American scientists Stanley Miller and Harold Urey used an apparatus similar to the one in the diagram below to simulate how life could have formed on Earth. The apparatus contained hydrogen gas, water vapor, ammonia, and methane. These gases were subjected to a spark, to simulate lightning. They found that organic molecules could form from the inorganic gases. Their theory of how life began, however, did not hold up under further testing. What crucial fact did they not have when they conducted their experiment?

F. There was no lightning in Earth's early atmosphere.

G. Ammonia and methane were not in the early atmosphere.

H. The chemicals used in the experiment were contaminated.

I. Organic molecules already existed before the atmosphere formed.