Oparin is best known in biology for proposing a hypothesis about which fundamental question regarding the early Earth?

Introduction / Context:
Alexander Oparin is a very important figure in the history of ideas about how life might have appeared on the early Earth. Long before modern molecular biology, he suggested that living cells could have arisen gradually from simple inorganic and organic molecules through a series of natural chemical changes. This exam question asks you to connect Oparin's name with the correct topic he worked on, and to distinguish it from other famous biological ideas such as the origin of species, one germ one disease and one gene one enzyme.

Given Data / Assumptions:

We are told that Oparin proposed a well known hypothesis. The options include different famous concepts: origin of species, origin of life, one germ one disease and one gene one enzyme. We assume standard textbook attributions for each scientific idea. The task is to identify which topic is most strongly and uniquely associated with Oparin.

Concept / Approach:
Oparin, along with J. B. S. Haldane independently, is credited with the chemical evolution or primordial soup hypothesis for the origin of life. He suggested that, on the early Earth, the atmosphere and oceans contained simple gases and molecules that, under the influence of energy sources such as lightning and ultraviolet radiation, gradually formed more complex organic compounds. These compounds could have concentrated in the oceans, forming a sort of primordial soup from which the first primitive cells eventually emerged. This focus on how life itself began clearly distinguishes his work from Darwin's origin of species and from later microbiology and genetics concepts.

Step-by-Step Solution:
Step 1: Recall that Darwin wrote On the Origin of Species and explained how new species arise by natural selection, not how life first appeared. Step 2: Remember that the phrase one germ one disease is associated with early germ theory and scientists such as Koch and Pasteur, not Oparin. Step 3: Recognise that the one gene one enzyme hypothesis is linked with Beadle and Tatum in biochemical genetics. Step 4: Link Oparin with the idea of chemical evolution and the primordial soup, which directly addresses the origin of life on Earth. Step 5: Therefore, the correct option must be the one that talks about the origin of life rather than origin of species or any other concept.

Verification / Alternative check:
When you read about the origin of life in standard biology books, you usually see the names Oparin and Haldane together with a description of early Earth conditions, reducing atmosphere and formation of organic molecules. The Miller Urey experiment is then introduced as a later test of this chemical evolution idea. In contrast, when you read about the origin of species, Darwin and Wallace are discussed. Germ theory centres on Pasteur and Koch, and the one gene one enzyme principle is linked to Beadle and Tatum. Because these associations are very consistent across textbooks, matching Oparin specifically with origin of life is a reliable choice.

Why Other Options Are Wrong:
Origin of species through natural selection is a concept developed by Charles Darwin, not Oparin. The one germ one disease principle belongs to the development of germ theory by Koch and others. The one gene one enzyme hypothesis is a result from Beadle and Tatum's work on Neurospora, not from Oparin's investigations. The option about blood circulation is unrelated to Oparin and refers instead to classical physiology discoveries. None of these alternatives capture Oparin's contribution to ideas about how life might have begun.

Common Pitfalls:
A frequent mistake is to loosely connect any origin type phrase with Darwin, because his book title contains the word origin. Another pitfall is to confuse origin of species with origin of life, even though they are very different questions. Students may also mix up historical scientists if they memorise names without linking them to specific ideas. To avoid these errors, remember a simple pairing: Darwin equals origin of species and natural selection, Oparin equals origin of life and chemical evolution of the primordial soup. Keeping this association clear will help you answer similar questions confidently.

Final Answer:
Oparin is best known for his hypothesis on the origin of life through chemical evolution on the early Earth.