The Precambrian era is said to have taken place during the early years of our planet. During this time, the levels of oxygen in the atmosphere were much greater than they are today.
This level of increased oxygen in the atmosphere is a direct result of the biological processes and the sheer number of certain organisms present in Precambrian-era oceans.
Though the options stated all depict ways in which oxygen entered the atmosphere during the Precambrian era, the increase of oxygen levels is credited to the cyanobacteria.
The cyanobacteria were among the few living organisms present on the Earth during this time. They are small algae that use photosynthesis to sustain themselves.
Photosynthesis is known to produce and release oxygen as a waste product. This and the vast number of cyanobacteria present at the time account for the increase in oxygen levels during this era.
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