Earth got hammered by cosmic rays 41,000 years ago due to a weak magnetic field 

 Earth's magnetic shield weakened around 41,000 years ago, resulting in increased bombardment of our atmosphere by energetic charged particles called cosmic rays. 

https://cdn.mos.cms.futurecdn.net/d3KZKeEPNAF6XfYa3PJE4H.png

https://www.space.com/earth-hammered-cosmic-rays-weak-magnetic-field