. Which of the following chemical was likely to be absent at the time of origin of life?


A. oxygen

B. water

C. hydrogen

D. ammonia

Best Answer

A

Sol.The first living beings were procaryotic like bacteria. They were single-celled. Nutritionally, they were chemoheterotrophs. Before the organic materials disappeared in the sea broth, new modes of nutrition developed. One of them was chemosynthesis. The organisms performing chemosynthesis are called chemoautotrophs. After that first photoautotrophs bacteria-like appeared. They were anaerobic and utilized hydrogen from sources other than water. Therefore, no oxygen was evolved and the atmosphere remained reducing in nature. It was cyanobacteria, which were present on earth about 3600 million years ago. They were photoautotrophs and they utilized water as hydrogen donor and evolved oxygen.

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