Question

The first organisms were ........... and not autotrophs.

• A. Heterotrophs
• B. Chemotrophs
• C. Phototrophs
• D. All of the above

Answer 1

The correct option is B Chemotrophs

• The organic molecules formed in the primitive oceans under primitive reducing conditions of earth served as the only source of carbon for the first life form of earth.
• The first life forms, bacteria like prokaryotic cells, originated from protocells in the Archaeozoic era were anaerobic chemo heterotrophs that used the organic component as a source of energy and carbon. The organisms that use organic/inorganic compounds as their carbon source are called heterotrophs while those that can synthesize their two organic compounds are known as autotrophs.
• Since the first life forms fed on available organic compounds, they were heterotrophs. Because of the limited supply of organic compounds, organisms evolved photosystems to capture sunlight to be used as a source of energy 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. phototrophs appeared much later in evolution.

Thus, the correct answer is option B.