Louis Pasteur's swan-neck flask experiment definitively disproved the theory of spontaneous generation, thereby supporting the principle that life arises only from pre-existing life. The correct option is (B).
For centuries, a widely accepted belief was that living organisms could spontaneously arise from non-living matter. This idea, known as spontaneous generation, suggested that maggots could appear from rotting meat, or mice from dirty hay. It was a powerful concept because it seemed to explain the appearance of life without needing a prior source. However, as scientific inquiry advanced, this theory faced increasing scrutiny. Louis Pasteur's brilliant experiments were designed to provide a conclusive answer to this long-standing debate.
His work was not directly about the theory of evolution in the sense of how species change over time, but rather about a fundamental aspect of life itself: its origin and continuity. By demonstrating that life does not spontaneously generate, he laid a crucial foundation for understanding how life propagates, which is essential for any theory of evolution. Evolution describes how existing life forms change; Pasteur's work clarified how existing life forms begin (from other life forms).
Here is how Pasteur's experiment addressed this question:
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The Prevailing Belief: Before Pasteur, many scientists, including some prominent ones, still believed in spontaneous generation. They argued that microorganisms, in particular, were simple enough to arise spontaneously from non-living broth or air. Experiments by others, like John Needham, seemed to support this by showing microbial growth in sealed, heated broths, though their methods had flaws.
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Pasteur's Hypothesis: Louis Pasteur hypothesized that microorganisms were present in the air and were responsible for contaminating broths, not that the broth itself generated life. He aimed to prove that life comes only from pre-existing life, a concept known as biogenesis.
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The Swan-Neck Flask Experiment: Pasteur designed a clever experiment using "swan-neck" flasks.
- He prepared nutrient broth (a liquid medium suitable for microbial growth) in several flasks.
- He then heated the broth in the flasks to sterilize it, killing any existing microorganisms.
- Crucially, he used two types of flasks:
- Some had long, S-shaped (swan) necks that were open to the air but trapped dust and airborne microbes in the bends of the neck.
- Others had straight necks, allowing direct exposure to airborne particles.
- He also included control flasks where the swan neck was broken off after sterilization, exposing the broth directly to the air.
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Observations and Results:
- In the swan-neck flasks, the broth remained clear and sterile for extended periods, even though it was exposed to air. The dust and microbes settled in the bends of the neck and could not reach the broth.
- If Pasteur tilted a swan-neck flask, allowing the trapped dust and microbes from the neck to come into contact with the broth, the broth quickly became cloudy with microbial growth.
- In the straight-neck flasks and the broken-neck control flasks, the broth quickly became cloudy with microbial growth, as airborne microbes had direct access. …