The Biggest Extinction You’ve Never Heard Of

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In the early 20th century, Swiss  paleontologist Hans Georg Stehlin   was digging through fossil collections  when he uncovered something strange. He noticed that there was a point in the fossil  record where most European mammals just…vanished,   only to be replaced

with others native to Asia. It turns out Stehlin had just discovered  the tip of something enormous:   an extinction that reshaped the entire planet. This was a moment when the Earth flipped  from a greenhouse to an icehouse and buried  

Antarctica under a thick sheet  of ice that’s still there today. And while you’ve probably never heard of this  extinction, it was the biggest one since the   end of the dinosaurs, nearly taking out our  primate ancestors, and by extension,

us. But while Stehlin knew he'd found something big,   he didn’t live to see the  true impact of his discovery. Because figuring out what caused all this   chaos – and how far it reached –  took scientists over a century.

With clues turning up everywhere  from the seafloor off Antarctica,   to the deserts of Mongolia, to a single line  of computer code run almost by accident. And the unlikely culprit? An entire continent drifting through  the wrong place at the

wrong time. The story Stehlin had stumbled into  was that, around 34 million years ago,   at the boundary between the Eocene  and Oligocene Epochs, at least 60%   of mammals suddenly disappeared  from the European fossil record. Herbivorous groups like the

early horse  relatives, the palaeotheres vanished,   along with the xiphodontids, which  looked like small antelopes.   And newcomers from Asia suddenly arrived,  including early deer-like ruminants,   pig-like entelodonts, and anthracotheres,  which were similar to hippos. Stehlin named it La Grande

Coupure,  French for “the great cut.” But he couldn’t answer the most important  question, which is: why did this happen? Over the next few decades, most scientists  simply thought that La Grande Coupure   was caused by species from Asia migrating 

into, and outcompeting those from, Europe. That was until 1973, when a paleontologist  studying rodent fossils proposed a new idea.   This extinction could have been the result of a  sudden and dramatic change in Earth’s climate. What he didn’t realize

was that on the  other side of the world, a research   vessel drilling into the seafloor was about to  find evidence that supported his hypothesis. This ship, called the Glomar Challenger,  gave scientists the ability to drill   cores of sediment

that unlocked tens of  millions of years of Earth’s history. And that same year, scientists collected samples   of the Eocene-Oligocene boundary from  between New Zealand and Antarctica. When they pulled this core up, it was full of   shells of tiny

marine organisms  called foraminifera, or forams. Now, as forams build their shells,  they combine calcium and carbonate   or bicarbonate ions to create calcium carbonate. That includes oxygen atoms  from the seawater around them,   meaning that their shells lock in a

snapshot  of what the water was like during that time. The forams from the more recent Oligocene end  of the core contained much more oxygen-18 – a   heavier version of oxygen – than  those at the older Eocene end. And

this shift matters because  it records Earth’s temperature   in a couple of ways, including tracking ice. Heavy oxygen is less likely to end up in glaciers. So when there’s more ice on land, the ocean  has higher concentrations of oxygen-18.

Also, forams in colder water are just more  likely to use oxygen-18 in their shells. So, more heavy oxygen in a foram means the water   was colder when it lived or more  ice existed on land at the time. In

this case, it was both. The scientists looking at the change in  ancient ocean chemistry were actually   seeing the entire planet suddenly get  colder and Antarctica freeze over. Over the next few decades, a clearer picture  slowly emerged of this

dramatic change. The Eocene Earth had been a lush and warm  place, nearly completely free of ice. Greenery covered the polar regions,   including Antarctica, which was home to  rich evergreen forests and shrublands. North America and Eurasia hosted tropical  and

subtropical forests and swamps. Without much ice, sea levels were high. And an inland seaway called the Turgai  Strait separated Europe from Asia. But this balmy climate didn’t last. First, around 34.2 million years ago, the  planet cooled over the

course of 40,000 years. During this time, the surface of the ocean  became a couple degrees Celsius colder. Tropical forests shifted to temperate  ones full of deciduous trees and conifers. Many regions also became more arid. But the most dramatic

change came  half a million years later when,   over another 40,000 years, a thick  sheet of ice built up on Antarctica. By the time these two events were over, the  surface of the ocean had cooled by as much   as

5 degrees Celsius, while on land, temperatures  in North America dropped by as much as 8 degrees. And as the true scale of the change came  into view, scientists began to wonder:   how did a planet-wide cold snap of this

magnitude  only cause grief for the mammals of Europe? Did the others somehow manage to get by unscathed? Then, in 1998, paleontologists examining fossils  collected over decades from across the Gobi   Desert in Mongolia noticed a similar pattern to  what

Stehlin had seen nearly 90 years earlier. They found that, before the Oligocene,  Asian landscapes were dominated by large   perissodactyls, relatives of horses and rhinos. And afterwards, small rodents and  lagomorphs, relatives of rabbits, took over. So La Grande Coupure

wasn’t actually  a one-way migration into Europe.   And as the Earth cooled and ice built  up on land, sea levels declined and the   Turgai Strait that had long separated Europe  and Asia closed, linking the two continents. Animals intermixed,

and those  who could survive the new cold   conditions took the place of those who could not. But the cold wasn’t just in Europe and Asia – it  affected other parts of the planet, too, right? Around this time, scientists

realized that 35%   of marine species had also gone  extinct during this transition. And in North America, other animals were hit hard,   causing a faunal turnover in groups  like gastropods and amphibians. Changes were more gradual in South America, but 

eventually the continent lost over half of its   mammal species too, just through a much slower  decline that unfolded over millions of years. The most important question still  didn’t have an answer though:   what caused the planet to  cool so

much all of a sudden? Well, in 2009, a team of scientists realized  that, at the Eocene-Oligocene boundary,   carbon dioxide, a powerful greenhouse gas,  had dropped by possibly up to a third. The clue also came from forams, specifically from  

the ratio of different versions of the  chemical element boron in their shells. This is affected by ocean acidity, which is set  by the amount of carbon dioxide in the atmosphere. So in the 1970s, forams had revealed  that the

