Final answer:
Cyanobacteria were the organisms responsible for the production of atmospheric oxygen on early Earth. They evolved from primitive phototrophs and through the process of photosynthesis, they released oxygen, which led to the transformation of the anoxic atmosphere.
Step-by-step explanation:
The organisms that were responsible for producing high levels of atmospheric oxygen on early Earth were cyanobacteria. Cyanobacteria evolved from early phototrophs and initiated the oxygenation of the anoxic atmosphere, fundamentally changing Earth's environmental conditions. These ancient microorganisms carried out photosynthesis and released oxygen as a by-product, transforming the planet's atmosphere and allowing the evolution of other life forms that depend on oxygen.
It is significant to note that for the first two billion years of Earth's existence, the Earth's atmosphere had no molecular oxygen. The first organisms were anaerobic, and autotrophic organisms like phototrophs appeared within one billion years of Earth's formation. Cyanobacteria evolved from these simple phototrophs and were pivotal in oxygenating the atmosphere, enabling the subsequent evolution of increasingly complex aerobic organisms and leaving an indelible mark on the biosphere.