British scientists wrote in the November 11, 2010 issue of the journal Nature and pointed out that they had found fossilized bacteria in Scottish rocks that might have lived about 1 2 billion years ago  This suggests that the concentration of oxygen on Earth has increased to the level needed for human evolution  This major event took place 1 2 billion years ago and it was 400 million years earlier than scientists previously thought
The new research is expected to give scientists a new understanding of the timetable for the evolution of the Earth's atmosphere and the life that depends on it for a living
Only the concentration of oxygen in the atmosphere increases to a critical point can some bacteria survive.