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In the earlier of Earth's years; the Hadean from memory; there was no water and hence no hydro- anything. This was the first 700 million years or so. (Between 4.5 and 3.8 billion years ago.)

But some very old banded iron deposits come from this early, indicating that these were precipitated by changing chemical conditions.

Another few hundred million years later, the blue-green algae appeared, and having invented photosynthesis with its attendant release of oxygen, created the large banded iron deposits. And some soft-bodied fossils, such as the Ediacaran.

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13y ago

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