Between two groups of islands in the Indian Ocean, there's a seemingly invisible line splitting up two totally different animal populations.

It became known as the Wallace Line, after the scientist who found it. And it took three branches of science to figure out why it exists.

A lot of the time, plant and animal communities will gradually change as you travel across continents or climates. But other times, you'll see a distinct border separating where species live.

That border can sometimes be caused by obvious things like mountains or rivers. But there's no obvious feature along the Wallace Line.

The study of how species are distributed around the world and why they live where they do is called biogeography. It's mainly a branch of biology, since how animals evolve and how good they are at moving around play a big role in where they ended up.

But other scientific disciplines like geology and climatology are important too. And biogeographers use all of these factors to divide the Earth into regions based on their plant and animal communities.

Today, the study of biogeography encompasses the whole globe, but it got its beginning when a naturalist in the 1800s noticed an odd quirk in the flora and fauna of the Indian Ocean. That naturalist was Alfred Russel Wallace, the father of biogeography, whose also famous for being the co-discoverer of natural selection along with some other guy named Charles Darwin.

Wallace traveled throughout the Amazon and Southeast Asia from the 1840s to 60s, searching for evidence of evolution. And he found plenty of it. 

One of the things he noticed was that features like rivers and mountain ranges often had different animals living on either side of them, even if the climate was similar on both sides.

That's because those kinds of physical barriers are one way new species can form -- one population gets split in two, and each one adapts to its new circumstances.

But Wallace also found something a little weirder: a sharp invisible line dividing the species on islands in and near Indonesia. On one side of the line, islands had animals more like those in Aisa. But the other side animals more like Australias.

The two continents are connected by a string of islands, but most of them are less than 160 kilometers apart. So you'd think there would be a gradual change from Asian fauna like tigers and elephants to Australian like kangaroos and platypuses as you hop from one island to the next.

Nope!

Instead, there's an abrupt switch that happens just east of the island of Borneo. And it's not like there's a bunch of mountains in the way either. Or anything else that he could see at the time. The change just kind of happens.

Wallace's idea about where the line was and how it formed were more or less correct. Even though he knew way less about the region's geography than we do now. That's why we still call it the Wallace Line.

He noticed that, for the most part, the water separating all of these islands was pretty shallow. But the water between the Asian- and Australian-like islands was much deeper.

Scientists already knew that the Ice Ages were a thing. So Wallace knew the sea levels would have gone down when all of that water froze into glaciers. He proposed that during the Ice Ages, the islands in the shallower water would have found themselves high and dry, connected to each other and their respective mainlands.

But the deeper water between the two groups of islands would have still been there, so it kept the two ecosystems separated, except for animals that could swim or fly.

Today's scientists still think that that's pretty much what happened. Nice job, Wallace. But now we also know why the water is deeper in that spot. It's where two tectonic plates, or huge layers of the Earth's crust, come together.

The details of plate tectonics weren't worked out until long after Wallace's death. But his observations got the ball rolling.

We now know that tectonic plates act as species-boundaries in other parts of the world too. And that's helped explain a lot of mysteries. Like why the animals of North and South America are so different, even though it's one giant piece of land.

Turns out, they weren't actually connected until two tectonic plates crashed into each other a few million years ago. After a lot of species had already evolved.

When you look at a map, it's easy to forget that the Earth has changed a ton over time. But those shifts in geology and climate are what shaped the animal communities we see today.

And the cool party is that the more we know about geology, the more we can figure out about biology, and vice versa.

You can't completely explain why different species live where they do without also knowing about things like Ice Ages and continental plates. Which kind of makes science one, big, nerdy family.

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