Origin of Earth & Early Atmosphere — NEET Biology
📚 Practice ConceptHuge clusters of galaxies comprise the universe. Galaxies contain stars and clouds of gas and dust. Considering the size of universe, earth is indeed a speck. The Big Bang theory attempts to explain to us the origin of universe. It talks of a singular huge explosion unimaginable in physical terms. The universe expanded and hence, the temperature came down. Hydrogen and Helium formed sometime later. The gases condensed under gravitation and formed the galaxies of the present day universe. In the solar system of the milky way galaxy, earth was supposed to have been formed about 4.5 billion years back. There was no atmosphere on early earth. Water vapour, methane, carbondioxide and ammonia released from molten mass covered the surface. The UV rays from the sun brokeup water into Hydrogen and Oxygen and the lighter H2 escaped. Oxygen combined with ammonia and methane to form water, CO2 and others. The ozone layer was formed. As it cooled, the water vapor fell as rain, to fill all the depressions and form oceans. Life appeared 500 million years after the formation of earth, i.e., almost four billion years back.
Which of the following statements regarding the oxygenation of Earth’s early atmosphere and primitive life forms are NOT correct? S1: Early Earth's atmosphere was largely reducing, containing gases like methane, ammonia, and water vapor but lacking free oxygen. S2: The earliest forms of life were most likely single-celled, non-photosynthetic organisms that thrived in a water environment. S3: Cyanobacteria were among the first organisms to perform oxygenic photosynthesis, leading to the accumulation of free oxygen in the atmosphere. S4: The released oxygen immediately formed the ozone layer, which then facilitated the direct origin of the first cellular forms on land. S5: The initial rise in atmospheric oxygen caused by photosynthetic life forms led to a significant decrease in temperature, allowing water vapor to condense into oceans.
View solution & NCERT explanation
Correct answer: C — S4 and S5 only
The core concept here involves the conditions of early Earth, the origin of life, and the process of atmospheric oxygenation. S1: 'There was no atmosphere on early earth. Water vapour, methane, carbondioxide and ammonia released from molten mass covered the surface.' (Page 111, Para 2). This describes a reducing atmosphere lacking free oxygen. Thus, S1 is correct. S2: 'The first non-cellular forms of life could have originated 3 billion years back. ... The first cellular form of life did not possibly originate till about 2000 million years ago. These were probably single-cells. All life forms were in water environment only.' (Page 111, Para 4; Page 112, Para 1). The earliest cells were likely heterotrophs or anoxygenic autotrophs before oxygenic photosynthesis became widespread. Thus, S2 is correct. S3: 'Some of these cells had the ability to release O2. The reaction could have been similar to the light reaction in photosynthesis...' (Page 121, Para 1). Cyanobacteria are prime examples of such organisms. This led to oxygen accumulation. Thus, S3 is correct. S4: 'Oxygen combined with ammonia and methane to form water, CO2 and others. The ozone layer was formed.' (Page 111, Para 2). The ozone layer formed *after* oxygen accumulation. 'All life forms were in water environment only.' (Page 112, Para 1). Life invaded land much later. Therefore, the ozone layer did not facilitate the direct origin of first cellular forms on land; rather, it protected existing life and allowed later colonization of land. Thus, S4 is not correct. S5: 'As it cooled, the water vapor fell as rain, to fill all the depressions and form oceans.' (Page 111, Para 2). The cooling of Earth's surface led to condensation of water vapor and ocean formation, which happened much earlier than the significant rise in atmospheric oxygen caused by photosynthetic life forms. Thus, S5 is not correct. Therefore, statements S4 and S5 are not correct. The final answer is C because S4 and S5 are the incorrect statements.
What does NCERT say about Huge clusters galaxies comprise?
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Through deep analysis of NEET and NTA, 176 of the 180 ReNEET 2026 (June 21) questions were already in the MedicNEET question bank before the exam.
176/180MedicNEET's Biology question bank is built from the same NCERT lines NTA picks repeatedly. Not random MCQs — questions crafted exactly like NTA crafts them.