Oxygen content used to be 35 percent...what changed?

Question

I read that about 350 million years ago, the oxygen content was about 35 percent, and now it is 21 percent. What happened to change it? If people had been living at that time, how would the higher oxygen content have affected them?

Answer 1

When the oceans first formed, the waters must have dissolved enormous quantities of reducing iron ions, such as Fe2+. These ferrous ions were the consequences of millions of years of rock weathering in an anaerobic (oxygen-free) environment. The first oxygen produced in the oceans by the early prokaryotic cells would have quickly been taken up in oxidizing reactions with dissolved iron. This oceanic oxidization reaction produces Ferric oxide Fe2O3 that would have deposited in ocean floor sediments. The earliest evidence of this process dates back to the Banded Iron Formations, which reach a peak occurrence in metamorphosed sedimentary rock at least 3.5 billion years old. Most of the major economic deposits of iron ore are from Banded Iron formations. These formations, were created as sediments in ancient oceans and are found in rocks in the range 2 - 3.5 billion years old. Very few banded iron formations have been found with more recent dates, suggesting that the continued production of oxygen had finally exhausted the capability of the dissolved iron ions reservoir. At this point another process started to take up the available oxygen.

Once the ocean reservoir had been exhausted, the newly created oxygen found another large reservoir - reduced minerals available on the barren land. Oxidization of reduced minerals, such as pyrite FeS2 , exposed on land would transfer oxidized substances to rivers and out to the oceans via river flow. Deposits of Fe2O3 that are found in alternating layers with other sediments of land origin are known as Red Beds, and are found to date from 2.0 billion years ago. The earliest occurrence of red beds is roughly simultaneous with the disappearance of the banded iron formation, further evidence that the oceans were cleared of reduced metals before O2 began to diffuse into the atmosphere.
Finally after another 1.5 billion years or so, the red bed reservoir became exhausted too (although it is continually being regenerated through weathering) and oxygen finally started to accumulate in the atmosphere itself. This signal event initiated eukaryotic cell development, land colonization, and species diversification. Perhaps this period rivals differentiation as the most important event in Earth history.

The oxygen built up to today's value only after the colonization of land by green plants, leading to efficient and ubiquitous photosynthesis. The current level of 20% seems stable.

And yes, i would certainly have affected humans!
Why does present-day oxygen sit at 20%? This is not a trivial question since significantly lower or higher levels would be damaging to life. If we had < 15% oxygen, fires would not burn, yet at > 25% oxygen, even wet organic matter would burn freely.
That's The Oxygen Concentration Problem.

Answer 2

That was then. This is now. 350 million was a LONG time ago. Things have changed.

Don't most people who "study" the ancient world model it like it is today?