Multiple choice

Which one of the following was not present in earth's atmosphere before the origin of life?

  1. Ammonia

  2. Free hydrogen

  3. Methane

  4. Free oxygen

Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

Before life, Earth's atmosphere had ammonia, methane, and free hydrogen, but no free oxygen. Free oxygen only appeared after photosynthetic organisms evolved and started producing it as a byproduct.