Question

The earliest type of organisms appeared in the ocean were
A. Autotrophs
B. Heterotrophs
C. Chemoheterotrophs
D .Photoautotrophs

Answer 1

Hint: The Earth is estimated to have formed 4.5 billion years ago. The atmosphere back then lacked oxygen and was a reducing one. Naturally organisms that can derive energy by reduction processes could have survived in such an atmosphere.

Complete answer:
The earth’s atmosphere in the primitive era was composed of carbon dioxide, nitrogen and hydrogen. The atmosphere was reducing in nature, as well as free of oxygen. The organic molecules that were present are known to have been formed in oceans under conditions of high temperature and pressure. These organic compounds, naturally formed the only source of carbon for the living organisms present. The organisms that are able to derive energy from organic sources could have been present during this time. At present, organisms that carry out life processes with energy from organic sources are called Chemoheterotrophs.
Chemoheterotrophs are heterotrophic in nature. They are the consumers that derive energy for body processes by consuming organic molecules. Chemoheterotrophs are known to use sulphur, hydrogen sulphide, ammonium to synthesize organic compounds by reduction of carbon dioxide.
Autotrophs are organisms that use carbon from carbon dioxide to form complex food particles like carbohydrates. Autotrophs occupy the producer level of the food chain. These organisms use either light or inorganic chemicals to produce their own food.
Autotrophs can be of two types – photoautotrophs and chemoautotrophs.
Photoautotrophs are organisms that use light as a source of energy to prepare their own food. Plants and algae belong to this group. Chemoautotrophs are organisms that derive their energy from oxidation of inorganic compounds. They are found in deep oceans and acidic springs. Methanogens and iron bacteria are examples of chemoautotrophs.
Heterotrophs are organisms that derive energy by consuming other plants or animals. These organisms are unable to make their own foods. Heterotrophs occupy the consumer level in the food chain. All animals belong to this category.
Thus, it can be deduced from the above discussion that the earliest atmosphere was a reducing atmosphere, where organisms that can reduce inorganic sources to derive energy could survive. Chemoheterotrophs derive their energy by consumption of organic compounds formed by reduction of carbon dioxide.

Hence, option (C) is correct.

Note: When the earth was formed, it contained inorganic compounds rather than oxygen. Therefore, organisms that appeared first on the earth were able to derive their energy for life processes by reducing inorganic compounds. Such organisms are known as chemoheterotrophs. They are able to reduce carbon dioxide with the help of reducing agents to form organic compounds.