Known as the Great Oxidation Event, this was not a sudden catastrophe but a planetary transformation lasting hundreds of millions of years. The oxygen produced by these ancient microbes reacted with iron dissolved in the oceans, creating the banded iron formations we see in rocks today. This process likely triggered a 'Snowball Earth' – a global glaciation – as oxygen removed methane, a potent greenhouse gas, from the atmosphere. While it caused a die-off of anaerobic life that couldn't adapt, i