00:00This invisible line divides the world into two parts,
00:05and most animals, fish, and birds, for some reason, can't cross it.
00:10The line is called the Wallace Line, and it runs between Australia and Asia.
00:15It has its name after Alfred Russell Wallace,
00:18a British scientist and explorer who discovered it in 1859.
00:23He traveled to Asia to explore a group of islands called the Malay Archipelago.
00:29As Wallace walked through the jungles, climbed hills, and sailed on boats,
00:34he noticed something like an invisible wall.
00:38On its west side, in places like Borneo and Sumatra,
00:42he saw animals you might find in Asia.
00:45There were big rhinos munching on leaves and monkeys swinging from tree to tree.
00:50The forests were full of life, with all kinds of animals you'd expect to see in a jungle.
00:57But in places like Sulawesi and Lombok,
01:01just across the imaginary line he came up with,
01:04Wallace found animals more like the ones in Australia and colorful birds like cockatoos.
01:11The behaviors of animals differ on different sides of the Wallace Line.
01:15Some creatures are great at traveling across tough barriers like water, but others are not.
01:21For example, bats can fly across open water,
01:25but they sometimes cross the Wallace Line.
01:28On the other hand, big animals that live on land like elephants usually stay on just one side of the line.
01:35Some birds are shy and like to stay hidden in thick bushes and trees to avoid predators.
01:40They wouldn't feel safe flying over wide stretches of water with no place to hide.
01:46Plants don't stick to the Wallace Line as strictly as animals do,
01:50because they spread in their own unique ways, like with seeds carried by the wind, water, or animals.
01:57One plant genus, the eucalyptus, mostly stays on the Australian side,
02:02but one eucalyptus tree has made its way to the Philippines on the Asian side.
02:08Underwater, the Wallace Line doesn't matter as much.
02:12The area between the Wallace Line and Australia is called the Coral Triangle,
02:17and it's bursting with life!
02:20Now, after more than 150 years, scientists think they finally have an explanation of the Wallace Line mystery.
02:28They believe it all started about 35 million years ago,
02:32when Australia broke away from Antarctica and slowly moved up to crash into Asia.
02:38When this happened, the crash created a chain of islands called the Malay Archipelago between Asia and Australia.
02:46This caused extreme changes in the weather, which made the area very different and affected which animals could live there.
02:53Researchers used a computer model that looked at how more than 20,000 species of animals from both sides of the Wallace Line would have behaved millions of years ago.
03:04When Australia started to drift away from Antarctica, it opened up a huge new era of deep ocean around Antarctica.
03:12This is where the biggest ocean current on Earth, called the Antarctic Circumpolar Current, began to flow.
03:19This swirling current made the planet's climate much cooler and completely changed how animals could live in different places.
03:26But not all animals were affected in the same way.
03:30Indonesia stayed warm and rainy, which was perfect for animals from Asia.
03:35These creatures could hop from island to island like stepping stones and make their way toward Australia.
03:41But animals from Australia had a harder time.
03:44Over millions of years, they had gotten used to living in cooler, drier places.
03:48So the tropical islands didn't suit them as well.
03:53Scientists believe if we can see how animals adapted to big changes in weather patterns millions of years ago,
03:59we might be able to predict how today's animals will cope with a similar situation.
04:07Other scientists after Wallace also tried to describe and explain the invisible natural border between Asia and Australia,
04:14and came up with their lines.
04:17The Weber Line, for example, is in a slightly different location than the Wallace Line,
04:22between the Oriental and the Australasian faunal regions within Wallacea.
04:27It's a group of mainly Indonesian islands that are separated by deep water straits from the Asian and Australian continental shelves.
04:36They were never connected to the big continents by land,
04:39so animals that could swim, fly or float across the water made it to these islands.
04:44That's why the creatures living there are so unique and come from both Asia and Australia.
04:50Weber's Line is where the balance tips.
04:53If you go one way, most of the animals are from Asia.
04:56If you go the other way, most of the animals are from Australia.
05:01Another important migration event in the history of our planet happened in the Americas.
05:07Around 30 million years ago, South America became a separate landmass as tectonic plates moved and pulled it away from Antarctica and Africa.
05:17Once it was on its own, the animals living there started to evolve in unique ways,
05:22and turned into creatures you couldn't find anywhere else in the world.
05:27There were animals we still see today, like opossums.
05:30Giant rodents like capybaras and chinchillas.
05:33Armadillos, anteaters and sloths.
05:38Some of the animals from those times are now extinct.
05:41There were giant ground sloths, bigger than cars, roaming around.
05:46There were glyptodonts, which were like giant armadillos with super tough, bony shells.
05:52There were also huge, scary birds that couldn't fly but could chase down their prey.
05:59Up in North America, animals could still travel to other parts of the world through the Bering Strait.
06:05So North America was full of different animals like deer, bison, horses, camels, and even mammoths and mastodons.
06:17They all lived alongside predators like big cats, wolves, and bears.
06:24About 3 million years ago, the continents of North and South America got connected by a strip of land called the Isthmus of Panama.
06:33It formed because the tectonic plates shifted again.
06:38Some major volcanic activity between 25 million years ago also contributed to the process.
06:45Animals started to move between North and South America,
06:48and this long process went down in history as the Great American Interchange.
06:54During the Ice Age, huge glaciers in the north grew and shrank over tens of thousands of years.
07:01When the glaciers were at their biggest, they held so much water that the sea levels dropped,
07:07and forests probably turned into wide, grassy plains.
07:11So animals had the perfect conditions to cross over.
07:15Giant ground sloths were among the animals that went up north from South America, all the way to Alaska.
07:23One special kind even roamed the chilly tundras alongside woolly mammoths and caribou.
07:30Lots of North American predators decided to explore the south.
07:35There were cats like cougars, jaguars, and saber-toothed tigers with their giant teeth.
07:41Wolves, foxes, otters, raccoons, and bears also joined the migration.
07:47The short-faced bear could stand up to 13 feet tall and weigh as much as a small car.
07:54South America also had some funny-looking animals called sanus, or South American native undulates.
08:01These were hoofed mammals that evolved into all kinds of weird shapes while South America was isolated.
08:09Big animals from North America, for example, relatives of elephants with strange tusks,
08:14came to the south and became some of the largest creatures around.
08:19The ancestors of today's llamas settled in the Andes Mountains,
08:23and deer, tapirs, and peccaries thrived in the forests.
08:28Animals kept moving back and forth between the continents for thousands of years.
08:33As the Ice Age ended, many of the bigger animals, like the giant ground sloths, couldn't adapt and disappear.
08:40It seems North American animals were better at adapting to their new homes.
08:45You can find armadillos, porcupines, and opossums from South America in North America.
08:51But many of South America's most famous animals like jaguars, llamas, and spectacled bears left North America long ago.
09:01Scientists think South America's animals, after being isolated for millions of years,
09:06had a harder time dealing with new challenges when their ecosystems changed.
09:13That's it for today. So hey, if you pacified your curiosity, then give the video a like and share it with your friends.
09:19Or if you want more, just click on these videos and stay on the Bright Side!
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