Where did the oxygen in the air come from?

Even a common school kid can answer that question.  Even these days, as dumbed down as our educational system has become.

The oxygen comes from photosynthesis, which takes carbon dioxide and water, produces carbohydrates and OXYGEN.

However, oxygen hasn't always been is such great quantities in the atmosphere.  As a matter of fact, it didn't begin to accumulate until about a billion years ago.  Since the Earth is over 4 billion years old, that means for 3/4ths of the Earth's history, the atmosphere didn't hold much oxygen.

So, what was in the atmosphere even when there was no oxygen?  Why, good 'ol carbon dioxide.  In fact, there had to be LOTS AND LOTS of the stuff.  In fact, so much carbon dioxide had to exist, that the man made global warming crowd would be astounded at the quantity.  If 20 percent of the atmosphere is oxygen today, then a large percentage had to be carbon dioxide in the geologic past.  There is no other means by which that conversion event could have taken place.

Yet, the oceans didn't boil over with all that carbon in the atmosphere.  It's a beautiful thing, as Rush once said.

If the Earth has already seen far greater concentrations of carbon dioxide than what is remotely possible by man's actions, then there is no real harm in it, now is there?