0:02 Since life has existed, so has
0:04 extinction. The complete and utter
0:06 eradication of a species, genus, and so
0:08 forth. And just as diverse as life has
0:10 become, so have the causes behind
0:12 extinctions. With some of the more
0:14 common tropes being space rocks not
0:16 following the speed limits, volcanoes
0:18 getting a little too hot, and natural
0:20 climate change. In other words, a lot of
0:22 the time, animals facing extinction are
0:24 really just duking it out with nature,
0:26 not with their fellow living beings.
0:27 However, given how long life's been
0:29 around, it has indeed occurred on
0:32 occasion of animals killing animals to
0:33 such an extent that it results in
0:35 extinction. But usually, it's quite
0:37 local and rather underwhelming. But once
0:39 upon a time, and actually not so long
0:41 ago, there was a bloody battle royale
0:44 which took place that led to not one but
0:46 multiple different extinctions and
0:48 permanently changed history forever with
0:50 its ramifications still even being seen
0:52 today. And this ancient war, you could
0:54 say, was all started by a girl, a girl
0:56 named Gabby. Just kidding. Gabby is not
0:58 a real person, but rather an acronym for
1:00 the great American biotic interchange.
1:02 Might not sound like the most exciting
1:04 thing ever, but trust me, it was a
1:07 chaotic bloody event. Today, if you take
1:08 a walk throughout the majority of North
1:10 America, especially the US, there's a
1:11 chance that at some point you may
1:13 encounter an apossum, a staple of urban
1:15 American wildlife that is nicely
1:17 adjusted to the continent. And key word
1:19 there, being adjusted, as it actually
1:21 doesn't have North American origins,
1:23 rather South American. Kind of odd,
1:25 right? And what you might also observe
1:27 walking around the US is that there
1:29 aren't any camels, llamas, or any kind
1:31 of camelid for that matter walking
1:33 about, which is what you'd expect. Not
1:35 very odd. Whereas in South America,
1:37 you'll find various kinds like the
1:38 iconic alpaca. And that's understating
1:40 it with these animals being near
1:42 synonymous with certain countries in
1:44 South America. And yet in reality,
1:45 camels are not from South America.
1:47 They're from North America. Kind of
1:50 shocking, right? Well, it turns out that
1:52 this example and the apossums are just a
1:54 couple of the clear signs that this
1:56 great interchange really happened. And
1:57 just telling you about these animals
1:59 without any more context is a bit
2:01 confusing. So, let's rewind the clock a
2:03 bit to see just how we wound up here.
2:04 And to achieve this, we need to go back
2:06 to the moment right before Gabby began,
2:08 which brings us to 3 million years ago.
2:11 In other words, the late plyene. At this
2:13 point, both North and South America were
2:14 still isolated from one another and
2:16 largely had been for about 100 million
2:18 years, allowing each to evolve their
2:20 very own unique life. And because of
2:22 this, taking a trip to either place,
2:24 despite their relative close proximity,
2:25 would end up in you having very
2:27 different experiences. South America in
2:29 particular would have been quite the
2:30 mind trip as it primarily was an
2:32 isolated island that had been so for
2:34 quite a while leading to many kind of
2:35 animals you simply could not find
2:37 elsewhere including endemic kind of
2:40 marsupials the sporacodons xenarth
2:43 leoperns estrapthes gonduan and the list
2:45 goes on. Some of the more famous
2:46 individual animals include the
2:48 enkyosaurike lipodants giant ground
2:51 sloths and numerous terirds that could
2:53 be found elsewhere but held unrivaled
2:55 diversity here. Now, if you were to
2:56 visit North America, you would have
2:58 found a whole lot less of these things.
