Cambrian explosion: First burst of animal evolution altered chemical makeup of Earth’s mantle

The Cambrian explosion 500 million years ago saw a huge variety of animals evolve – and also led to carbon being buried in the seabed and ultimately carried into the planet’s mantle

By Michael Marshall

When animal life exploded in the oceans more than 500 million years ago, it changed the face of the planet. Now it seems the effects of that burst of evolution reached thousands of kilometers into Earth’s heart.

“We can link a major event that is happening at the Earth’s surface with a fundamental change in the deep Earth,” says Andrea Giuliani at ETH Zurich in Switzerland.

A huge range of animals evolved during the “Cambrian explosion”, which is thought to have begun about 541 million years ago. While some animals probably existed beforehand, the Cambrian explosion saw the emergence of many familiar groups like arthropods – which includes insects and spiders – and animals with backbones.

Giuliani and his colleagues now say they have evidence this evolutionary blossoming had effects thousands of kilometers inside Earth.

The team studied rocks called kimberlites, which are carried to the surface from deep inside the planet. “If we look at kimberlites, we can potentially get a more pristine signal of the deep Earth than using other magmas [molten rocks that have since cooled],” says Giuliani.

They analysed 144 kimberlites and related rocks from 60 locations worldwide. In each kimberlite, the team looked at the mix of different types, or isotopes, of carbon. The two most common forms are carbon-12 and carbon-13, with living organisms generally absorbing the former.

Giuilani’s team found that carbon-12 levels rose in kimberlites younger than 250 million years, probably due to huge amounts of organic matter being buried in sea-floor sediments during the Cambrian explosion.

Some of this material was later carried into the deep Earth via tectonic plate movement. Plates can get forced down in a process called subduction, ending up in Earth’s mantle.

It then takes a long time for this material to travel to the surface in rocks like kimberlite. “The minimum time is about 250 million years or so,” says Giuliani. Very little organic matter is thought to have been deposited 1 billion to 550 million years ago, making the Cambrian explosion the only plausible source of the organic carbon, according to Giuliana.

Journal reference: Science AdvancesDOI: 10.1126/sciadv.abj1325