Redrawing the geological timeline of Earth’s first billion years is casting new light on whether life emerged on land or in the oceans
EARTH is 4.5 billion years old. It formed at the same time as the sun and the other planets in our solar system when thousands of rocks, large and small, collided and merged. Then came a hammer blow. A smaller planet seems to have struck Earth with such force that our world’s entire surface melted and huge amounts of material were blasted into orbit, eventually forming the moon. This explains why Earth’s first 500 million years are called the Hadean aeon after the mythical Greek underworld. But eventually, the vast sea of hot magma cooled and solidified into rock. Oceans formed. Land masses emerged.
The young planet would still have looked profoundly different from today. There was no oxygen in the air, so lots of metallic iron was dissolved in the water, staining the oceans green. Levels of atmospheric methane were perhaps high enough to colour the sky orange. And any land would have been barren rock, mostly dark black and grey.
How this otherworldly landscape came into being has long been a mystery. When did the oceans emerge? How deep were they? Was there always exposed land above the waves, or was Earth once a true water world? Now, a sketchy picture is starting to emerge. It doesn’t just reveal surprises about the making of our planet. Understanding when Earth first got dry land also shapes ideas about how life began. Some scenarios for its origin require exposed surfaces, so they can’t be true if everything was underwater. The question of Earth’s first land is therefore also a question about the origins of every living thing, including us.