Question

which of the following was most likely responsible for the fact that organic molecules no longer spontaneously form on earth?
the amount of uv radiation increased
the amount of oxygen in the atmosphere increased.
water accumulated in the oceans.
earths surface temperature warmed.

Explanation:

• UV radiation increase: UV radiation can break down molecules. But in the early Earth, organic molecules formed despite some UV. Now, oxygen presence is more crucial.
• Oxygen increase: Oxygen is a highly reactive gas. In the early Earth, the atmosphere was reducing (low oxygen). Organic molecules could form. Now, the atmosphere is oxidizing (high oxygen). Organic molecules would react with oxygen (e.g., combustion) rather than form spontaneously.
• Water in oceans: Water is a medium for many reactions. Its accumulation in oceans doesn't prevent organic molecule formation. In fact, early organic reactions occurred in water.
• Earth's surface warming: Temperature affects reaction rates. But the key factor is the oxidizing atmosphere (due to oxygen) rather than just temperature.

Answer:

The amount of oxygen in the atmosphere increased.