Scientists believe that oxygen first showed up in the atmosphere about 2.7 billion years ago. They think it was put there by a one-celled organism called "cyanobacteria," which had recently become the first living thing on Earth to make oxygen from water and sunlight.
The rock record provides a good bit of evidence that this is so. But one of these rocks has just gotten a great deal more slippery, so to speak.
Full story at
Caltech 
