Q.Explain the Oparin-Haldane hypothesis for the chemical origin of life on primitive Earth. What did Oparin and Haldane propose about the composition of the early atmosphere, and why was the absence of free oxygen important to their hypothesis?

Of the given planets, Mars is regarded as having the best possibility of life.

Mars lies relatively near the Sun's habitable conditions and shows evidence of past water (dry river channels, polar ice), a thin atmosphere, and a surface that could once have supported microbial life, which is why it is the focus of the search for extraterrestrial life. Mercury is too hot and airless, Venus has a crushing, extremely hot CO2 atmosphere, and Jupiter is a giant gas planet with no solid surface — all hostile to life. Hence Mars is the answer.

Free oxygen (O2) was absent from the early Earth's atmosphere.

According to the Oparin-Haldane theory of chemical evolution, the primitive atmosphere was a reducing one containing gases such as water vapour, methane (CH4), ammonia (NH3), hydrogen (H2), carbon dioxide (CO2) and nitrogen, but no free molecular oxygen (O2). Free oxygen built up much later once photosynthetic organisms (cyanobacteria) evolved and released it. Hence O2 was not present in the early period.

The Miller-Urey experiment simulated early Earth conditions using a mixture of methane (CH₄), hydrogen (H₂), ammonia (NH₃), and water vapour (H_2O) subjected to electrical sparks (simulating lightning) at a temperature around 800^circC. The correct option is (B).

The Russian biochemist Alexander Ivanovich Oparin proposed the theory of chemical evolution of life and authored the book 'The Origin of Life' (1924/1938).

Oparin proposed that life originated on primitive Earth through a gradual chemical evolution of simple inorganic molecules present in the early reducing atmosphere into complex organic molecules, and eventually into the first primitive self-replicating cells. His hypothesis, independently supported by J.B.S. Haldane, forms the basis of the widely accepted Oparin-Haldane theory of the origin of life, and was later experimentally supported by the Miller-Urey experiment (1953). Charles Darwin proposed the theory of natural selection (not origin of life per se), and de Vries proposed the mutation theory of evolution.

The early Earth's atmosphere was 'reducing' precisely because it lacked free molecular oxygen; its dominant gases were methane, ammonia, hydrogen and water vapour, not O2 or N2.

According to the Oparin-Haldane theory of chemical evolution, life originated in a primitive atmosphere very different from today's. This atmosphere is called a reducing atmosphere because it was rich in hydrogen-containing gases — chiefly methane (CH4), ammonia (NH3), hydrogen gas (H2), and water vapour (H2O) — and almost completely lacked free oxygen (O2). Free O2 only accumulated in Earth's atmosphere much later, after the evolution of photosynthetic cyanobacteria (the 'Great Oxidation Event'). Nitrogen gas (N2), while present in trace amounts from volcanic outgassing, was not the atmosphere's chemically defining feature in the way CH4/NH3/H2 were — these are the exact gases the Miller-Urey experiment used to simulate the primitive atmosphere. Hence, among the given options, neither (a) O2 nor (b) N2 correctly answers the question.

Earth's primitive atmosphere was reducing (no free oxygen); free O2 accumulated only later, after photosynthetic organisms evolved.

According to the chemical evolution theory (Oparin-Haldane hypothesis), the primitive Earth's atmosphere was a REDUCING atmosphere, containing gases like methane (CH4), ammonia (NH3), hydrogen (H2), and water vapour, but essentially no free molecular oxygen (O2). This reducing, oxygen-free environment was essential because it allowed simple organic molecules (amino acids, sugars, nucleotides) to form and accumulate from inorganic precursors under the influence of UV radiation, lightning, and heat (as demonstrated experimentally by Stanley Miller and Harold Urey) without being oxidised/destroyed. Free O2 only became a significant atmospheric component much later, after the evolution of oxygen-releasing photosynthetic organisms (cyanobacteria).

Oparin proposed that organic molecules aggregated into coacervates as a step toward the first living cells.

A. I. Oparin, in his theory of chemical evolution/origin of life, suggested that in the primitive ocean, simple organic molecules (formed from inorganic precursors under early-earth conditions) aggregated and organised themselves into small, membrane-bounded, colloidal clusters called coacervates. These proto-cell-like droplets could concentrate molecules, carry out rudimentary chemical reactions, and are considered a plausible precursor stage to the first true cells (protobionts).

'Origin of universe' matches with 'Big-Bang' — the widely accepted theory for how the universe began.

The Big Bang theory proposes that the universe originated from a single, extremely dense and hot point (singularity) that underwent a massive explosion roughly 20 billion years ago, after which the universe has been continuously expanding and cooling ever since. This explosive event led to the formation of galaxies, stars and eventually planets like Earth, and is the currently accepted scientific explanation for the origin of the universe. It matches Column B item (1) Big-Bang.

The Earth is estimated to have formed about 4.5 billion years ago, along with the rest of the solar system.

According to the widely accepted origin-of-the-universe/solar-system model taught alongside the theory of evolution, the universe originated about 20 billion years ago (Big Bang), while our solar system — including Earth — condensed out of a solar nebula roughly 4.5 billion years ago. Early Earth had no atmosphere in the modern sense; volcanic activity released water vapour, methane, ammonia, and other gases that later formed a reducing atmosphere in which chemical evolution (leading to the origin of life) is thought to have occurred.

The origin of the universe is explained by the Big Bang theory, which states that the universe originated from a single, immensely hot, dense point through a massive explosion roughly 15–20 billion years ago.

The theory that describes the huge explosion leading to the origin of the universe is the Big Bang theory. According to this theory, the universe began as an extremely small, hot and dense singularity, which underwent a colossal explosion (the 'Big Bang') roughly 15–20 billion years ago. This explosion caused rapid expansion, and matter (initially as hydrogen and helium) began to condense over time under gravity, eventually forming galaxies, stars, and planets — including the special conditions on early Earth that later allowed the origin of life.