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