Question

Anaerobes were the first organisms to appear on the primitive earth because

• A. They had no enzyme system for oxygen utilization.
• B. Free oxygen could not be assimilated.
• C. Free molecular oxygen was absent.
• D. They were simple in organisation.

Answer 1

The correct option is C Free molecular oxygen was absent.

The first life form, bacteria like prokaryotic cell, originated from protocell in the Archaeozoic era (3800 million yrs ago, the era of the dawn of life). The first life forms were anaerobic as the primitive atmosphere was reducing and oxygen was absent. They were chemoheterotrophs that used the organic component as a source of energy and carbon. Due to the limited supply of organic compounds, organisms evolved photosystems to capture sunlight to be used as a source of energy thereby leading to the origin of photoautotrophs. The organisms that use light as a source of energy and carbon dioxide as a chief carbon source are referred to as photoautotrophic organisms. These photoautotrophs use water as an electron donor during photophosphorylation that results in the production of oxygen. Accumulation of oxygen in the atmosphere, later, led to the origin of aerobic forms which were more efficient in energy production as compared to their ancestral anaerobic forms. Thus, the correct answer is option C.