How did the at­mos­phere change over time?

Al­most all liv­ing crea­tures on Earth de­pend on oxy­gen, but has the at­mos­phere al­ways been able to sup­port life?

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Earth’s ear­li­est at­mos­phere had al­most noth­ing in com­mon with the one that we live in to­day. The first at­mos­phere was a thin mix­ture of the very light gases of hy­dro­gen and he­lium. The light gases quickly dis­ap­peared into space and were re­placed by a brand new at­mo­spheric make-up of meth­ane, am­mo­nia, ni­tro­gen, CO , and lots of wa­ter vapour. In this cock­tail of gases, the first life on Earth be­gan to de­velop, but the air did still not con­tain any oxy­gen, as oxy­gen mol­e­cules would im­me­di­ately re­act chem­i­cally with meth­ane, etc.

Earth’s present at­mos­phere is a con­tin­u­a­tion cre­ated by pho­to­syn­thetic aquatic life known as cyanobac­te­ria. They pro­duced so much oxy­gen that the chem­i­cal re­ac­tions could not use all of it, so there was a sur­plus.

The ox­i­da­tion of the at­mos­phere be­gan about three bil­lion years ago, and now, most life forms de­pend on oxy­gen.

Oxy­gen first be­gan to ac­cu­mu­late in the at­mos­phere around three bil­lion years ago.

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