I’ve just had chance to check in a geology book and in that one it says, as I explained earlier that the early earth was devoid of oxygen until stromatolite type bacteria evolved ca 3.5 billion years ago. The book says the atmosphere at that time was probably Nitrogen and CO2 + traces of others but of course proportions are virtually impossible to determine. However, these mutated bacteria started to produce oxygen from CO2 and water by the photosynthetic (i.e. sunlight/uv) method that plants use today. Early emissions of oxygen were soaked up by iron containing rocks to form the rust-banded formations I mentioned. Gradually, the oxygen level in the atmosphere rose from an estimated 0.2% at 3-3.5 billion years ago to about 17% at the end of the so-called vendian geological period at 650 million years ago. So obviously it hasn’t changed much since.
The arrival of oxygen on the atmospheric scene had another useful side effect for life on earth. Before this time, there was no protection for life near the ocean surfaces because intense UV had no barrier to reaching the ocean surface. However, with oxygen present, UV interacted with it to produce ozone which in turn absorbed UV light – thus stopping it from blasting life on the ocean surface. Hence the modern worry about the ozone ‘hole’ that developed over the Antarctic a decade or so ago, but is now thankfully reported to be re-filling.
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