Answer:
Hey There!!
You can say that...
Oxygen was added to early earth’s atmosphere through PHOTOSYNTHESIS!
Step-by-step explanation:
The introduction of organisms which respired with the Sunlight and CO2 through PHOTOSYNTHESIS had brought a change in the Earth's early atmosphere...
PHOTOSYNTHESIS produces OXYGEN as a waste product (it's not needed by the organism)
This Oxygen is then released into the atmosphere where it was either converted into OZONE or left to move around the rest of he atmosphere...
I HOPE THIS HELPS WITH YOUR WORK!
:D