Answer
b.
Work Step by Step
Although many bacteria are able to carry out photosynthesis, only cyanobacteria release oxygen into their environment. This process made it possible for organisms that use oxygen to thrive.
About 2.5 billion years ago, the oxygen produced by cyanobacteria started accumulating in the early atmosphere. Because oxygen was a poison to many species at that time, only those individuals that were able to neutralize the detrimental effects of oxygen survived. Some of these organisms could even use oxygen to release energy from compounds.
Additionally, the oxygen that accumulated in the atmosphere formed an ozone layer that could protect organisms living on land. Organisms became shielded by this layer from detrimental ultraviolet radiation when before, organisms had to hide in the sea to escape the radiation.
