Rusty, iron-rich layers have been found in ocean-floor sediment deposits dating back some 3.2 billion years, 200 million years earlier than cyanobacteria were previously thought to appear. And while cyanobacteria didn't really take off until about 2.3 billion years ago (during what is dubbed the "Great Oxygenation"), the presence of rust in striped rocks from a site in South Africa makes it clear that there was oxygen (and therefore oxygen producers) on Earth 3.2 billion years ago.
The team behind the discovery determined that the rust formed under an oxygen concentration of 0.1%; while this doesn't sound like much, all other rocks studied from the same time period show 0% oxygen.
What this means for Earth's early history, and cyanobacteria's place in it, is still to be determined; it is possible that this is an isolated incidence in our planet's early history (as cyanobacteria didn't become commonplace until nearly a billion years later). But whatever the implications are, it shows how some cyanobacteria were capable of surviving in the UV-laden, asteroid stricken early Earth.
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