About 2.6 million years ago, there may have been a supernova (or other event) that caused a burst of comic rays lasting 100k years.

15 minute video

I follow this guy, Paul Fellows, on YouTube, and he has re-awakened an interest in astronomy for me during the last few months. He may not have a very large subscriber count, but is well regarded by several.

For those who do not like watching video, here is a link describing some of the research mentioned ,

DNA-busting radiation from star-killing supernova could have influenced evolution on Earth