Zoology - Evolution Question with Solution | TestHub

Primitive earth atmosphere has actively reducing gases such as hydrogen, carbon monoxide, and hydrogen sulfide that made the atmosphere reducing types. Also, it had no free oxygen. The reduced chemicals energy proved positive environment for the development photosynthetic cyanobacteria. These were able to make use of sunlight as source of energy and evolve the metabolic pathways of photosynthesis. This led to gradual increase in oxygen in the atmosphere and change the earth atmosphere to the current oxidizing type from reducing type.