“As the geologist [[Shanan Peters]] calculated: ‘if every living thing other than humans burned up, oxygen levels would fall from 20.9% to 20.4%’. **It would also take millions of years to deplete the globe’s oxygen supply by any notable amount**. The oxygen in our atmosphere came from phytoplankton in the oceans, millions of years ago. Before then, Earth’s atmosphere had none; microorganisms lived anaerobically – meaning they didn’t need oxygen – or were ‘extremophiles’ living in intense environments, fuelled by elements such as sulphur. “**Around 2.5 million years ago, the Earth had its ‘Great Oxidation Event’ where cyanobacteria – the first organisms to photosynthesise – started to convert CO2 into O2**. That’s where most of our oxygen comes from, and it’s very hard to significantly change this balance. ([Location 1956](https://readwise.io/to_kindle?action=open&asin=B0C3X6X695&location=1956))” --- **Tags** -- [[quotes]], [[photosynthesis]], [[carbon-dioxide]], [[climate-change]], [[anaerobic]] **Source** -- [[260103133118 - B - Not the End of the World]]