Question

What caused oxygen to form in the atmosphere?
No links or websites just explanation..

Answer 1

Well, nobody knows for sure, but the most likely theory is that photosynthesis by cyanobacteria (blue-green algae) was what started the rise in atmospheric oxygen levels; some could also have been released by ultraviolet radiation from the sun separating water molecules in the early oceans into hydrogen and oxygen.

Answer 2

Today oxygen makes up about 20 percent of Earth's atmosphere, but for the first two billion years after Earth formed, its atmosphere was anoxic (oxygen-free). About 2.3 billion years ago, oxygen increased from a trace gas to perhaps one percent of Earth's atmosphere. Another jump took place about 600 million years ago, paving the way for multi-cellular life forms to expand during the Cambrian Explosion.

Lol, I know you said no links.... But read more here:
http://www.learner.org/courses/envsci/unit/text.php?unit=1&secNum=6

Answer 3

Simply photosynthesis. The water cycle and carbon cycle help

Answer 4

why dont you accept messages