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In between two of the islands of Indonesia,\h\h
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there’s an ancient line that\h
is both real, and not real.
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You can’t see it, but it’s there all\h
the same. If you stood on the coast of\h\h
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Bali and looked east to the shores of\h
Lombok, you’d be staring right at the\h\h
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line’s narrowest point: a 32 km stretch\h
of water that seems pretty unassuming.
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This invisible barrier weaves its way\h
through the entire Malay Archipelago,\h\h
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the largest collection of islands on the planet.
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See, on the western side, the animal life\h
is characteristic of Asia, featuring rhinos,\h\h
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elephants, tigers, and woodpeckers, to name a few.
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But cross the line, and things suddenly change.\h
You won't find those same species on the eastern\h\h
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side. Instead, the islands have a totally\h
different cast of ecological characters,\h\h
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including marsupials, Komodo\h
dragons, cockatoos, and honeyeaters.
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This is what scientists call\h
a biogeographic boundary,\h\h
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the meeting point of two regions of\h
biodiversity that are highly distinct.
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And this particular line, called the Wallace Line,\h
is perhaps the sharpest and most iconic of all.
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So how did this invisible line come to be? Why\h
does it shape the distribution of so many species?\h\h
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How did we figure out the path it takes?
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And how can it be both real and imaginary?
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The Wallace line was first sketched out in\h
1859 by a guy named, wait for it, Wallace.
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Alfred Russel Wallace to be\h
exact, a British naturalist who\h\h
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you might have also heard of as the\h
co-discoverer of natural selection.
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That concept came to Wallace in\h
a literal fever dream as he lay\h\h
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bedridden with malaria during part of his\h
eight-year trip around the Malay Archipelago.
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And while he’d end up being\h
overshadowed by Darwin on that front,\h\h
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the second-best idea he had on that trip\h
was the existence of the Wallace line.
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I mean...
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...you know, two good ideas in one trip.
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This idea helped to forever establish\h
him as the father of biogeography,\h\h
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the study of the distribution of living things.
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He had spent his voyage observing and\h
collecting as many species as he could,\h\h
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hopping from island to island across\h
almost the entire archipelago.
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And it was as he moved east from Bali to\h
Lombok that he first noticed something\h\h
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intriguing. Even though the islands\h
were separated only by a narrow strait,\h\h
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the change in animal life wasn’t gradual\h
and subtle, it was sudden and distinct.
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Wallace saw the differences\h
in animal life between the\h\h
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two as being even more striking\h
than between England and Japan!
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It was birds that initially caught\h
his attention. Certain species that\h\h
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were plentiful on Java and Bali - like the\h
yellow headed weaver, coppersmith barbet,\h\h
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and the Javanese three-toed woodpecker\h
- didn’t exist at all on Lombok.
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And this abrupt shift extended to\h
mammals and even many insects, too.
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Almost as if an invisible barrier\h
was separating two different worlds.
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But why? And how?
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The biogeographic line that he drew,\h
which others would tweak in later years,\h\h
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didn't just reflect the proximity of the islands.
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In fact, some islands on opposing\h
sides of the line are closer to each\h\h
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other than many islands on the\h
same sides are to one another.
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So Wallace realized that other, more mysterious\h
forces must be in play. Like, say, geology.
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He recognized that the geological\h
past shapes the biological present.
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And, because the distribution of living species\h
today partly reflects ancient geological events,\h\h
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he saw biogeography as a way\h
to uncover epic chapters of\h\h
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the planet’s history that might\h
otherwise have been unknowable.
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These concepts are easy to take for\h
granted today - we talk about them\h\h
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all the time here on Eons - but they were\h
still fairly new ideas in Wallace’s day.
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And by taking this perspective, he concluded\h
that the western islands must have once all\h\h
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been connected to each other, and to the Asian\h
mainland. While today they are surrounded by\h\h
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shallow seas, this is only the result of\h
a geologically recent rise in sea levels.
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How else could the big animals of that\h
side, like tigers and rhinos and tapirs,\h\h
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have ended up on the islands? Because they’re now\h\h
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separated by expanses of water that are\h
way too wide for those species to cross.
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He had a similar thought about how\h
the islands east of Java and Borneo\h\h
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had formed - at least some of them were the\h
remnants of a former Australian continent.
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Wallace had a hunch that,\h
throughout all of that change,\h\h
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deeper waters with strong currents between\h
the two regions must have prevented many\h\h
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species from crossing from continent to\h
continent when sea levels were lower.
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And this is still preventing many\h
species from crossing today when\h\h
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sea levels are higher and the continents are\h
fragmented into neighboring groups of islands.
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Even many flying bird and\h
insect species obey the line,\h\h
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ones that aren't capable of crossing\h
those stretches of open ocean.
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Wallace had pulled together many pieces of\h
the puzzle, but he and other scientists at\h\h
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the time were missing one key idea to\h
complete the picture… Plate tectonics.
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The surface of the planet is not static,\h
of course, it’s dynamic. It’s made up of\h\h
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individual large sections or plates that move\h
and collide over vast stretches of geologic time.
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It’s yet another concept that’s\h
easy for us to take for granted,\h\h
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but which is actually a relatively recent\h
addition to our understanding of the world.
