When a Tiny Land Bridge Triggered an Ice Age

0:00 Thank you to Brilliant for supporting PBS.

0:04 Just over three million years ago, a giant ground sloth wandering through

0:08 a tropical forest did something that seemed unremarkable.

0:12 It kept walking.

0:13 But these particular steps were anything but [music] ordinary.

0:16 Because this sloth was doing something almost no

0:18 big mammals had done for millions of years.

0:21 It was walking from South America to North America.

0:24 The land it walked on, the ismas of Panama,

0:26 had only recently emerged from beneath the ocean,

0:29 connecting the two continents after a 20 million-year separation.

0:32 And suddenly, South American creatures, from terror birds to giant sloths,

0:36 found themselves with an entire new continent to explore,

0:39 while North America's grumpers and saber-tooth cats migrated south.

0:42 On land, the ismas kicked off possibly the greatest

0:45 natural experiment in the history of life on Earth,

0:48 the so-called Great American Biotic Interchange, or Gabby.

0:51 But in the water, this narrow strip of land did something completely different.

0:55 It divided.

0:56 The ismist split the Atlantic and Pacific into two separate oceans.

0:59 And that division would have a much bigger effect

1:01 on the planet than the land connection ever did.

1:04 Because this tiny strip of land

1:06 probably changed ocean currents around the globe,

1:08 transformed climate patterns, and even helped trigger an ice age.

1:15 In the middle of the 19th century,

1:17 a naturalist studying fish in Central America noticed something odd.

1:20 Of the more than 150 species he

1:22 observed off Panama's [music] eastern and western coasts,

1:25 over one-third seemed to be identical.

1:27 So the oceans must have been connected at some point.

1:29 Not long after, Alfred Russell Wallace, yes, the Wallace line guy,

1:33 noticed a pattern in American fossils that would later

1:36 be recognized as evidence of the great American biotic interchange.

1:39 So scientists began to piece together that an ismas had risen

1:42 to [music] connect the continents and close off that once flowing seaway.

1:45 And over the next century, as plate tectonics came into focus,

1:48 they started to figure out the general picture of how this happened.

1:52 See, after South America broke away from Gonduana in the late messoic era,

1:55 it had been isolated for millions of years,

1:58 giving rise to bizarre creatures like car-sized gipadons and those terror birds.

2:02 Meanwhile, the Feralon plate was slowly subducting

2:04 beneath the Caribbean and South American plates,

2:07 triggering vcanism that built a chain of islands, the Panama Ark.

2:10 And over millions of years, that arc crept eastward,

2:13 eventually colliding with South America [music]

2:15 and driving uplift that pushed some of the seafloor above sea level while

2:19 also causing more volcanism that helped build land.

2:22 Eventually, this combination of volcanic activity and tectonic

2:25 collision pushed enough land up to form the ismas,

2:28 transforming the Panama Ark from an island chain into a land bridge.

2:31 But scientists still hadn't figured out one thing.

2:34 When did it finally become a complete bridge,

2:36 one that sloth could just meander [music] across?

2:38 The first solid evidence came from seafloor cores

2:41 in the 1970s which revealed that groups of tiny

2:43 marine organisms called forammenifera began diverging from each

2:47 other around 3 and a half million years ago.

2:49 Then throughout the 1980s and into the '90s,

2:52 scientists looked at DNA from marine species on both sides of Panama.

2:55 They used the fact that DNA accumulates changes

2:58 over time at a known rate to compare

3:00 how different the Atlantic and Pacific populations had

3:02 become [music] and estimate when the population split.

3:05 This method is often called the molecular clock.

3:07 And these molecular clocks also pointed to around 3 million years ago.

3:10 And the fossil record on land seemed to agree.

3:13 There were tons of terrestrial vertebrates

3:15 crossing between the continents at this time.

3:16 So a mystery solved, right?

3:18 The ismas closed about 3 million years ago.

3:20 The timeline [music] made sense, explaining why the Gabby happened when it did

3:23 and why the oceans changed when they did.

3:25 For nearly 40 years, scientists accepted it.

3:28 But then in 2015, two studies dropped

3:30 just days apart that completely contradicted this timeline.

3:34 The first study compiled fossil records from hundreds of sites across

3:36 both continents looking for patterns

3:38 that previous smaller studies might have missed.

3:40 They discovered that some animals hadn't waited

3:42 until 3 million years ago to cross.

3:45 Instead, there was evidence of earlier pulses of migration

3:47 at [music] both 20 and 6 million years ago.

3:50 And that same study found that the sea creatures backed up this timeline, too.

