When Plate Tectonics Switched On, Twin Quasars at Cosmic Dawn and the First Animal Ever Named

Read the episode (Narration script)

Several of today's stories are about things that went missing. A chemical signature that vanished from the world's rocks, a fish that keeps vanishing a kilometre down, and some spiders that vanished from a museum. To balance the books, there are also two black holes where even one was hard to explain.

Start with the rocks. Earth is the only planet we know of with plate tectonics, the slow conveyor that builds mountains, opens oceans and drags old crust back down into the interior. Venus and Mars don't have it. When Earth got it is one of the long arguments in geology, with estimates billions of years apart, and the reason is awkward: plate tectonics destroys its own evidence. Old crust gets recycled.

A team at Curtin University in Western Australia, led by Janne Liebmann, has gone looking with lead. Uranium decays into lead at a steady rate, so a block of crust that sits around for hundreds of millions of years builds up its own distinctive mix of lead isotopes. Melt that block to make new granite, and the granite inherits the mix, like a family nose.

Writing in Proceedings of the Royal Society A, the team gathered lead measurements from thousands of rocks spanning almost four billion years. In the older ones, on ancient continental cores around the world, the same inheritance keeps turning up: these rocks were made by re-melting a stock of crust that first formed between 4 and 3.7 billion years ago. For over a billion years, the planet kept reworking the same old material, more or less in place.

Then, between about 2.75 and 2.63 billion years ago, the signature largely disappears. In younger rocks it doesn't come back. A statistical test for sudden shifts finds the same break in the full collection and in regional subsets.

The team's reading is that the ancient stockpile was finally being carried away and recycled, and that magma-making moved to the edges of plates, as it works today. Co-author Chris Kirkland calls it the clearest geological evidence yet for when the tectonic engine switched on. A disappearance is firmer evidence that one regime ended than of exactly what replaced it, and the people who favour an earlier start won't pack up. But it's a sharp line, in a record that offers very few.

Much further back, and much further away. A quasar is a supermassive black hole feeding so quickly that the hot gas spiralling into it outshines its whole galaxy. Finding them in the universe's first billion years is already a headache, because there seems too little time to grow a black hole that big.

Now there are two, side by side. Silvia Onorato of NOIRLab and colleagues were going through James Webb Space Telescope data on about 200 early quasars when a second one showed up beside a quasar known since 2004. The Gemini North telescope in Hawaii confirmed it. We see the pair about 900 million years after the Big Bang, roughly 18,000 light years apart, which is less than the distance from here to the centre of our own galaxy. One black hole is about one and a half billion times the mass of the Sun, the other about 400 million, and both are swallowing gas at around the fastest steady rate theory allows.

Going by how quasars cluster on larger scales, the odds of catching a pair this tight in a survey that size come out hundreds of times too small, though the team notes those predictions are shaky at such short distances. Their suggestion, in Astrophysical Journal Letters, is that colliding young galaxies funnel gas to both black holes at once. If so, there should be more pairs to find.

Back on Earth, and under it. Swordfish spend their days hunting at around 600 metres and their nights near the surface. But satellite tags occasionally record one plunging past a thousand metres. Ciara Willis, working at Woods Hole Oceanographic Institution, and colleagues pooled tag records from 34 North Atlantic swordfish, more than 3,600 days in all, and confirmed just 29 of these dives. They were brief, mostly in daylight, and tended to happen near ocean fronts and major currents. That fits hunting or fleeing a predator less well than a third idea, they report in Marine Biology: the fish is taking a quick vertical sample of the water during a long migration, possibly to get its bearings. How that would work, nobody knows yet.

And finally, a housekeeping job 269 years in the making. Scientific naming of animals officially begins with Linnaeus in 1758, with one exception: a Swedish book on spiders by Carl Clerck, from 1757. That makes the first spider in Clerck's book, an orb weaver called Araneus angulatus, the first animal species ever formally described.

Every species name is meant to be pinned to a reference specimen, and Clerck's are lost. So Jonathan Coddington of the Smithsonian and colleagues caught eight spiders on a Swedish island in the Baltic, matched them to Clerck's drawings and to the species' genome, and designated one male as the new standard. It matters practically, because DNA suggests as many as four unnamed species may be hiding under that one name.

The authors note that if anyone ever needs an objective answer to the question "what's an animal?", there is now one available. It's a spider. Specifically, that one. See you tomorrow.