow explains the new evidence:

Now, researchers from the University of Kansas in Lawrence think they have found a possible answer. Physicist and co-author Adrian Melott says that he began suspecting a galactic cause after noticing a 2005 paper that calculated that the drop in species diversity occurs regularly on a time scale of tens of millions of years, which—for a cyclical event–is too long for something happening within the solar system. So he and Kansas colleague Mikhail Medvedev began examining the possibilities. At about the same time as the drops in biodiversity, the researchers determined, the sun reaches the highest point in its orbit relative to the galactic plane, where most Milky Way stars reside. At that point, the scientists report in the 1 August Astrophysical Journal, the solar system is closest to an incoming source of potentially lethal cosmic rays created by interactions between the Milky Way’s magnetic field and radiation generated by a cluster of nearby galaxies.

Here’s the journal article by Medvedev and Melott. The amount of Cosmic Rays at those positions explains the mass extinctions.