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Cyanobacteria

a string of cyanobacteria
Argonne National Lab

This blue-green microbe called a cyanobacterium was likely the first photosynthetic organism—and it was a game changer in the story of life on Earth. Over millions of years, oxygen continued to accumulate in the atmosphere, thanks to cyanobacteria and other photosynthetic organisms. It was this oxygen that would allow complex life to thrive in the millennia to come.