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
The cosmic ray theory is pretty wild - these muon particles can penetrate deep underground and even through ocean water, potentially causing DNA mutations at rates 20-30x higher than normal background radiation durng that period.