Question

How has the free O₂ been released into the earth’s atmosphere as there was no free O₂ at the time of origin of life?
(A) By catabolic activity of animals
(B) By photosynthetic activity
(C) By heating of the earth
(D) None of the above

Answer 1

Hint: The contemporary theory of chemical evolution is predicated on the idea that on a prehistoric planet, a combination of simple chemicals joined together to form increasingly complex molecular structures, from which the first living cell finally emerged (s).
Step by step solution:
Chemical reactions that took place in the ocean during the early history of life led to the synthesis of complex organic molecules from simple inorganic chemicals. In the evolution of life on Earth, chemical evolution is regarded as the initial stage. The hydrogen atmosphere was replaced with a mixture of water vapour, carbon monoxide, carbon dioxide, nitrogen, and methane. The oceans were created as the Earth cooled, causing water vapour to condense and torrential rains to fill its basins. The changes in molecule concentrations produced by chemical evolution are substantially greater than those produced by selection alone. Contrary to biological evolution, however, chemical evolution is not unbounded. Darwinian evolution is frequently seen as a distinguishing characteristic of life. On the other hand, in evolutionary biology, the theory that the organic building blocks of life were produced when inorganic molecules came together is referred to as "chemical evolution" most frequently. Chemical evolution, also known as abiogenesis, may have been the origin of life on Earth.
Cyanobacteria were the first organism to photosynthesize and produce oxygen on primitive earth, this shifted conditions on primitive earth from reducing to oxidising and thus chemical evolution stopped as it cannot occur in an oxidising atmosphere, now biological evolution started.
Hence, option (B) is correct.
Note: The idea of chemical evolution describes how life first emerged from non-living substances. According to the constantly changing environmental circumstances, first inorganic chemicals and then organic compounds were produced.