3:01 Instead, finding camelids, deer, horses,
3:03 probosidians, and carnivorans that came
3:06 in the form of cougars, jaguars,
3:08 cannids, bears, lions, and of course,
3:11 saber-tooth cats. So, really, they
3:12 couldn't have been more different or
3:14 separate. Yet, spoiler alert, things
3:16 were about to change. And actually,
3:18 small signs of the upcoming chaos were
3:20 already visible. First skirmishes, as I
3:22 like to call them, had already taken
3:24 place for a very long time. tracing as
3:26 far back as 40 million years ago. And
3:28 what I mean by this is that some animals
3:29 had already invaded through something
3:31 called oceanic dispersal. And most of it
3:32 seems to have been aimed at South
3:34 America. But just like a true surprise
3:36 attack that you would expect from a real
3:38 war, it wasn't from the north that these
3:39 skirmishers came. Rather, they got
3:41 flanked and they came from the east.
3:43 Because as it turns out, most of the
3:45 pre-inchange invaders weren't actually
3:47 North American. Rather African fauna.
3:49 And they mainly included rodents, bats,
3:52 and tortoises. And this may seem rather
3:54 dubious considering the giant distance
3:56 between the two continents. Yet long
3:58 ago, the two were much closer than now
3:59 with a distance likely having been
4:01 smaller and the Mediterranean Sea is
4:03 wide, allowing these animals to raft or
4:05 island to hop their way over. And even
4:06 though this was just the rising action,
4:09 not the climax, so to speak. The arrival
4:10 of these fauna had their own
4:12 ramifications as they outco competed
4:13 many of the South American animals
4:15 namely small marsupials and underwent
4:17 immense diversification of their own
4:19 leading to things like the capiara new
4:21 world porcupines new world monkeys
4:24 chinchillaas and more. Now this
4:25 continued for millions of years but then
4:27 about 7.3 million years ago North
4:30 America made its likely first attack and
4:32 it came in the form of unnamed proyanid
4:34 the group to which animals like raccoons
4:36 belong to. And similar to the earlier
4:38 African arrivals, it was done through
4:40 oceanic dispersal or island hopping and
4:42 represent perhaps the first carnivorous
4:44 placental mammal to ever step foot on
4:46 the continent. And seeing that predatory
4:48 placental mammals are quite successful
4:50 and pretty much everywhere today, it
4:51 perhaps comes as no surprise that this
4:53 prosyanids arrival sent shock waves
4:55 across the continent, especially because
4:56 a part of their diets would have
4:58 consisted of the eggs of endemic
5:00 animals. Which is even why a handful of
5:02 scientists believe that the sebids, i.e.
5:04 the continent's largest predator for
5:05 millions of years might have been
5:07 indirectly erased by these new faces
5:09 with them eating all their unhatched
5:11 children. Not very nice of them. Shortly
5:13 after that, hognos skunks and sigma
5:15 dantine rodents grabbed the next raft
5:18 over causing further change and damage.
5:19 Yet, this was not an attack without a
5:21 response, for there is partial evidence
5:23 that both continents traded warning
5:25 shots, I guess you could say, as terra
5:26 birds and ground sloths in at least
5:27 small numbers might have briefly
5:29 inhabited the southernmost parts of
5:30 North America before the true
5:32 interchange. But whatever the case, all
5:34 of these skirmishes were really just an
5:36 appetizer, a taste of the true war that
5:38 was to come. Because somewhere along the
5:40 way, the two continents were called in
5:42 for support and actually started butting
5:44 heads, literally and figuratively, as
5:46 their tectonic plates had collided. And
5:48 this collision started a long period of
5:50 uplift while also setting off numerous
5:52 chains of underwater volcanoes that
5:53 continuously pumped new land and islands
5:55 to the surface, which culminated in the
5:57 formation of the ismas of Panama about
5:59 2.7 million years ago. You know that
6:01 narrow ship of land that connects the
6:03 continents. And this really cranked up
6:05 the craziness all the way to 100. As
6:07 with its creation, for the first time in
6:10 a long time, North America and South
6:11 America were connected again by a
6:13 physical landbridge, allowing for the
6:15 unparalleled movements of animals, and
6:17 thus the floodgates of chaos were
6:19 opened. At first, animals from both
6:21 sides rushed across at a near equal rate
6:24 with camelids, deer, horses, and
6:26 probosidians moving into South America.
