Answer:
The atmospheric oxygen initially came from the water molecule
Step-by-step explanation:
Condition on the primitive Earth , about 4 billion year ago were such which favored chemical evolution. . When the temperature of the Earth was less than 100 degree Celsius. Its atmosphere had nitrogen in the form of ammonia, carbon in the form of carbon dioxide and oxygen in the form of water vapour. There was no free oxygen available and so the atmosphere was reducing.
Then spontaneously biomolecules originated which further gave rise to protobionts. These protobionts evolved into several unicellular organisms among which one was cyanobacteria.
Cyanobacteria are considered as the organisms which added the first molecule of free atmospheric oxygen into the atmosphere. They carried out photosynthesis and split the water molecules to release oxygen as the bye product of the process.