The earliest oxygenic phototrophs are thought to be a. purple sulfur bacteria. b. green nonsulfur bacteria. c. purple nonsulfur bacteria. d. cyanobacteria. e. green nonsulfur bacteria and purple nonsulfur bacteria.

Option (D).

Explanation:

Cyanobacteria (also known as blue-green algae) represent a group of bacteria which make their food by the process of photosynthesis, during which they use carbon dioxide and release oxygen.

Due to their ability to produce oxygen as a byproduct of photosynthesis, Cyanobacteria are thought to be the earliest oxygenic phototrophs that changed the early low-oxygen atmosphere (reducing atmosphere) into an oxidizing atmosphere. It led to change in Earth's living forms and evolution.

Thus, the correct answer is option (D).

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