According to a new computer model, early life helped shape Earth’s continents:
The pair included biological weathering in one modelled Earth, and left it out of another. Press play on both Earths at 4 billion years ago, and for the first 1.5 billion years, there is very little difference between them. By about 2.5 billion years ago, early continental plates emerge above the ocean, regardless of life. But then, everything diverges.
On live-Earth, algae, bacteria and more complex life colonise the new land, erode it and dump masses of sediment into the ocean. The sediment – 40 per cent water by weight – is eventually pulled down more than 100 kilometres beneath the surface by early subducting tectonic plates, where piping hot temperatures release the trapped water. The hydrated mantle is viscous and more buoyant, so it rises and bursts through the surface in volcanic eruptions that add to the continental plate. “This is the major factor for how life enhances the continental formation rate,” says Höning.
In the dead Earth model, hardly any continents emerged.
Critics argue that too little is known about plate formation as yet to be sure of life’s role, but concede it is a valuable hypothesis.
However, let’s not overlook this aspect: If Spohn and Höning’s model turns out to be persuasive, it will impact sky’s-the-limit claims about life-friendly planets outside our solar system. In the traditional model, we look for planets hospitable to life. In their model, life creates the hospitality, at least on land. So life comes first.
Whether the bar is now higher or lower one can debate, but it is definitely a different model.