3:53 Taking a wider look at genetic analyses,

3:55 they found that some marine life actually began evolving

3:58 separately around the time of those pulses of migration,

4:01 as if something was already disrupting the flow between the oceans.

4:04 The land migrations and marine divergences

4:06 challenged the idea that the terrestrial

4:08 groups migrated and marine groups split about 3 million years ago.

4:12 The second study turned to geology,

4:13 specifically tiny crystals called zirkons that form when volcanic rocks cool.

4:18 Because here's the thing, are you ready?

4:19 Panama's rocks have a very distinctive signature.

4:22 They're around 50 million years old and they formed

4:25 in a specific volcanic environment that gives them a unique chemical makeup.

4:29 It's like a geological fingerprint that's only found in Panama.

4:32 So when the researchers looked at 15 million-year-old

4:34 river and shallow marine sediments in South America, they found something weird.

4:39 Zirkons with Panama's distinctive signature mixed

4:41 in, which meant that rivers were

4:43 already carrying Panama rock debris to South America 15 million years ago.

4:47 And for rivers to be eroding Panama and flowing to South America,

4:50 Panama had to be above water and connected by land.

4:53 Both of these studies pointed to the idea that maybe

4:55 the ismas existed as early as 15 million years ago,

4:58 if not earlier, way before anyone thought.

5:01 But scientists now faced a puzzle.

5:02 If the ismas formed that long ago,

5:04 why did most land animals wait millions of years to cross?

5:08 Because it's clear that the vast

5:09 majority of crossings happened much more recently.

5:11 And how are marine species still mixing

5:13 between oceans until 3 million years ago?

5:15 The evidence pointed to two different timelines entirely and both

5:18 couldn't be right unless scientists were asking the wrong questions.

5:22 Many answering the question of when did

5:24 the estim form required asking a more specific question.

5:27 How exactly did the final closure happen?

5:30 And the solution came from a surprising place.

5:32 Looking at a modern example of what ancient Panama might once have been like,

5:36 the Indonesian archipelago.

5:38 You see, between the Sunda continental shelf

5:39 on the Asian side [music] and the Sahul

5:41 continental shelf on the Australian side sits

5:44 a group of deep water islands called Waclesia.

5:46 Yep.

5:46 Named after [music] the same Wallace,

5:48 Waclesia is also a volcanic arc that creates both

5:51 a partial barrier and a partial connection between two regions.

5:54 These Indonesian islands form a chain with gaps between

5:56 them affecting migration on water and land in interesting ways.

6:00 Like we talked about in our Wallace line episode,

6:02 even narrow gaps between the islands with strong ocean

6:05 currents rushing through them can prevent land animals from crossing.

6:08 Gaps like the Wallace line, for example,

6:10 are enough to confine most creatures to their respective

6:12 sides despite being only about 30 km across.

6:15 And that's because powerful ocean currents

6:17 can help create these invisible barriers.

6:19 Yet, some creatures do make it across, at least occasionally,

6:22 and marine life can still mix through the channels between islands.

6:25 So by 2016, scientists realized what would it

6:28 mean if Panama didn't form all at once,

6:30 but instead as a complex chain of islands

6:32 first that slowly closed over millions of years.

6:34 This would explain everything in the conflicting data.

6:37 There could be early islands above water separated

6:39 by deep channels that still allowed for submarine

6:41 mixing while strong currents held off most

6:43 of the land crossings until the very end.

6:45 Armed with this new model,

6:47 scientists could finally piece together a more detailed picture

6:49 of the closure with a timeline to go with it.

6:52 Most scientists now agree that 24 million years ago,

6:55 the Panama Ark riding on the Caribbean

6:57 plate first collided with South America underwater.

7:00 At this point, the Atlantic and Pacific oceans

7:02 were still connected with deep channels open between them.

7:05 But by sometime between 15 and 9 million years ago,

7:08 some islands had pushed above sea level,

7:10 which is why there's evidence of rivers from those zirkons.

7:12 Deep channels, however, continued to separate them.

7:15 Then by around 9 million years ago,

7:17 the deepest passages closed and deep water critters became

7:20 cut off while shallow water creatures continued to move across.

7:23 As more and more land emerged from beneath the sea and got closer together,

7:26 some animals were able to island hop across,

7:29 explaining those early migration pulses.

7:31 And only around 3 million years ago did the ismas appear to close completely,

7:35 forming a continuous bridge that allowed sloths

7:38 and many others to wander across freely.

7:40 Now, it's important to note that this is

7:41 still an active area of scientific debate.