6:28 While South America sent gypadons,
6:30 ground sloths, and various unullet to
6:32 represent their herbivores. While in the
6:34 predator department, it was mainly the
6:35 terror birds that formed the front
6:37 lines, whilst cougars, jaguars,
6:40 saber-tooth cats, canines, and bears
6:42 marched south. Now, setting up a little
6:44 watching post in the ismas of Panama
6:45 would have been absolutely insane in
6:47 these first days given just how narrow
6:48 the strip of land was. And you could
6:50 have seen some of the most iconic
6:52 animals of the Cenazoic meet for the
6:53 very first time, which for them was
6:55 probably like seeing aliens. I mean,
6:57 just imagine how confused these animals
6:59 must have been when a giant terird met a
7:01 smileon or when a smileon encountered a
7:03 car-sized armadillo. I know I sure as
7:05 heck would have been very confused. And
7:07 this confusion seemed to do the trick as
7:09 for a while things remained quite
7:11 neutral with various groups streaming in
7:13 and out in both directions. Yet, it
7:15 wouldn't take too long for the tides to
7:17 shift in favor of one continent. You
7:19 see, for the South American animals that
7:20 reached the higher latitudes of the
7:22 North, they started to encounter a major
7:24 obstacle, which was that all the niches
7:26 they had occupied in South America were
7:28 already dominated by animals in the
7:30 North. And worst yet, they found
7:32 themselves unable to outspecialize the
7:34 North American competition, leading to
7:35 many of the ways of South American
7:37 creatures to die off, while those who
7:38 were able to somewhat establish
7:40 themselves found it very difficult to
7:42 diversify. Take the terirds for example.
7:44 Flightless apex predators that
7:45 essentially made the rules in South
7:48 America. It could reach gigantic sizes.
7:51 And yet only one genus, the Titanis, is
7:52 known from the north and its range is
7:54 restricted to the most southern US
7:56 states with bones only being recovered
7:58 from Florida, Texas, and California. And
8:00 this sort of restriction is also seen in
8:02 the Gipodons, those giant armadillos who
8:04 in South America had become widespread
8:06 and highly diversified. But in the
8:08 north, only the genus Gipthereum,
8:10 managed to secure roots. a nearly half a
8:12 ton member whose range was even more
8:13 limited than in South American
8:15 commanders, the terabirds, with fossils
8:17 only being known from Mexico and Texas.
8:19 But with that said, there was actually a
8:21 third South American animal that did do
8:23 relatively well, and that was Sid the
8:25 sloth. In other words, massive ground
8:28 sloths, which of the three animals I've
8:29 mentioned so far was largely the most
8:31 successful invader, as four different
8:33 lineages are known to have successfully
8:35 occupied the north with even one genus,
8:38 the 3 m or 10-ft long megalonics, having
8:39 successfully made their way all the way
8:41 up to Alaska, where it might have even
8:42 briefly crossed into Asia via the
8:44 Bearing Strait. And this surely would
8:46 have been a sight for sore eyes, as top
8:48 of being tall, it was a thick boy, too,
8:50 weighing well over 1 ton. And across the
8:52 three other lineages, they could be even
8:53 bigger. And the reason they're able to
8:55 be so successful, relatively speaking,
8:56 is because it's thought that these guys
8:58 ability to rear up on their hind limbs,
9:00 thus reaching great heights, gave them
9:02 an advantage over both other South
9:04 American migrants and also the native
9:06 large megapana like the elephant today.
