How far back in time would we be able to go and still breathe our planet’s air?

Focus-Science and Technology - - Q & A - DAVID KNOWLES, VIA EMAIL

To­day, oxy­gen makes up roughly 21 per cent of our air, but it was vir­tu­ally non-ex­is­tent in Earth’s early at­mos­phere. Soon af­ter the ad­vent of pho­to­syn­the­sis 2.4 bil­lion years ago, oxy­gen lev­els crept up to 1 or 2 per cent – if you were to breathe this air, you would die al­most im­me­di­ately. Be­tween 850 and 600 mil­lion years ago, oxy­gen con­cen­tra­tions in­creased steadily from 2 to about 10 per cent: still not enough for hu­mans to sur­vive on. Fast for­ward to 400 mil­lion years ago and you could just about breathe but might feel dizzy and con­fused on about 16 per cent oxy­gen. Around 300 mil­lion years ago, oxy­gen lev­els reached a hu­man-friendly 19 per cent and have not dropped be­low since.


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