7:43 While most researchers agree with this model

7:45 of the ismas closing gradually over millions of years,

7:48 they're still arguing about the exact timing of different stages.

7:51 But this new model helped scientists develop more

7:53 detailed hypotheses about what happened next and why.

7:56 Because while scientists had long suspected that the ismas

7:59 played a role in altering the global climate,

8:01 this new timeline helped clarify that it was

8:03 the final closure that had the biggest [music] impact.

8:05 You see, for millions of years,

8:07 warm equatorial waters had flowed westward around the globe,

8:10 including through the seaway between the Atlantic and Pacific.

8:13 But suddenly, in geological terms at least, that path was blocked,

8:17 forcing these warm waters to flow north instead.

8:19 And one hypothesis suggests that this deflection

8:22 strengthened what we now call the Gulf Stream.

8:24 Caribbean waters became warmer and saltier.

8:27 And crucially, all that warm water now flowed toward Greenland and the Arctic.

8:31 Warmer water meant more evaporation,

8:32 [music] and more evaporation meant more water vapor in the atmosphere.

8:36 Vapor that atmospheric circulation was carrying toward the poles.

8:39 For the first time in millions of years,

8:41 northern latitude saw enough snowfall that not

8:44 all of it melted during the summers.

8:46 And as the snow accumulated year after year, it compacted into ice.

8:50 By 2.7 million years ago, major glaciation had begun in the northern hemisphere.

8:55 The ice age had arrived.

8:56 Now, the ismas definitely wasn't the only factor at play here.

9:00 Global ice ages are complicated, it turns out.

9:02 For example, during the period that the ismas was forming,

9:05 CO2 levels were also declining,

9:06 which played a role in the global cooling that had already begun.

9:09 And because there are competing factors,

9:11 scientists still debate how much of a role

9:13 the ismas played in intensifying those global changes.

9:16 Some scientists have even argued that the ismas actually

9:18 played no role at all in triggering the ice age,

9:21 pointing to models that fail to show this as a straightforward cause and effect.

9:24 The fact that it's been so difficult

9:26 to pin down the exact chronology of the closure

9:28 means it also has been really hard

9:29 to establish causal connections that result from that closure.

9:32 But while a scientific debate continues,

9:35 many researchers agree that this narrow strip of land [music]

9:37 probably played a key role in fundamentally reorganizing Earth's climate.

9:41 In fact, this tiny piece of land likely shaped the climate we have today.

9:44 But this geological saga wasn't some sudden

9:47 crash of continents that suddenly built the ismas.

9:49 Instead, it was a 12 million-year process of gradual closure,

9:53 islands rising, channels narrowing,

9:55 and current squeezing through ever smaller gaps.

9:57 And when the ismas finally closed completely,

9:59 [music] this small strip of land barely more than 50 kilometers across

10:03 at its narrowest seems to have had cascading effects across Earth's systems,

10:07 managing to not only reorganize life on two continents,

10:10 but also redirecting ocean currents and helping to trigger an ice age,

10:14 reminding us that sometimes relatively small changes

10:17 can leave big marks on the planet's history.

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11:19 One group of North American animals

11:20 that flourished in South America were the dogs,

11:23 but their fascinating evolutionary story was overlooked

11:25 by Charles Darwin during his famous voyage,

11:27 [music] which is dumb because his boat was literally named the Beagle.

11:30 Learn more in our episode,

11:32 Darwin missed an example of evolution right under his nose.

11:35 Also thanks to this month's eontologists who help connect us.

11:38 Jake Hart, John Davis, Ing, Addie, Carl Wolfful, Juan M,

11:43 Jackie Scott Rston, Andy, Eric Higgins,

11:46 Raphael Hassa, Melanie Lamb, Carnival, and Steve.

11:51 Become an Eonite at patreon.com/eons and you can get fun perks

11:55 like access to exclusive polls and videos from the Eons team.

11:59 And as always, thanks for joining me in the Andy and Eric Hagen studio.

12:02 Subscribe at youtube.com/eons for more tectonic tales.

12:13 I can cry on Q.

12:14 Do you want me to cry?

12:15 I'll do it.

12:16 I don't think that's relevant.

12:17 Just give me 45 seconds.

12:18 Can you uncry [laughter] crying right now?

12:24 Yeah.

12:24 Can I [laughter] Oh my gosh.

12:26 Suck them back in.

12:27 Suck them in.

12:28 Yeah.

12:28 No, I wasn't.

12:29 It's like a geological fingerprint that's only found in Panama.

12:32 See, I can turn it on and turn it off just like that.

12:34 [laughter]