Earth cooled suddenly,   and almost four decades later, they revealed how. But what caused the carbon dioxide  to drop in the first place? Well, it turns out the answer had  actually been proposed a year earlier:   the entire continent of

India was to blame. See, before India rammed into Asia, it was an  island traveling north through the Tethys Ocean. This journey was powered by the rapid subduction  of the tectonic plate that formed the seafloor. But that ancient ocean

was filled with life,   including plankton with calcium  carbonate shells like forams. When these died, their shells fell  to the seafloor and eventually were   subducted along with the rest of the plate. As they heated up, carbon dioxide  was released

and the planet warmed. That was one part of what kept the  Earth so toasty during the Eocene. But around 50 million years ago, India slammed  into Asia and that seafloor suddenly disappeared. Now, that was all still 15 million

years  before the extinction, and just stopping   this greenhouse gas release wouldn’t be enough  to explain the sudden drop in temperature. But those shells weren’t working alone. See, the massive volcanic eruptions known   as the Deccan Traps were triggered as 

India moved over the Réunion hotspot,   resulting in 1.5 million square kilometers  of lava flooding across ancient India. And this was only a few hundred thousand years   before the asteroid impact  that killed the dinosaurs. While the eruption wasn’t the

primary  cause of that extinction, it lit a fuse   that caused the Eocene-Oligocene  extinction 32 million years later. See, all that lava cooled and formed  massive plains of the rock basalt. And as India moved through the warm equatorial  Asia,

this rock went through a chemical  reaction called silicate weathering. This is when carbon dioxide in the atmosphere  dissolves in rainwater and forms carbonic acid,   which reacts with the basalt, creating calcium and  bicarbonate ions that wash away into the

ocean. Carbon atoms that were once in the atmosphere  could end up back in the shell of a plankton,   which falls to the seafloor when it dies – a full   circle journey for the carbon that was  released during the

Eocene subduction. And basalt weathers 5 to 10 times faster  than continental rocks like granite,   so when India spent 20 million years near  the equator with a ton of fresh basalt,   it was the perfect combination  for extreme weathering. And

once India’s voyage came to a sudden end,  that collision pushed up the Himalayan Mountains,   which exposed even more rocks to weathering. So for tens of millions of years, India was  basically a carbon dioxide factory that all   of a

sudden not only shut off, but started  rapidly sucking it back out of the atmosphere. Now, there are a lot of other factors  that played into the exact timing   of the plummeting temperature – like  Earth’s position relative to the

sun   and changes in ocean currents as Antarctica  separated from South America and Australia. But India’s journey is what set the stage for  the cooler world that has persisted ever since. Despite figuring out the root  causes of this global

shift,   though, scientists still thought that Africa  had been spared from the extinction event. The continent sat close to the equator, after all,   so many reasoned that it didn’t experience  such a sudden drop in temperature. But in 2021, scientists

realized African fauna   hadn’t been spared from the extinction at  the Eocene-Oligocene boundary after all. The discovery came from a  paleontologist named Erik Seiffert. He had been playing around with evolutionary  trees on his computer when a particular line   of

code produced a graph that showed  lineages of African rodents were,   in fact, devastated by the extinction. In a way, he was making the same  connections Stehlin had over a   century prior, just now aided by a computer. Together with

a team that included colleague  Dorien de Vries, who was already studying   fossilized teeth from the area, they  investigated further and found that a   whopping 63% of the mammal lineages they studied  were lost across Africa and the Arabian Peninsula.

This hits pretty close to home, given that our own  primate ancestors were evolving during this time. And once they looked at fossilized  primate teeth more closely,   they discovered that, 30 million years  ago, our ancestors nearly didn’t make it.

There was an evolutionary bottleneck – and so  much diversity was lost that, at the low point,   only a single type of anthropoid tooth  – the group that includes us – remained. That tooth type would go on to determine

the diets  and habitats for the anthropoids that followed,   some of which would eventually evolve  into apes and, ultimately, humans. So a mystery that began with  one paleontologist carefully   cataloging fossils took scientists  over a century to fully unravel. And

we’d learn that this event, caused  by one continent’s ill-timed drift,   created a perfect storm that froze  Antarctica, reshaped life across the globe,   and set the stage for our own evolution –  admittedly after a bit of a close shave.

Wanna know about India’s solo trip  to Asia that started with a breakup? Check out our episode, “When India  Was An Island” to learn more.   And thank you to this month’s Eontologists! Addie, Amanda Seabock, Annie &  Eric Higgins,

Benjamin Birdsall,   Carl Woelfel, Jackie Scott-Ralston, Jake Hart,  John Davison Ng, JuanM, Melanie Lam Carnevale,   Nico Robin, Raphael Haase, Tony  Dai, Victor Angkico, and S.T.E.V.E.   Become an Eonite at patreon.com/eons  and you can get fun perks, like getting   a

monthly digital puzzle featuring  paleoart commissioned just for Eons! And as always thanks for joining me  in the Annie and Eric Higgins Studio. Subscribe at youtube.com/eons  for more tours through time. At the boundary between the  Eocene and Oligocene epochs...

It's a potayto, potahto? Mmhmm,  like "niche", "nitch"? Mmhmm, yeah. I like "epics" because then you  can do puns, instead of "eepox". Lame. Along with the xiphodontids... I said it right, but I got scared. [laughs]