Final answer:
The Miller-Urey experiment did not include oxygen as part of the mixture simulating the Earth's early atmosphere.
Step-by-step explanation:
The Miller-Urey experiment used a mixture of ammonia, hydrogen, methane, and water to simulate the Earth's early atmosphere. These experiments demonstrated that organic compounds, including amino acids, could spontaneously form under such conditions. Notably, oxygen was not included in this mixture as it was not considered a major component of the early atmosphere. The absence of oxygen in the experiment aligns with the hypothesis that the early Earth's atmosphere was a reducing environment, which would have been conducive to the formations of these organic compounds.