When a Tiny Land Bridge Triggered an Ice Age

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Just over 3 million years ago,

a giant ground sloth wandering through a tropical forest did something

that seemed unremarkable – it kept walking.

But these particular steps were anything but ordinary.

Because this sloth was doing something almost no big mammals had done for millions

of years – it was walking from South America to North America.

The land it walked on – the Isthmus of Panama – had only recently

emerged from beneath the ocean,

connecting the two continents after a 20-million-year separation.

And suddenly, South American creatures – from terror birds to giant sloths – found

themselves with an entire new continent to explore,

while North America’s gomphotheres and saber-toothed cats migrated south.

On land, the isthmus kicked off possibly the greatest natural experiment in the history

of life on Earth,

the so-called Great American Biotic Interchange, or GABI.

But in the water, this narrow strip of land did something completely different:

it divided.

The isthmus split the Atlantic and Pacific into two separate oceans.

And that division would have a much bigger effect on the planet than the

land connection ever did.

Because this tiny strip of land probably changed ocean currents around the globe,

transformed climate patterns, and even helped trigger an ice age.

In the middle of the nineteenth century,

a naturalist studying fish in Central America noticed something odd.

Of the more than 150 species he observed off Panama’s eastern

and western coasts,

over one-third seemed to be identical –

so the oceans must have been connected at some point.

Not long after, Alfred Russel Wallace – yes,

the Wallace Line guy – noticed a pattern in American fossils

that would later be recognized

as evidence of the Great American Biotic Interchange.

So scientists began to piece together

that an isthmus had risen to connect the continents

and close off that once-flowing seaway.

And over the next century, as plate tectonics came into focus,

they started to figure out the general picture of how this happened.

See, after South America broke away from Gondwana in the late Mesozoic Era,

it had been isolated for millions of years – giving rise to bizarre creatures

like car-sized glyptodonts and those terror birds.

Meanwhile, the Farallon Plate was slowly subducting beneath the Caribbean and South American plates,

triggering volcanism that built a chain of islands – the Panama arc.

And over millions of years, that arc crept eastward,

eventually colliding with South America

and driving uplift that pushed some of the seafloor above sea level

while also causing more volcanism

that helped build land.

Eventually, this combination of volcanic activity

and tectonic collision pushed enough land up to form the isthmus – transforming the

Panama arc from an island chain into a land bridge.

But scientists still hadn’t figured out one thing:

when did it finally become a complete bridge,

one that sloths could just meander across?

The first solid evidence came from seafloor cores in the 1970s,

which revealed that groups of tiny marine organisms called foraminifera began diverging from each

other around 3.5 million years ago.

Then, throughout the 1980s and into the ‘90s,

scientists looked at DNA from marine species on both sides of Panama.

They used the fact

that DNA accumulates changes over time at a known rate to compare how different

the Atlantic and Pacific populations had become,

and estimate when the populations split.

This method is often called the ‘molecular clock,’

and these molecular clocks also pointed to around 3 million years ago.

And the fossil record on land seemed to agree – there were tons of

terrestrial vertebrates crossing between the continents at this time.

So mystery solved, right?

The isthmus closed about 3 million years ago.

The timeline made sense,

explaining why the GABI happened

when it did and why the oceans changed

when they did.

For nearly forty years, scientists accepted it.

But then, in 2015,

two studies dropped just days apart that completely contradicted this timeline.

The first study compiled fossil records from hundreds of sites across both continents,

looking for patterns that previous, smaller studies might have missed...

They discovered that some animals hadn’t waited until 3 million years ago to cross.

Instead, there was evidence of earlier pulses of migration,

at both 20 and 6 million years ago.

And that same study found that the sea creatures backed up this timeline, too.

Taking a wider look at genetic analyses,

they found that some marine life actually began evolving separately around the time of

those pulses of migration –

as if something was already disrupting the flow between the oceans.

The land migrations and marine divergences challenged the idea that the terrestrial groups migrated,

and marine groups split, about 3 million years ago.

The second study turned to geology – specifically,

tiny crystals called zircons that form when volcanic rocks cool.

Because here's the thing, are you ready?

Panama's rocks have a very distinctive signature - they're around 50 million years old

and they formed in a specific volcanic environment

that gives them a unique chemical makeup.

It's like a geological fingerprint that's only found in Panama.

So when the researchers looked at 15-million-year-old river

and shallow marine sediments in South America,

they found something weird – zircons with Panama's distinctive signature mixed in.

Which meant that rivers were already carrying Panama rock debris to South America 15

million years ago.

And for rivers to be eroding Panama and flowing to South America,

Panama had to be above water and connected by land.

Both of these studies pointed to the idea

that maybe the isthmus existed

as early as 15 million years ago –

if not earlier – way before anyone thought.

But scientists now faced a puzzle.

If the isthmus formed that long ago,

why did most land animals wait millions of years to cross?

Because it’s clear that the vast majority of crossings happened much more recently.

And how were marine species still mixing between oceans until 3 million years ago?

The evidence pointed to two different timelines entirely.

And both couldn’t be right… unless scientists were asking the wrong questions.

Maybe answering the question of 'when did the isthmus form' required asking a more

specific question:

how, exactly, did the final closure happen?

And the solution came from a surprising place – looking at a modern example

of what ancient Panama might have once been like:

the Indonesian archipelago.

See, between the Sunda continental shelf on the Asian side

and the Sahul continental shelf on the Australian side,

sits a group of deep water islands called Wallacea – yep,

named after that same Wallace.

