Frenzied star-making in the Milky Way Galaxy starting about 2.4 billion years ago had extraordinary effects on life on Earth. Harvests of bacteria in the sea soared and crashed in a succession of booms and busts, with an instability not seen before or since. According to new results published by Dr. Henrik Svensmark of the Danish National Space Center, the variability in the productivity of life is closely linked to the cosmic rays, the atomic bullets that rain down on the Earth from exploded stars. Most likely, the variations in cosmic radiation affected biological productivity through their influence on cloud formation. Hence, the stellar baby boom 2.4 billion years ago, which resulted in an extraordinarily large number of supernova explosions, had a chilling effect on Earth probably by increasing the cloud cover.
http://www.space.dtu.dk/English/Research/Research_divisions/Sun_Climate/SC_The_Milky_Way_shaped_life_on_Earth.aspx