Question

What gas do some scientists think was was missing from Earth's early atmosphere?

Answer 1

Oxygen is the gas that some scientist think was missing from the Earth's early atmosphere.

Explanation:

Oxygen in the atmosphere and dissolved in the oceans is mainly a result of photosynthesis.

These scientists think that the complex process of photosynthesis took millions of years to evolve.

So the early atmosphere of the earth according to the standard neo darwinian model had no oxygen, because it took millions of years for the most simple life to begin and that life only used anaerobic fermentation to product energy.

Plants and photosynthetic bacteria evolved much later according to this world view. So there could be no oxygen in the early atmosphere.