ANSWER is the Absence of oxygen. Earth was formed about 4.6 billion years ago.It was a hot mix of gases and solids and almost no atmosphere.The surface was in the molten stage. After the cooling of Earth, an atmosphere developed which has mainly gases and spewed from volcanoes.It contains hydrogen sulfide, methane, and ten to as much carbon dioxide.The idea of the prebiotic Earth as a ROILING organic “soup” with a highly reduced atmosphere of gases.It was rich in ammonia, methane, water vapor, and noble gas NEON but it lacked free oxygen.It is ASSUMED that after millions of years first biological production of oxygen was DONE by unicellular organisms and it eventually led to the accumulation of oxygen in the atmosphere.