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3. The earliest life forms we know of were microscopic organisms (microbes) that left signals of their presence in rocks about 3.7 billion years old.
4. There is suggestive evidence that photosynthetic organisms were present approximately 3.2 to 3.5 billion years ago, in the form of stromatolites, layered structures similar to forms that are produced by some modern cyanobacteria, as well as numerous microfossils that have been interpreted as arising from phototrophs
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