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The Hadean Eon is the first eon in Earth’s history as a planet. It begins at the time when the planet’s crust formed, and extends to about 3.8-3.5 billion years ago. Early in the Hadean the oceans formed as the atmosphere cooled and water condensed. Late in the Hadean life had begun, and left traces in forms of particular forms of carbon isotopes deposited in the oldest sedimentary rocks (from about 3.7-3.9 billion years ago). The Apex cherts from Pilbara Supergroup (northwest Australia) have fossils of what is presumably cyanobactera in rocks that are 3.47 billion years old, among the oldest fossil records of life on Earth. During the Hadean Eon there was probably intense ultraviolet radiation hitting the planet’s surface since there was little oxygen in the atmosphere (and thus, very little or no ozone layer).
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