9:08 However, while South America did
9:09 struggle there for the most part, there
9:11 was a certain area where their soldiers
9:13 excelled, so to speak, and that was in
9:16 the small animal department. As
9:17 mentioned at the start, apossums are
9:19 found all over North America, and they
9:21 were one of the many to come on over
9:22 during the interchange, largely owing
9:24 their success to their extremely
9:26 adaptive diets, ability to scavenge, and
9:28 of course, play dead. That definitely
9:29 helped confuse a few hungry mouths along
9:31 the way. And two other little guys that
9:33 did nearly just as well, included the
9:35 smaller armadillos and porcupines, whose
9:37 unique defenses and size proved enough
9:38 for them to carve out their own niches
9:40 in the north. And then on top of this, a
9:43 few amphibians, reptiles, and birds,
9:44 including hummingbirds and parrots,
9:46 found decent success as well. But past
9:48 these animals, there really wasn't that
9:50 much gain for South America. And I don't
9:51 think it's too much of a stretch to say
9:53 that in this interchange, North America
9:55 definitely came out on top, as its
9:57 invading animals proved far more
9:59 successful. And not only that, but
10:01 ravaged much of South America's
10:03 ecosystems. And out of all the areas, it
10:05 was the South American predators that
10:07 likely suffered the most. Prior to the
10:09 interchange, the South American Predator
10:10 Guild was largely made up of
10:12 non-placental mammals, large predatorial
10:14 birds, and a myriad of nasty reptiles.
10:16 And each of these groups was successful
10:17 and lethal carnivores within their own
10:19 right. And yet, they all met their match
10:21 when the North arrived. Short-faced
10:24 bears, direwolves, and saber-tooth cats
10:26 were all part of the exchange, and each
10:28 brought their own problems, big ones at
10:30 that. The bears had great size,
10:31 hibernation, and generalist teeth. The
10:33 direwolves brought their complex social
10:35 packs, and the Macyrodants had their
10:37 size and giant saber-like teeth,
10:38 allowing each of them to carve huge
10:40 chunks out of different niches. And of
10:42 course, you also can't forget that
10:43 otter, raccoons, and skunks entered as
10:45 well, causing their own mischief and
10:48 upending things that much more. And it's
10:49 not over yet, as on top of these
10:51 animals, it's ultimately the Great
10:53 American interchange that resulted in
10:55 not only every canine, but every feline
10:56 we know from South America today,
10:58 including animals regarded as Amazonian
11:01 icons like jaguars. And thus with all
11:03 this influx of carnivores, it's quite
11:04 easy to see how the continent's
11:06 predatory guild was completely flipped
11:07 on its head. And originally it was even
11:09 thought that North American carnivores
11:10 made many of their South American
11:13 counterparts go fully extinct just by
11:15 existing. Namely, a varying amount of
11:17 native reptiles and then also the
11:19 legendary terirds. But with that said,
11:21 in recent years, new debate has sprung
11:22 up pushing back on this idea a little
11:24 bit. But it does seem that for the very
11:26 least the great American interchange
11:28 reshuffled the predator guild and led to
11:30 a drastic decrease in numbers with
11:31 terabird fossils for example becoming
11:33 distinctively more rare only after the
11:35 landbridge formed while the diversity
11:37 greatly slowed as well. And the waves of
11:38 North American predators didn't just
11:40 flip the script for the predator guild
11:42 but also the herbivorous one too with
11:44 the influx of killers taking a toll in
11:46 certain groups. Not to mention that the
11:47 arrival of North American herbivores
11:49 also influenced things as well. And of
11:51 all those impacted here, it seems like
11:53 the South American native unullet, or
11:56 Sanu for short, drew the shortest stick.
11:57 This collection of mammals included five
11:59 orders of now extinct herbivores that
12:01 were placental, typically being a bit
12:02 larger, with some even being
12:04 multi-toned, and in many cases through
12:06 convergent evolution, looked similar to
12:08 herbivores you would find elsewhere on
12:10 the planet. And for almost 60 million
12:11 years, these guys have been living it up
12:14 in South America. But now the party was
12:16 coming crashing down as they were unable
12:18 to compete with the invaders and huge
12:20 portions of them were swiftly replaced.
12:21 In fact, it was only a handful of
12:23 families within these orders like
12:26 macroconids and toxidants that held on
12:28 to the very end. And of the families
12:30 that did survive to the end, that is,
12:31 they tended to be even bigger than the
12:33 norm. Thus leading to the hypothesis
12:35 that perhaps their size helped them put
12:37 up a better defense. Furthermore, it
12:38 seemed that in general, many South
12:40 American mega herbivores fared quite
12:42 well against the invading northerners.
