Final answer:
The Earth's atmosphere during the archaic era was low in oxygen due to chemical reactions with the crust that removed oxygen as quickly as it was produced by plants. Evolution of cyanobacteria and geological changes led to an increase in oxygen production, setting the stage for the evolution of aerobic life forms.
Step-by-step explanation:
In the archaic era, the Earth's atmosphere contained a low percentage of oxygen due to chemical reactions with the Earth's crust. Although plants were releasing oxygen through photosynthesis, this oxygen was quickly removed by reactions with minerals and compounds in the crust. Geological studies of ancient rocks reveal that for about 2 billion years, the atmosphere was anoxic, with no abundant free oxygen present.
During this era, anaerobic organisms that do not require oxygen for growth were the only forms of life able to exist. It was the evolution of cyanobacteria, an ancestral form of blue-green algae, which marked the beginning of the atmosphere's oxygenation. Cyanobacteria's photosynthetic activities gradually increased the oxygen production, and as a result, more oxygen started to accumulate in the atmosphere.
Simultaneously, increased geological activity contributed to significant erosion, burying plant carbon and preventing it from recombining with oxygen to form CO₂. The slow yet steady increase in oxygen levels, combined with geological changes, set the stage for the evolution of other life forms that could use oxygen for more efficient metabolic processes.