It is often debated (usual answer asteroid) what caused the Cretaceous-Tertiary extinction that signaled the end of the dinosaurs 65 million years ago

The Permian-Triassic (250 million years ago) extinctions were worse: No class of life was spared from the devastation. Trees, plants, lizards, proto-mammals, insects, fish, mollusks, and microbes -- all were nearly wiped out. Roughly 9 in 10 marine species and 7 in 10 land species vanished.

"Deep inside Permian-Triassic rocks, scientists have found soccer ball-shaped molecules called "fullerenes" (or "buckyballs") with traces of helium and argon gas trapped inside. The fullerenes held an unusual number of 3He and 36Ar atoms -- isotopes that are more common in space than on Earth. Something, like a comet or an asteroid, must have brought the fullerenes to our planet."

They fail to mention cosmic rays. Near the end of these papers one reads:

"These variations in 3He concentration are caused by fluctuating sedimentation rates, sediment focusing, and/or variability in the IDP flux to Earth. Most of this IDP 3He in the GPC core is produced by high-energy cosmic ray bombardment of silicates..."

NASA reports that comets could have delivered important organic molecules to Earth -- sowing the seeds for life.

Genesis by comets is a controversial idea, but it has received an important boost with the knowledge that a NASA supported experiment has revealed that complex molecules hitchhiking aboard a comet could have survived an impact with Earth. The next test will be bacterial spores...