12:44 And this particularly extended to the
12:46 giant xenarthans like ground sloths and
12:48 armadillos. And in fact, these guys
12:50 managed to hold their ground so well
12:52 that the mecha herbivore space was just
12:54 about the only one North Americans could
12:56 not crack with only a handful of gumpers
12:58 and camelids being successful enough to
13:00 survive their migration south. And even
13:02 after 2 million years of the land bridge
13:04 being around, more than 75% of the
13:05 herbivores in South America who weighed
13:07 over 1 ton were native. And when it
13:09 comes to the reason why Xenarthans were
13:11 so successful beyond their size, one
13:13 explanation is often their seemingly
13:15 overkill level of defense. with gipadons
13:17 having their thick armor and in some
13:19 cases literal spiked clubs. Whereas
13:20 ground sloths had not only their stature
13:22 with one member in fact being the
13:24 largest mammal to ever walk on two legs,
13:26 but also had deceptively sharp claws
13:28 that were pretty massive. Especially
13:29 when you consider the fact that they
13:31 were attached to quite powerful arms. If
13:33 you get hit by this, game over. But with
13:35 that all said, while the mega herbivores
13:38 did do well, it was really an unlikely
13:39 group that survived the best, and that
13:41 is marsupials, which were explicitly
13:44 small. These guys did fantastic when it
13:46 came to withstanding the onslaught. As
13:48 besides a few groups dying out, they
13:50 continue to persist and importantly did
13:52 so in large abundance. And opposed to
13:54 the South American mega herbivores, they
13:56 are of a select handful of animals which
13:58 existed not only before the interchange
14:00 but still exist today. Whereas on the
14:02 flip side, not a single one ton mammal
14:04 remains on the continent today. Well,
14:06 unless you're counting domesticated
14:08 animals. But overall, despite the small
14:10 winds for South America here and there,
14:12 North America, as mentioned, was
14:13 definitely the winner of the great
14:14 American interchange as not much of its
14:16 native wildlife went extinct. Whereas
14:18 South America was fundamentally changed
14:20 by the event. And according to some
14:22 research, the continent was actually
14:24 doomed before Gabby even occurred. And
14:26 it all comes down to just one thing,
14:28 geography. Just like a real war,
14:29 geography played a huge part in this
14:31 exchange. It might have been what gave
14:32 the North Americans such a massive
14:34 advantage, doing so in two distinct
14:37 different ways. The first reason was the
14:39 climate of each continent. Now, you'd
14:40 intuitively think this would be a 50/50
14:42 situation, whereas South America equals
14:44 warm and North America equals colder,
14:46 but it's not as simple as that, as a
14:48 climate asymmetry exists between the
14:50 two. You see, if a North American animal
14:52 travel down to South America, they would
14:54 hit tropical warm conditions before they
14:55 even stepped foot on the continent. As
14:57 in the lower parts of Central America,
14:58 things are already starting to heat up
15:00 and humidify. Meaning, by the time they
15:02 make it over, you already know this
15:03 animal can withstand tropical moist
15:05 conditions. Otherwise, they would have
15:06 died off already someplace in the
15:08 central regions. However, on the flip
15:10 side, if a South American animal travels
15:12 north, they will still experience a
15:13 preferred climate once entering North
15:15 America, as the nearby Panama is already
15:17 considered North America and retains a
15:19 simmer climate to many of the southern
15:20 countries. And you think this would be
15:23 good news, and it is in the short term.
15:24 Yet, the moment an animal hits the
15:26 transmexican volcanic belt, things
15:28 quickly change and the weather gets much
15:30 drier and colder, leading to a very rude
15:32 wakeup call and making their chances of
15:35 getting to, say, the US extremely hard.
15:36 And another way to say this is that
15:38 before a North American animal reached
15:40 its final destination, you already knew
15:41 it could handle the new climate. Whereas
15:43 for animals traveling upwards, you
15:45 didn't. As upon entering North America,
15:46 they still would not experience the
15:48 shift in climate, which is not true the
15:50 other way around. And this plays a big
15:52 part in explaining why the Gabby ended
15:54 up the way it did. Yet the study
15:55 suggested one more way in which
15:57 geography altered the outcomes. And this
15:59 reason apparently had an even larger
16:02 impact. So what is this factor you ask?
