00:00Deep inside the heart of the Amazon rainforest lies a cave so massive, it defies logic.
00:08Stretching for 2,000 feet, this tunnel is so big a human could easily walk around inside of it.
00:15Its walls are etched with deep, mysterious claw marks.
00:18But this is just one of many.
00:21If you travel to the south of Brazil, you'll stumble upon over 1,500 similar caves.
00:27Ranging from modest, tiny burrows to massive passageways.
00:32These are not your average spooky, bat-filled caverns.
00:36They're what researchers call paleoburrows, also known as ancient underground structures dug by creatures so colossal, they make modern elephants look small.
00:48Whatever creature dug those tunnels displaced 800 elephants' worth of dirt.
00:52I know, I know, it still looks like a normal cave, but experts noticed this one was special because the walls were too neat and rounded.
01:01And the floor was too smooth.
01:04Besides, no water or erosion leaves such signs of serious digging power.
01:09These Brazilian caves are the largest geological records of animal activity on the planet.
01:14It's like stumbling into a prehistoric house made by creatures the size of cars.
01:21To unlock this mystery, researchers analyzed the scratch marks on the walls and realized they had probably been made by giant ground sloths.
01:30Despite being huge, these animals were not earthbenders, they had to work in groups if they wanted to dig such long tunnels.
01:48This is why scientists believe many sloths shared the same tunnel and dug it together.
01:54Even some giant armadillos may have helped with the digging.
01:58After all, they were ridiculously skilled excavators and lived there too.
02:04Okay, but why would a giant sloth or armadillo need a 2,000-foot-long underground lair?
02:10Great question.
02:12And the truth is, scientists aren't totally sure.
02:15It doesn't make sense for them to dig tunnels this massive just to escape predators.
02:21Especially since the tunnels were big enough for a saber-toothed tiger to crawl in, for example.
02:27So, unless they installed prehistoric security systems, this theory has holes.
02:32Ha ha ha.
02:33Pun intended.
02:34The best bet is probably climate.
02:37Back then, everything was drier and colder.
02:39And some species of giant sloths were living their best lives back when Earth was transitioning between cold and warmer periods.
02:48Their big bodies usually protected these animals from temperature changes.
02:52But drastic fluctuations probably made them hide in order to survive.
02:57Besides, just like modern sloths, giant sloths probably had a hard time maintaining their body temperature.
03:02And going underground for a while is a great way to protect your babies and save some energy.
03:09After all, the bigger you are, the more energy you spend.
03:14Some scientists believe these tunnels might have had rotating tenants.
03:19One sloth moves in, digs a bit, leaves some scratch marks, moves out.
03:23And sometime later, another one crashes there and adds its own decor touches.
03:27A hostel for ancient megafauna.
03:31All evidence points to a more timeshare situation than a prehistoric private property.
03:36After all, these creatures weren't exactly social butterflies.
03:40Most sloths and armadillos usually keep things strictly one-on-one.
03:44Just mom and baby.
03:45Yeah, dad sloths stuck around only for the good part.
03:51But you probably noticed one tiny detail here.
03:54These guys are not around anymore.
03:56They went extinct 11,000 years ago, right around the time humans started to get popular on Earth.
04:03When another ice age was ending.
04:06In fact, 80% of all mammals that were heavier than 2 pounds disappeared then.
04:11The first culprit is, of course, the usual suspect.
04:16Humans.
04:17Around 10,000 years ago, a wave of skilled hunters using distinctive tools like spear points spread across North America.
04:24The theory goes that American animals weren't used to this new predator, making them easy targets.
04:30Once the big plant eaters started vanishing, the big meat eaters followed.
04:35They didn't have enough food to survive.
04:37And it wasn't just excessive hunting that caused some damage.
04:41Early humans may have also harmed ecosystems in ways that made survival harder for large species.
04:47But wait, there's another suspect.
04:51Climate change.
04:52Around the same time, Earth's climate got a major shakeup.
04:56The climate was going from super cold to only slightly cold.
05:00Just so you know, Earth's temperature was about 46 degrees Fahrenheit, making seasons change and plants perish.
05:06And if plants are no longer around, big animals that eat them, like our friends, giant sloths, have less nutrition, making them even more vulnerable.
05:18Many scientists now think that both human impact and climate change played a role, just in different ways, depending on the region.
05:25In some places, humans may have tipped the balance.
05:28In others, nature itself did the job.
05:31Either way, it was a perfect storm that ended the age of giants.
05:34Turns out, being huge isn't always a plus.
05:40Giant animals tend to live longer and reproduce slowly, which makes them sitting ducks when disaster strikes.
05:46Besides, it takes a long time for these guys to bounce back.
05:51After dinos disappeared, for example, it took 15 million years for new mega mammals to stomp onto the scene.
05:58The last major extinction happened around 12,000 years ago.
06:02Not really enough time for brand new massive beasts to evolve.
06:07Today's biggest ground animals, like elephants, rhinos, and bison, aren't newcomers.
06:12They're actually the tough survivors of that ancient chaos.
06:16And, well, at least we still live alongside the blue whale, the biggest mammal ever.
06:21But why did some prehistoric animals get so gigantic in the first place?
06:28Well, being big has some perks, in spite of all the extinction thing.
06:33It keeps you safe from predators and helps you win the fight for food.
06:37If plant eaters bulked up, predators had to level up, too, just to keep up.
06:42And the environment plays a big role.
06:45You see, cold-blooded animals might grow bigger in hot places to stay cool,
06:49and warm-blooded giants thrive in cold places to stay warm.
06:53One paleontologist even suggested that some dino-sized vegetarians needed those massive bellies
06:59just to digest the tough, woody plants of their time, as if they had a salad bar inside their bodies.
07:06There's also the thing with oxygen.
07:11Back then, Earth's atmosphere had way more of it.
07:14That's a big deal, especially for creatures with less efficient breathing systems, like insects.
07:20Higher oxygen levels meant their bodies could handle more growth and more activity,
07:24kind of like nature's turbo mode.
07:27We also have to discuss bones, specifically the lighter ones.
07:31Many huge dinosaurs had air sacs built into their skeletons,
07:35just like modern birds.
07:37This made their bones strong, but not ridiculously heavy,
07:40meaning they could grow super tall or long without collapsing under their own weight.
07:46Those air sacs also helped with cooling down,
07:49which is super important when you're basically a walking oven of muscle.
07:53On the flip side, modern mammals are limited by gravity and biology.
07:58Bigger bodies need stronger hearts, lungs, and blood pressure just to keep functioning,
08:03and land doesn't offer much support.
08:07This also explains why whales got even bigger than dinosaurs.
08:12The ocean acts like a natural support system,
08:14which pretty much means that buoyancy helps hold up all that mass,
08:18so blue whales can hit a jaw-dropping 330,000 pounds without turning into pancakes.
08:25On land, that kind of size would be a serious health hazard.
08:29So, ancient giants weren't just freaks of nature.
08:33They were the result of the right mix of air, food, physics, and environment working together in just the right way.
08:41The whole extinction event surely is a bummer for giant sloths that worked as gigantic engineers of nature,
08:47but it surely prevented us from sharing the planet with insects the size of crows,
08:51like some types of griffin flies or millipedes reaching almost 6 feet in length.
08:57Yikes!
08:58Now, if you'll excuse me, I'm going to appreciate modern sloths,
09:02slow, small, and thankfully incapable of terraforming the landscape and giving me insect-related panic attacks.
09:09That's it for today.
09:13So, hey, if you pacified your curiosity, then give the video a like and share it with your friends.
09:18Or, if you want more, just click on these videos and stay on the bright side!
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