Multiple choice

In the atmosphere of the primitive earth there was no

  1. ammonia

  2. carbon dioxide

  3. nitrogen

  4. oxygen

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

Primitive Earth had a reducing atmosphere composed of ammonia, methane, hydrogen, carbon dioxide, and nitrogen - but NO free oxygen. Oxygen only appeared later when photosynthetic cyanobacteria evolved and began producing it as a byproduct. This oxygen eventually accumulated to form our current oxygen-rich atmosphere.