Question

if cyanobacteria had failed to appear on earth, how would the evolution of earths atmosphere most likely be different?
earths atmosphere would contain more oxygen and less carbon dioxide.
earths atmosphere would lack significant amounts of nitrogen.
earths atmosphere would still be composed mainly of hydrogen and helium.
earths atmosphere would consist of volcanic gases and water vapor.

Explanation:

Cyanobacteria were crucial in the oxygen - rich atmosphere development through photosynthesis. Without them, the atmosphere would not have been oxygenated and would have remained in a pre - oxygenated state dominated by volcanic gases and water vapor. Volcanic activity released gases like carbon dioxide, sulfur dioxide, and water vapor into the early atmosphere, and without cyanobacteria's photosynthetic oxygen production, this composition would persist.

Answer:

Earth's atmosphere would consist of volcanic gases and water vapor.