Traces of oxygen were probably generated in the early atmosphere through the breakdown of water molecules into oxygen and hydrogen by ultraviolet light in a process called

Photosynthesis.

Explanation:

Photosynthesis may be defined as the process by which the plants prepare their own food by the use of sunlight and chlorophyll. The light energy is converted into chemical energy for food storage.

The early environment of the Earth's atmosphere is highly reducing. The photosynthesis at an early environment causes the formation of the traces of oxygen. The water molecule will breakdown into the hydrogen and oxygen by the use of the ultraviolet light.

Thus, the answer is photosynthesis.

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hope that !         : )