2.3 billion years ago: Mass extinction by snowball Earth. 2.1 billion years ago: From prokaryotes to eukaryotes. 2.3 billion years ago, the Milky Way Galaxy collided with a dwarf galaxy resulting in a supernova explosion that produced abundant cosmic rays. Due to the degeneration of the heliosphere, cosmic rays struck the Earth, promoting the formation of clouds which eventually enveloped the Earth Due to the thick cloud envelope, the Earth experienced global glaciation, called the Snowball Earth, which caused global mass extinction. However, some Prokaryotes survived the Snowball Earth to evolve into larger life as an endosymbiotic system. At last, eukaryotes appeared.