Fire into water (Image: Patrick McFeeley/NGS/Getty)

REWIND Earth’s story 3 or 4 billion years, to when life was emerging. The surface of our planet was starting to cool but still piping hot – possibly about 200 °C. Early, unstable continents may have been forming. Now imagine life doesn’t emerge, and press play.

This is what a new computer model allows us to do, offering the first glimpse of the role that life played in shaping the surface of our blue marble. The results paint a picture of a lifeless Earth as an unrecognisable water-world with very few continents, if any at all.

“It is the first time the biosphere has been taken into account when modelling the interior evolution of the Earth,” says Tilman Spohn of the Institute of Planetary Research in Berlin, Germany.


The key process in the model he built with colleague Dennis Höning is biological weathering of land, by which organisms eat up rocks and create sediments that wash into the oceans and pile up several metres thick. “Just think about lichens that cover bare rocks and provide continuous contact of water with the rock. Or bacteria that produce acids and dissolve rock,” Spohn says.

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 model, live-Earth settles into an equilibrium where plate tectonics create as much continental crust as it destroys. About 40 per cent of the surface is covered in continent, much like today.

The big surprise came from dead-Earth. There, the mantle is drier, so continental crust is produced much more slowly. The planet becomes a stable water-world, with very little continental land (Planetary and Space Science, doi.org/p4r).

A lifeless planet Earth would have been a stable water-world, with very little continental land

Spohn says the difference might have been even more dramatic. With less water inside the mantle, stable continents might never have formed at all. Instead, a vast global ocean might have been dotted with volcanoes.

The modelling suggests that there is a tipping point in the amount of sedimentation beyond which large continents become almost inevitable, and then remain in place. How much life was required to tip Earth into a continental state is not known.

“It’s a great thought experiment,” says geochemist Mark Harrison from the University of California, Los Angeles. But he says too little is known about the processes that drive continental formation to know how relevant the findings are to Earth. Nicolas Flament at the University of Sydney in Australia agrees that more work is needed: “Matching the model to Earth’s history is a challenge because of the lack of consensus on lots of things: when continents emerged; when terrestrial life formed; and how much water is stored in the Earth’s mantle, even today.”

Norm Sleep, a geophysicist at Stanford University, California, however, says the model fits with what we know about the evolution of Earth, and says the process it describes is backed by indirect evidence of life in early subducted rocks. In particular, the presence of aluminium oxide in continental granite is an indication that life was eroding land and dumping sediment into subduction zones, he says. Such oxides comes from clay, which is mostly produced by the biological weathering of rock.

But the field is rife with conflict. Robert Hazen, a mineralogist at George Mason University in Fairfax, Virginia, is sure that life has affected the deep insides of the planet, but says there’s no good evidence of life that could have contributed this sort of weathering so long ago. He says chemical weathering from lifeless sources – like acid rain from the carbon-rich atmosphere – could also have done the trick. “I hope this paper challenges scientists to think about the co-evolution of the geosphere and biosphere,” says Hazen.

This article appeared in print under the headline “Life gave Earth its continents”