Biology · Evolution · NEET
No. This is the single most tested point. Early Earth had NO free oxygen. NCERT clearly says there was no atmosphere at first, then gases like water vapour, methane, CO2 and ammonia covered the surface. Free O2 came much later - only after oxygen-releasing (photosynthetic) cells evolved, about 2000 million years ago. So the primitive atmosphere was oxygen-free (anoxic).
Water vapour, methane (CH4), carbon dioxide (CO2) and ammonia (NH3). These were released from the molten mass of the cooling Earth. Note: oxygen is NOT in this list. Also note nitrogen gas (N2) is not listed by NCERT here - do not add gases NCERT does not mention.
A reducing atmosphere has hydrogen-rich gases (methane, ammonia) and NO free oxygen, so it donates electrons/hydrogen rather than accepting them. Early Earth was reducing because free O2 (an oxidising gas) was absent. This is important because organic molecules like amino acids can only form and survive in a reducing atmosphere - free oxygen would have destroyed them. This is why the Miller-Urey experiment used a reducing gas mixture.
Earth formed about 4.5 billion years ago. Life appeared about 500 million years after Earth formed, i.e. almost 4 billion years ago. First cellular life appeared about 2000 million years ago, and some of those cells could release O2.
UV rays from the Sun broke water vapour into hydrogen and oxygen; the lighter H2 escaped into space. The remaining oxygen combined with ammonia and methane. As Earth cooled, the water vapour condensed and fell as rain, filling the depressions to form the oceans. The ozone layer also formed during this cooling phase.
Today's atmosphere is oxidising (rich in free O2). Free oxygen would break down the delicate first organic molecules before they could combine into life. Life could only originate in the early oxygen-free, reducing atmosphere. This is a favourite reasoning question in NEET.
Following are the two statements regarding the origin of life: (i) The earliest organisms that appeared on the earth were non-green and presumably anaerobes. (ii) The first autotrophic organisms were the chemoautotrophs that never released oxygen. Of the above statements which one of the following options is correct?
Try the real previous-year questions from this chapter — each with the answer and a full solution.
No. Free oxygen was absent. The early atmosphere had water vapour, methane, CO2 and ammonia, making it a reducing atmosphere. Oxygen was released later by early cells.
Water vapour, methane (CH4), carbon dioxide (CO2) and ammonia (NH3), released from the molten mass. Oxygen is not one of them.
Earth is about 4.5 billion years old. Life appeared about 500 million years after Earth formed, roughly 4 billion years ago.
UV rays from the Sun broke up water vapour into hydrogen and oxygen. The lighter hydrogen escaped to space, and the oxygen combined with ammonia and methane. Later the ozone layer formed.
A reducing (oxygen-free) atmosphere lets organic molecules like amino acids form and survive. Free oxygen would oxidise and destroy them, so life could not have started in an oxidising atmosphere.
