The origin of complex life (snowball earth) (repost) 


Life on Earth goes back at least two billion years, but it was only in the last half-billion that it would have been visible to the naked eye. One of the enduring questions among biologists is how life made the jump from microbes to the multicellular plants and animals who rule the planet today. Now, scientists have analyzed chemical traces of life in rocks that are up to a billion years old, and they discovered how a dramatic ice age may have led to the multicellular tipping point.