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.