Why did nearly all life on Earth die 250 million years ago?S

Among paleontologists, it's sometimes called the "Great Dying." Roughly a quarter of a billion years ago, 90-95 percent of all life on Earth died out. It took 30 million years for the planet to recover. What happened?

Most people are familiar with the extinction event 65 million years ago that wiped out the dinosaurs. But the Great Dying was much more devastating. It left almost nothing alive.

Welcome to the Permian

Let's start with the lay of the land. The era before the Great Dying - also known as the Permo-Triassic Extinction - is called the Permian, and it was a time of rapid animal evolution, including mammal-reptile hybrids called synapsids that looked sort of like giant lizards - some even had big sails on their backs. These early mammals roamed a massive landmass called Pangaea, while the one, giant ocean called Panthalassa teemed with sea creatures, from tiny single-celled organisms to trilobytes and large fish. On land, vast forests of giant ferns were giving way to trees similar to the ones we have on Earth today, dropping seeds in order to reproduce.

Why did nearly all life on Earth die 250 million years ago?S

Basically there was an entire ecosystem of plants and animals on sea and land that you would hardly recognize as earthly - it was as if our planet wasn't really our planet at all. And then a series of catastrophic events managed to destroy most of the life that existed.

The extinction event

Looking at the fossil record, it's clear that there was an abrupt, massive decline in animal diversity.

Why did nearly all life on Earth die 250 million years ago?S

In this chart, you can see that there were actually three die-offs during the Permian, but the one at the end of the Permian and the beginning of the Triassic, 250 million years ago, was extreme. Writing in the Proceedings of the Royal Society B, Earth scientists Sarda Sahney and Michael J Benton call it "the most devastating ecological event of all time." They explain the chart above, which looks at die-offs of tetrapods (four-legged creatures):

Global diversity (dashed line) and mean alpha diversity (solid line) of Permo-Triassic tetrapod families. Extinctions are labelled as 1, Olson's extinction; 2, end-Guadalupian extinction; and 3, end-Permian extinction.

The seas were hit as hard as the land. Sahney and Benton continue:

The impact of the end-Permian event was devastating. In the sea, the level of species loss was 80–96%, and blastoid echinoderms, tabulate and rugose corals, graptolites, trilobites, eurypterids, acanthodians and placoderms disappeared entirely. On land, the dominant Glossopteris flora was replaced, eight orders of insects became extinct and two-thirds of tetrapod families were lost. The only tetrapod lineages to survive were procolophonoids, dicynodonts, and presumably therocephalians, cynodonts, and archosauromorphs, and their Triassic recovery was slow.

Put another way: It's likely that 9 out of 10 marine species and 7 out of 10 land species went extinct. Moreover, this was the only extinction event on Earth that destroyed many species of insects along with other animals.

When you look at the geologic record, there is simply nothing there at the hinge between Permian and Triassic:

Mega-volcanoes and a giant asteroid impact

So what happened? Nobody is completely sure, but there is evidence to support two major catastrophes, both of which would have led to devastating climate change. At the end-Permian, giant mega-volcanoes began to erupt in the Siberian region. Hillel Hoffman writes in National Geographic:

A deadly epoch of volcanic eruptions left a million-square-mile (2.6-million-square-kilometer) fingerprint in Siberia. Below the town of Norilsk lies a two-and-a-half-mile-thick (four-kilometer-thick) pile of lava, overgrown by conifers. Geologists call this vast lava field the Siberian Traps. It wasn't produced by one volcano. "Thick, pulsing flows of glowing magma gushed out from numerous broad, flat volcanoes," said geologist Paul Renne of the Berkeley Geochronology Center. "Hundreds of cubic miles spread across Siberia-enough to cover the Earth to a depth of about 20 feet (6 meters)."

At the same time, there is now ample evidence that an enormous, 9 kilometer asteroid hit the ocean. According to NASA, geologists studying rocks from the Permian period have discovered a well-known fingerprint left by objects that originate from space: "soccer ball-shaped molecules called "fullerenes" (or "buckyballs") with traces of helium and argon gas trapped inside."

Deadly climate change

So you've got massive volcanic eruptions, spewing tons of sulfur and greenhouse gas into the atmosphere. Billowing clouds cut plants off from life-giving light, and acid rain pours from the skies. The ozone layer is shredded. Then you've got this major asteroid impact, whose heat is so intense that it ignites forests. The burning trees release carbon dioxide and other toxins. The end result? A long-term transformation in the Earth's climate, similar to what environmentalists predict in a worst-case scenario for our near future if we continue to burn fossil fuels and release other toxins. Carbon dioxide levels rise, oxygen levels fall, and animals and plants die off by the millions.

Here is one interpretation of the sequence of events:

Not all scientists agree that the asteroid impact caused the volcanic eruptions. Whether the volcanoes or the asteroid came first, it's certain that the Great Dying was caused by the buildup of carbon dioxide both in the sea and on land. The devastation was so complete that the period following the end of the Permian has a 10-million-year "coal gap" when no coal was formed at all. Scientists speculate that's because there was simply not enough plant life to create the coal.

The survivors

So, who survived the Permian-Triassic Great Dying? The fossil record is full of fungi, which thrive on dead bodies. And one group of those reptile-mammal tetrapods, called Lystrosaurus, began to thrive. About the size of pigs, Lystrosaurs were burrowing animals who ate plants. They had a lumbering, splayed gait and beaks that probably allowed them to chomp on rough vegetation. And they sported short, wiggly tails. Early in the Triassic, Lystrosaurs represented 90 percent of all animal life on land. They were alone on a dead world.

Why did nearly all life on Earth die 250 million years ago?S

Why did these guys - our distant ancestors - survive when so many of their fellow creatures didn't? No one is entirely sure. It's possible that their lungs, adapted for life in the tunnels they dug for shelter, were suited to an atmosphere with less oxygen. And it's also possible it was sheer luck.

Their progeny fanned out across the southern part of Pangaea, which gradually separated from the Northern half of the supercontinent and became its own continent, packed with dinosaurs and proto-mammals. Though life never completely died out, it took 30 million years for the planet to have a complete ecosystem again, packed with predators and herbivores and a wide range of flora and fauna.

The Permian - never forget

What does the Great Dying tell us about our place in Earth's current ecosystem? Most importantly it reminds us that our existence is short, contingent, and precarious. Giant extinction events are the norm in geologic history. Indeed, the Triassic period ended with another mega-extinction event, as did the Jurassic period. The Earth has been many different planets through the eons, and the catastrophic changes it goes through on a regular basis mean we'll be lucky to survive as a civilization.

More specifically, the Permo-Triassic extinction event proves that climate change caused by greenhouse gasses can kill nearly every creature on the planet. Regardless of how that greenhouse scenario starts - whether it's a massive volcano or centuries of burning fossil fuels - climate change kills more effectively than a meteor impact. Remember: the meteor and volcanoes only started the cycle of death - the Great Dying was probably as comprehensive as it was due to atmospheric changes.

Many people writing about the Great Dying claim that its most profound lesson is that life always finds a way to survive. But looked at another way, this long-ago moment in geological time teaches us that we're under constant threat of being wiped off the face of the planet with balls of fire and poison gas.

Survival is as mysterious as the Permo-Triassic Extinction itself. Life always finds a way. But it may not be our life, on this planet, in this tiny sliver of linear time.