Rocks from western Australia called the Warrawoona Group show fossilized prokaryotes:

http://earth.usc.edu/~geol150/evolution/precambrianfossils.html

These rocks are 3.5 billion years old!

More extreme -- but less convincing -- is an argument that the isotopic makeup of the Isua (Greenland) metamorphosed sedimentary rocks indicates that living organisms had to be present when those sediments were deposited ... around 3.85 billion years ago!

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