Wallacea is also a volcanic arc

that creates both a partial barrier

and a partial connection between two regions.

These Indonesian islands form a chain with gaps between them,

affecting migration on water and land in interesting ways.

Like we talked about in our Wallace Line episode,

even narrow gaps between the islands – with strong ocean currents rushing through them

– can prevent land animals from crossing.

Gaps like the Wallace Line, for example,

are enough to confine most creatures to their respective sides – despite being only

about 30 kilometers across.

And that’s because powerful ocean currents can help create these invisible barriers.

Yet some creatures do make it across, at least occasionally.

And marine life can still mix through the channels between islands.

So by 2016, scientists realized:

What would it mean if Panama didn't form all at once,

but instead as a complex chain of islands first,

that slowly closed over millions of years?

This would explain everything in the conflicting data… There could be early islands above

water,

separated by deep channels

that still allowed for some marine mixing –

while strong currents held off most of the land crossings until the very end.

Armed with this new model,

scientists could finally piece together a more detailed picture of the closure,

with a timeline to go with it.

Most scientists now agree that, 24 million years ago, the Panama Arc,

riding on the Caribbean Plate, first collided with South America underwater.

At this point, the Atlantic and Pacific oceans were still connected,

with deep channels open between them.

But by sometime between 15 and 9 million years ago,

some islands had pushed above sea level –

which is why there’s evidence of rivers from those zircons.

Deep channels, however, continued to separate them.

Then, by around 9 million years ago,

the deepest passages closed and deep water critters became cut off,

while shallow water creatures continued to move across.

As more and more land emerged from beneath the sea and got closer together,

some animals were able to island-hop across – explaining those early migration pulses.

And only around 3 million years ago did the isthmus appear to close completely,

forming a continuous bridge that allowed sloths and many others to wander across freely.

Now, it's important to note

that this is still an active area of scientific debate.

While most researchers agree with this model of the isthmus closing gradually over millions

of years,

they're still arguing about the exact timing of different stages.

But this new model helped scientists develop more detailed hypotheses about what happened next

– and why.

Because, while scientists had long suspected

that the isthmus played a role in altering the global climate,

this new timeline helped clarify

that it was the final closure

that had the biggest impact.

You see, for millions of years,

warm equatorial waters had flowed westward around the globe – including through the seaway

between the Atlantic and Pacific.

But suddenly – in geological terms at least – that path was blocked,

forcing these warm waters to flow north instead.

And one hypothesis suggests

that this deflection strengthened what we now call the Gulf Stream.

Caribbean waters became warmer and saltier – and, crucially,

all that warm water now flowed toward Greenland and the Arctic.

Warmer water meant more evaporation.

And more evaporation meant more water vapor in the atmosphere – vapor

that atmospheric circulation was carrying toward the poles.

For the first time in millions of years,

northern latitudes saw enough snowfall that not all of it melted during the summers.

And as the snow accumulated year after year, it compacted into ice.

By 2.7 million years ago, major glaciation had begun in the Northern Hemisphere.

The ice age had arrived.

Now, the isthmus definitely wasn't the only factor at play here – global ice

ages are complicated,

it turns out.

For example, during the period that the isthmus was forming,

CO2 levels were also declining,

which played a role in the global cooling that had already begun.

And because there are competing factors,

scientists still debate how much of a role the isthmus played in intensifying those

global changes.

Some scientists have even argued

that the isthmus actually played no role at all in triggering the ice age,

pointing to models that fail to show this as a straightforward cause and effect.

The fact that it’s been

so difficult to pin down the exact chronology of the closure means

that it has also been really hard to establish casual connections

that result from that closure.

But while the scientific debate continues,

many researchers agree that this narrow strip of land probably played a key role

in fundamentally reorganizing Earth's climate.

In fact – this tiny piece of land likely shaped the climate we have today.

But this geological saga wasn't some sudden crash of continents

that suddenly built the isthmus.

Instead, it was a 12-million-year process of gradual closure – islands rising,

channels narrowing, and currents squeezing through ever-smaller gaps.

And when the isthmus finally closed completely,

this small strip of land – barely more than 50 kilometers across at its

narrowest – seems to have had cascading effects across Earth's systems.

Managing to not only reorganize life on two continents,

but also redirecting ocean currents and helping to trigger an ice age.

Reminding us that, sometimes,

relatively small changes can leave big marks on the planet’s history.

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One group of North American animals that flourished in South America were the dogs,

but their fascinating evolutionary story was overlooked by Charles Darwin during his famous voyage.

Which is dumb because his boat was literally named the Beagle.

Learn more in our episode,

“Darwin Missed An Example of Evolution Right Under His Nose”.

Also thanks to this month’s Eontologists, who help connect us!

Jake Hart, John Davison Ng, Addie, Carl Woelfel, JuanM, Jackie Scott-Ralston,

Annie & Eric Higgins, Raphael Haase, Melanie Lam Carnevale, and S.T.E.V.E.

Become an Eonite at patreon.com/eons and you can get fun perks,

like access to exclusive polls and videos from the Eons team!

And as always thanks for joining me in the Annie and Eric Higgins Studio.

Subscribe at youtube.com/eons for more tectonic tales.

I can cry on cue, do you want me to cry?

I'll do it.

Just give me 45 seconds.

But can you un-cry?

Yeah!

Can I un-cry?

It's like a geological fingerprint that's only found in Panama.

See, I can turn it on and turn it off just like that?