Question

When oxygen appeared in atmosphere?

Answer 1

Most scientists believe that for half of Earth's 4.6-billion-year history, the atmosphere contained almost no oxygen. Cyanobacteria or blue-green algae became the first microbes to produce oxygen by photosynthesis, perhaps as long ago as 3.5 billion years ago and certainly by 2.7 billion years ago.

Hope it's helpful

Answer 2

$\huge\underline\mathrm{\red{AnSweR}}$

Most scientists believe that for half of Earth's 4.6-billion-year history, the atmosphere contained almost no oxygen. Cyanobacteria or blue-green algae became the first microbes to produce oxygen by photosynthesis, perhaps as long ago as 3.5 billion years ago and certainly by 2.7 billion years ago.