Supernovae showered Earth with radioactive debris

A series of massive supernova explosions near our solar system showered the Earth with radioactive debris between 3.2 and 1.7 million years ago. The scientists found radioactive iron-60, relatively recent in astronomical terms, in sediment and crust samples taken from the Pacific, Atlantic and Indian Oceans.
The scientists believe the supernovae in this case were less than 300 light years away, close enough to be visible during the day and comparable to the brightness of the Moon.
Although Earth would have been exposed to an increased cosmic ray bombardment, the radiation would have been too weak to cause direct biological damage or trigger mass extinctions. The resulting exposure to radioactive debris could have had significant effects on the evolution of life on Earth.

There were two papers published today in Nature associated with this discovery.

http://www.nature.com/nature/journal/v532/n7597/full/nature17196.html
http://www.nature.com/nature/journal/v532/n7597/full/nature17424.html

Also, here's accompanying news article: http://www.nature.com/nature/journal/v532/n7597/full/532040a.html