16:04 Isolation. The isolation levels of each
16:06 continent. As stated before, South
16:08 America for a very large chunk of its
16:10 history was socially distancing itself.
16:11 And the last time it was connected to
16:13 one of its brethren, it was to places
16:15 like Antarctica and Australia, two
16:17 relatively inhospitable and smaller
16:19 continents, relatively speaking. On the
16:21 other hand, North America, while not
16:23 being what I'd say is social, did have
16:24 various points connected directly to
16:27 Eurasia via landbridge, which in turn
16:28 connects to Africa. And what this did
16:30 was that it resulted in a lot of
16:32 migration back and forth between North
16:33 America and the rest of the world,
16:35 welcoming in new faces and exposing its
16:37 animals to a lot more competition and
16:39 threats. And then you also add in the
16:40 fact that North America is larger than
16:43 South America too by almost 40% that is.
16:45 So in other words, North America tougher
16:47 competition, a tougher climate, and a
16:49 larger map, resulting in a brutal battle
16:51 royale over time. And thus, by the time
16:53 the interchange occurred, the animals in
16:55 North America were arguably much more
16:57 meaner and tougher than the somewhat
16:59 chiller animals down south, who evolved
17:00 with less threats, a smaller playing
17:02 area, and were unaware of the carnage
17:04 awaiting for them. And obviously this is
17:06 kind of sad because we're never able to
17:09 see the megapana of South America. But
17:10 selfishly, I guess we should kind of be
17:12 happy for this event because its effects
17:14 ultimately played a big part in how
17:16 nature was when humans first arrived
17:17 with the date being about 23,000 years
17:19 ago for North America and 20,000 for
17:21 South America, give or take. And
17:22 something you may have noticed
17:24 throughout this video is that many of
17:25 the animals I talked about which found
17:28 success or survived, like the mccyrodons
17:30 or certain xenarthans, are no longer
17:33 around. So why is that? Well, it
17:34 probably has nothing to do with the
17:36 actual great interchange, but instead a
17:38 series of events sometimes referred to
17:40 as the late plyosene extinctions.
17:42 Basically, starting the late plyosine
17:43 and lasting to the holysine, the world
17:45 saw a huge eradication of megapaa. And
17:47 no, it wasn't just in these two
17:49 continents, but worldwide. And in the
17:52 Near Arctic, Neotropic and Australasia,
17:53 the total extinction rate for megapana
17:56 was above 50%. And for large to
17:58 giantized megapana, the rate was even
18:00 abyssally higher. And for years and
18:01 years, researchers have sought to
18:03 explain the causes behind these
18:05 mysterious vanishings. And while no
18:06 conclusive cause has ever been agreed
18:08 upon, the main idea does usually relate
18:10 to us, mainly due to our over hunting
18:12 and landscape alteration. While then
18:13 climate change is also cited as
18:15 contributing cause, as around that time
18:16 things started to heat up and lots of
18:18 the ice sheets were treated. But like
18:20 many paleo events, you also have the
18:22 smaller, less supported hypotheses,
18:23 which are admittedly a bit more
18:24 interesting as some of the alternatives
18:26 suggest that it could have been a global
18:28 disease that could transmit through many
18:29 kinds of animals. an extraterrestrial
18:32 impact, a pesky space rock which is
18:34 known as the younger dryass hypothesis,
18:35 and a weakening of Earth's magnetic
18:37 fields that could have caused a huge
18:40 flux in UVB radiation. But I digress.
18:42 These alternatives are, to be honest,
18:44 not very supported. But regardless, I
18:46 think the most fascinating takeaway here
18:48 is the fact that unlike most extinction
18:50 events, this was not caused by mother
18:51 nature, but rather by the simple
18:53 presence of other animals, showing you
18:56 just how devastating competition can be.
18:58 And on a flip side, why it's very very
19:00 important not to introduce non-native animals.
