Question

According to heterotroph hypothesis, the source of food for the earliest form of life consisted of
A) Algae
B) Bacteria
C) Micro-organisms
D) organic compounds

Answer 1

Hint: The Urey-Miller experiment demonstrated how using electricity and a concentrate of simple inorganic compounds, organic molecules formed naturally without any other kind of interference.

Complete Answer:
- Algae are autotrophs. They are able to synthesise their own food molecules using photosynthesis using energy from sunlight and simple inorganic and organic atoms and molecules. Algae would also increase available free oxygen in the atmosphere as a by-product of photosynthesis. Option A is not correct.
- Bacteria can be chemi-osmotic and photosynthetic (which are often classified as blue-green algae). However, most bacteria are autotrophs which require organic compounds as food. Earliest archaea would not have had bacteria as a source of food either. Option B is incorrect.
- Micro-organisms include algae, bacteria, archaea, and protozoans. The earliest heterotrophs would not have had these available as a source of food. Option C is not correct.
- According to the Haldane-Oparin hypothesis, life originated in a chemical soup, with inorganic atoms gradually forming molecules, and then simple organic compounds were formed. Early organisms were heterotrophs that were unable to synthesise their own food. They had to therefore get nutrition from simple organic compounds that were present in their environment. The first organisms did not synthesise their food in the way that photosynthetic plants do. Option D is the correct answer.

Hence the correct answer is option D.

Note: The conditions as the earth was forming were not conducive to supporting any life that required oxygen. The atmospheric became like it is today as a result of the continual activity first of photosynthetic microorganisms, and the green plants. Even today, the maximum amount of photosynthesis occurs in the oceanic phytoplankton.