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In fact, plate tectonics only\h
became widely accepted in the\h\h
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late 1960s – more than half a\h
century after Wallace’s death.
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And also, around the time when I was born.
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Which means, I'm...
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...I'm as old as plate tectonics...
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We now know that plate tectonics shapes our\h
planet in many ways, including forming and\h\h
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deforming continents, raising up island\h
chains, and building mountain ranges.
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And studies have shown the Malay Archipelago\h
to be one of the most complex tectonic\h\h
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regions in the world, a meeting point of\h
multiple plates all jostling for space.
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And this is responsible for not\h
only the area’s many volcanoes and\h\h
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frequent seismic activity, but also the\h
peculiar contrasts of its animal life.
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Because by the 1980s, scientists were able to\h
say with confidence that the Wallace line is,\h\h
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at its core, a result of plate tectonics.
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Wallace had correctly identified\h
that two former continuous land\h\h
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masses had existed on either side\h
of this line in the deep past.
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Today, we know them as the paleocontinents of\h
Sunda in the west and Sahul in the east, both of\h\h
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which existed during the ice ages when more water\h
was locked up in ice and sea levels were lower.
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Wallace didn't know it, but\h
while they’re pretty close now,\h\h
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the two partly-sunken continents\h
used to be much, much further apart.
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The Sahul continent of the\h
eastern side of the line,\h\h
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encompassed Australia, Tasmania,\h
New Guinea, and the Aru islands.
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And it only approached the Asian Sunda\h
continental shelf in the west around 20\h\h
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to 25 million years ago in the late\h
Oligocene or early Miocene epoch.
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This was a result of the Australian\h
plate slowly drifting north over tens\h\h
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of millions of years after breaking\h
away from Antarctica in the south,\h\h
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bringing its distinctive community of\h
birds, reptiles, and marsupials with it.
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So even though the species of each side are\h
neighbors now, they’d been evolving separately\h\h
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for eons, their two worlds only colliding\h
fairly recently in evolutionary terms.
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And in between them, immediately east of\h
the line, a complex force of plate tectonics\h\h
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created a chain of new islands in an area of\h
the archipelago that’s now called Wallacea.
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These oceanic islands differ from\h
the continental islands that flank\h\h
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them in that they were never connected\h
to either of the greater land masses.
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They were ecological blank\h
slates waiting to be filled\h\h
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in with whatever creatures could make it there.
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And those ended up being mostly\h
species from the Australian side,\h\h
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seeing as the Wallace line acted as a\h
barrier to Asian species moving east.
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Take, for instance, the Komodo dragon,\h\h
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a giant monitor lizard that today lives on\h
a handful of islands in eastern Indonesia.
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Their fossils first appear in mainland\h
Australia more than 3 million years ago\h\h
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in the Pliocene epoch, only reaching\h
their current Indonesian island homes\h\h
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in the Wallacea region around 1 million years ago.
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And even now, the deep waters with strong\h
currents that weave between the two regions,\h\h
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including the strait between Lombok and Bali,\h
still limit the dispersal of many species across\h\h
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the line, keeping the differences in their\h
evolutionary history so strikingly visible.
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This is what created the stark contrast\h
bisecting the jungle of islands that\h\h
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Wallace first sketched out in 1859, and\h
that still fascinates biogeographers today.
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Wallace’s invisible line may\h
not be real in a physical sense,\h\h
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but it shows just how loudly ancient\h
geological events can echo through time,\h\h
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and how they shape the diversity and distribution\h
of life in strange and contrasting ways.
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And while Darwin might get virtually\h
all the credit as the guy who figured\h\h
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out how species came to be,\h
Wallace is still recognized\h\h
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as a pioneer in figuring out how\h
species came to be where they are.
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So plate tectonics also explains\h
why Earth has supercontinents!
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You can celebrate this fact with our\h
Saga of the Supercontinents poster\h\h
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that features four of these\h
continental configurations.
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Available now at DFTBA.com.
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And thanks to this month’s\h
top-of-the-line Eontologists!
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I'm gonna have to set up a meeting with Kallie,\h\h
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and me, and Joann from HR, because it's not\h
okay that she's making me say these puns.
\h
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Raphael Haase, Jake Hart,\h
Juan M, Annie & Eric Higgins,\h\h
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John Davison Ng, and Melanie Lam Carnevale.
Become an Eonite at patreon.com/eons and you\h\h
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can get fun perks like submitting a joke\h
for me to read. Here’s one from Sonja.
\h
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You can’t blame barnacles for being\h
clingy… They’re just a tiny shellfish.
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Once I get...
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Once my lungs fill back\h
up, I can finish that joke.
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You can't...you can’t blame barnacles for being\h
clingy…they’re just a tiny shellfish.
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Yeah...
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Ok...
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Laugh doesn't lie, if it makes me laugh,\h\h
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then it must be a joke right?
And as always thanks for joining\h\h
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me in the Adam Lowe studio. Subscribe at\h
youtube.com/eons for more epic epochs.
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Alfred Russel Wallace, to be exact...
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Alfed Russel Wallace...
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Alfred Wussel Wallace, to be exact...
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It was birds that initially...
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It was birds that-
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It was bir- eh-
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...Oh my god.
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Ok mouth, you don't like me and I\h
don't like you, let's just do this.
16229
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