How did earth's atmosphere become oxygen-rich

Cyanobacteria or blue-green algae produce oxygen

Most scientists consider that toward half of Earth's 4.6-billion-year antiquity, the atmosphere carried about no oxygen. Cyanobacteria or blue-green algae became the leading microbes to generate oxygen by photosynthesis, possibly as long ago as 3.5 billion years ago and certainly by 2.7 billion years ago

in situ hybridization has been widely used for research applications, including clinical cytogenetics, gene mapping, tumor biology and studies of chromosome evolution.