Question

What essential gas of today's modern atmosphere was absent from earth's early atmosphere?

Answer 1

Oxygen was not present in the early atmosphere.

Explanation:

Oxygen is the product of photosynthesis. When photosynthetic organisms first evolved and started producing oxygen, much of it reacted with dissolved iron in the oceans. Only once the iron content dropped did oxygen begin to build up in the atmosphere.

Although essential to us, oxygen is a highly reactive gas and would have been toxic to early life. It is thought that the build up of oxygen in the oceans and atmosphere lead to the evolution of eukaryotes.