03/12/2010 (12:20 pm)
Oxygen was naturally in the atmosphere when the earth was formed. What happened to it - it is not anymore?
The earth's early atmosphere was a mixture of nitrogen, carbon dioxide, water vapour, methane and possibly ammonia. There was no oxygen,
Oxygen is a very reactive element, and usually readily combines with other elements to form oxides. So any oxygen in the early atmosphere would have rapidly reacted with other materials.
So how did the free oxygen get there?
There was only one possible source: life.
Soon after the earth cooled down enough for the seas to form, early primitive live forms evolved. thes were probably bacteria, similar to today's 'blue-green algae'. They didn't need oxygen, but used carbon dioxide and water (and possibly nitrogen rich ammonia) to produce the first sugars and amino acids.
They gave off oxygen as a waste product.
These bacteria were very plentiful, and after a long time (perhaps a billion years), they had produced so much oxygen, that free oxygen began to accumulate in the oceans and in the atmosphere.
Other life forms were able then to evolve.
You could say; were it not for those primitive cyanobacteria, there would be no oxygen in the atmosphere, and life would not have evolved.
There is good evidence to support this theory; look up
'banded iron formation' (rocks that were formed ithosede early years of life on earth) for more information.
It still makes 21% of earth's atmosphere....
But yes earlier it might be in larger quantities earlier. The depletion might be due to respiration.
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