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3500 million
1000 years = 1 millennium so 3500 years = 3.5 millennia.
Remembering that there was no year zero, 3500 years before 2014AD would have been 1487BC.
It is impossible to be entirely accurate, but general consensus seems to be around 3500 million years ago, based on algae remains.
0.0035
3500 years
3500
3500
about 3500 years ago
3500
350
Slightly more than 3000 million years ago, perhaps even as long as 3500 million years ago shortly after the earth's crust cooled just enough for bodies of liquid water to accumulate on its surface.