The first organisms to appear on primitive earth were anaerobes because ..............

The primitive atmosphere of earth was reducing atmosphere that lack molecular oxygen but was rich in water vapour, carbon monoxide, carbon dioxide, nitrogen and hydrogen. The high temperature, UV light and lightening facilitated reaction between any free oxygen and the prevailing reduced substances to form oxides (iron oxides, silicon oxides). This makes option B incorrect as there was no free oxygen and option A correct. Absence of molecular oxygen allowed origin of anaerobic organisms that carried out fermentation of organic compounds to release energy. Evolution of photoautotrophs, that used water as electron donor during photophosphorylation and produce oxygen, added free oxygen to the earth atmosphere. This makes option C incorrect. Correct answer is A.