A2.1.4 Evidence for the origin of carbon compounds Notes

The Miller-Urey Experiment: Simulating Early Earth

1. Miller and Urey proposed that Earth’s early atmosphere was reducing, containing gases such as methane (CH₄), ammonia (NH₃), hydrogen (H₂), and water vapor (H₂O).
2. They reasoned that these gases could react to form organic compounds when energy was supplied.

Experimental Setup

1. The experiment used a closed system of two connected flasks to simulate early Earth conditions.
2. Steps:
   1. Evaporation: Water was boiled to create water vapor (simulating the ocean).
   2. Gas Mixture: The vapor mixed with methane, ammonia, and hydrogen (simulating the atmosphere).
   3. Energy Source: Electrical sparks represented lightning, providing energy for reactions.
   4. Condensation: A condenser cooled the vapor, recycling liquid water back into the flask.

Results

1. After several days, the solution turned reddish-brown and chemical analysis revealed over 20 amino acids and other simple organic molecules.
2. This demonstrated that organic compounds could form spontaneously under simulated early Earth conditions.

Significance

1. The experiment showed that:
   1. Complex organic molecules (like amino acids) can form from simple inorganic gases.
   2. Abiogenesis is chemically plausible, life’s building blocks could arise naturally.

Strengths of the Experiment

1. Proof of Concept: Provided the first experimental evidence that organic molecules can form spontaneously under prebiotic conditions.
2. Controlled Simulation: The closed system allowed precise control of variables like gas composition and energy input.
3. Reproducibility: Repeated experiments (with different gases and energy sources) consistently produced organic compounds, supporting the robustness of the findings.

Limitations and Criticisms

1. Questionable Atmosphere Composition:
   1. Later evidence suggests early Earth’s atmosphere was neutral or mildly oxidizing, dominated by CO₂ and N₂, not strongly reducing gases.
   2. Under such conditions, amino acid formation is far less efficient.
2. No Direct Evidence for Life:
   1. The experiment produced amino acids, but not nucleotides or self-replicating molecules.
   2. It demonstrated chemical possibility, not the full origin of life.