Too small to see
Can something too small to see change a planet?
Great Oxidation Event, about 2.4 to 2.3 billion years ago · Oceans and air · Drawing. Not a photograph.
Generated interpretation of iron-rich shallows. Banded iron formations are the rock evidence; this picture is not a photograph of them.
First look
Can something too small to see change a planet?
Wonder. Then look. This is not a test.
For a long time the air had almost no free oxygen.
Microbes that make oxygen by photosynthesis changed the chemistry of oceans and sky.
The change took a vast amount of time and was not a single afternoon.
First look is enough. Keep looking is if you want more.
Keep looking
How would we know? Open only if you want the evidence.
Keep looking · A later rise too
Oxygen rose again much later, in the Neoproterozoic. Complex animals live in the aftermath of more than one atmospheric story. Scientists still debate the exact levels and the exact triggers.