Earth’s mass extinctions share a surprising hidden pattern

Every mass extinction comes with a named culprit. At the end of the Cretaceous it was an asteroid; at the end of the Permian, a long run of volcanic eruptions in Siberia.

A new study of the past 539 million years shows what condition the planet was in when each one arrived.

Earth’s climate and carbon cycle had only five long-lived states in all that time, and life was at its most fragile during the switches between them.

The team came together at a study group in Edinburgh hosted by the International Centre for Mathematical Sciences, where mathematicians, paleontologists and climate modelers worked the same problem.

Ivan Sudakow, a mathematician at The Open University in the UK, is the lead author of the study. In an interview with Earth.com, he explained the evolutionary implications plainly.

Andrej Spiridonov, a professor of paleontology and Earth systems science at Vilnius University, built the part of the analysis that scores the health of the biosphere.

“There is predictability in the long-term evolutionary dynamics, and it stems from the coupling with long-term carbon cycle and climate states,” Spiridonov said.

Earth’s climate changed in jumps

Chemists read two ratios in the carbonate rock that formed on ancient sea floors. One tracks roughly how warm the ocean was, the other how carbon moved through it.

The team spaced both records evenly, 1,140 points, one every 472,000 years, back to 538.5 million years ago. Then they compared each moment with all the others.

Drawn as a grid, the answers come out in blocks. Inside one, the system keeps returning to conditions it has been in before.

The boundaries fell near 447, 358, 239 and 57 million years ago. A geologist would recognize the sequence: the hot early Paleozoic, a late Paleozoic ice age, the greenhouse world of the dinosaurs, and today’s colder world.

The team also plotted temperature against carbon dioxide.

Each state clusters in its own region of that graph rather than along one curve. If that holds, the warming from a given rise in CO₂ depends on which state the planet is in.

Only four states could be checked that way, because the CO₂ record doesn’t reach past about 420 million years.

Warning signs before the biggest crises

A system near a switch behaves in a telling way. Small disturbances stop fading as fast, so each wobble in a record looks more like the wobble before it.

Researchers who study modern tipping points watch for exactly that. The team measured it here inside a window 20 million years wide, where a rising value means the system is slower to recover from a disturbance.

Those warning intervals don’t fall randomly. They cluster around the end-Ordovician, the late Paleozoic, the end-Triassic and the warm spike between the Paleocene and the Eocene.

On geological time the switches between climate states were fast. States that had lasted tens to hundreds of millions of years gave way over a few million. On the time scale of an animal’s life, these were lengthy changes.

“These transitions were incredibly slow on the organismal time scales, and they would have been imperceptible for animals living during these time periods,” Spiridonov told Earth.com.

He said the warning signals imply strong flickering between climate states, some of it severe enough to be called a mass extinction.

Vulnerability peaks match the Big Five

Counting extinctions alone misses half of what a stressed biosphere does. The team combined the rate at which genera vanished with the rate at which new ones appeared, then weighed that turnover against the diversity alive at the time.

A high score means life was turning over fast relative to the variety it had in reserve.

A genus is a group of closely related species, and the fossil record tracks genera far more reliably than individual species.

The counts here came from marine invertebrates logged in the Paleobiology Database. Nothing that lived on land is in that tally.

The team ran the same test with two rival counting methods, which disagree about how to handle a patchy fossil record. The two agreed.

Vulnerability peaked in the early and middle Paleozoic. It peaked again at the Permian-Triassic boundary, where the end-Permian extinction killed as much as 95 percent of species. From the Early Triassic onward the score fell, and kept falling.

Every interval of elevated extinction the team examined scores above zero, with one exception. The smallest, in the mid-Pliocene about 5 to 2 million years ago, doesn’t.

The Cambrian world stayed under strain

The oldest of the five climate states runs from 539 to 447 million years ago, the stretch that includes the Cambrian explosion. Its scores stay high the whole way, on a par with the values around the Permian-Triassic crisis.

Asked by Earth.com why that stretch stands apart, Spiridonov said the climates then were both extremely warm and unstable.

Genera appeared and vanished at high rates while global diversity stayed relatively low, and that is the combination the index scores worst.

The warm world ended in a long cooling and the end-Ordovician ice age. Spridonov said the same warning signs turn up in evolutionary rates before the Late Ordovician mass extinction, which wiped out an estimated 85 percent or more of species.

“Apparently, the first of the classical ‘Big Five’ mass extinctions potentially could have been predicted in advance,” he added.

The 450 million years after that extinction repeat themselves far more. Spiridonov said that difference is probably related to land plants taking hold on the continents, which changed the way that carbon moves through the whole system.

Today’s biosphere is the calmest on record

Run the same score across the last 57 million years and it stays below zero the whole way, still falling at the end. No other interval scores that low.

“The Cenozoic Era is essentially ‘anti-Cambrian’ with respect to vulnerability,” Spiridonov noted.

He said life should handle a severe disturbance better against this background. Diversity should climb back faster than it would have in other periods, as far as anyone can tell.

The authors give a plain reason for the decline. Diversity has kept building while extinction rates eased, and today’s genera tend to be long-lived ones.

None of that is a statement about a single human century. The finest step in this climate record is about half a million years wide.

Climate state boundaries need more testing

A switch between states was not the direct cause of every mass extinction. On the authors’ reading, the long-term structure sets the background stress level that a volcanic province or an asteroid arrives into.

They built the method for global records rather than a single cliff face, where missing layers can imitate a worldwide change.

Deep-time records are unevenly sampled and imperfectly dated, so both chemical records had to be smoothed and filled in first. The boundaries also don’t line up with every class of extinction the same way.

Underneath all of it is a more basic question. Nobody has worked out which feedbacks keep a state stable for tens of millions of years. What mix of slow pressure and sudden shock flips it out of one is also unsettled.

Next, the authors want to test different background conditions with the same short disturbance and see whether the resulting extinction changes in size.

For now, the study shows how the planet changed over time, but not what kept those conditions stable for millions of years.

The full study was published in the journal Nature Communications.

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