Answer:
The correct answer is option - microorganism.
Explanation:
The initial life form on earth was bacteria. These bacteria could live and survive in a toxic atmosphere at that time scale. Researchers and scientist believe that life in the earth were prokaryotes with no cell membrane and cell nucleus most probably bacteria.
All other organisms evolved later when there are enough oxygen and no toxicity in the environment.
Thus, the correct answer is option - microorganism.
