What is 21 percent of the atmosphere?

Answer 1

Earth began to cool than water vapor from the atmosphere condensed and fell as rain. Earth than had liquid water and energy from lightening and the sun, which may have played a major role in the origin of oxygen and water build up in the atmosphere. :) D I hope this answered your guy's question

Answer 2

We must blame the algae for that. About 2.4x109 years ago these guys invented photosynthesis. From this invention, all the green plants arose.

The algae sucked up all (well most) of the methane, and CO2 from the atmosphere, and changed it from an atmosphere of perhaps 2% oxygen to one of over 20% oxygen. The Oceans would earlier have been a greenish colour because of the dissolved iron, but with the new oxygen, the iron was precipitated out. We know this from studying banded iron deposits.

All this oxygen his poisoned the anoxic bacteria, and was a major geological event.

But if you have understood the Greenhouse Gas theories, you'll quickly realize that mopping up all the greenhouse gases would remove that effect from the Earths climate, and the Earth went into about 1x109 years (billion) of Snowball Earth, a period when average temperatures at the Equator went to -50oC! And all the oceans were frozen.

However, the volcanoes continued to produce CO2 and methane, and gradually the temperature of the earth rose again. Which was a GOOD THING.

And soon the pre-Cambrian period when lots of soft bodied biota appeared, but of these we have only a poor record.

Answer 3

21% (~20.95%) of Earth's atmosphere comprises oxygen.

Answer 4

21 percent of the Earth's atmosphere is composed of oxygen.