The Miller-Urey experiment was important because it showed .
1. exactly how life began on Earth
2. when life first began on Earth
3. that is was possible to form organic molecules from inorganic molecules
4. the kinds of rocks that would contain fossils
5. all of these

The correct answer is 3. The Miller-Urey experiment was important because it showed that is was possible to form organic molecules from inorganic molecules .

Explanation:

The Miller-Urey experiment was a famous experiment from 1953 in which Stanley Miller and Harold Urey demonstrated that under certain circumstances complex amino acids can arise spontaneously. The experiment is seen as an indication of how life on Earth may have originated from inorganic substances, the so-called primordial soup, and it was the first attempt in that direction.  

In the experiment, a mixture of methane, ammonia, water vapor and dihydrogen is heated and exposed to electrical discharges, which were to simulate lightning on the early Earth. It is believed that the friction of the dust particles released during volcanic eruptions was responsible for this lightning. The composition of the mixture was chosen based on what was assumed at that time about the atmosphere of the young Earth before there was life.  

After running continuously for a week with their arrangement, Miller and Urey found that 10 to 15% of the carbon in the system was now in organic compounds. Two percent had formed amino acids, including 13 of the 22 currently used by living cells, glycine being the most common. Sugars and lipids were also formed. However, the building blocks of DNA and RNA (nucleotides) were missing in their entirety. Simple versions of nucleotides such as nucleosides (nucleotides without the phosphate group) were also not found.