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Go to Brilliant.orgSciShow to learn how you can take your STEM skills to the next level! [ intro ] In a way, humans are part of the one percent. The one percent of species that hasn't gone extinct... yet, that is.

There have been five major extinction events on our planet which, combined, have wiped out at least 99 percent of all species that have ever lived. Lucky for us, humans weren't around during any of them. Because, well, there's a decent chance we wouldn't have survived if we were.

But, obviously, some animals did make it through. In fact, some animals, like jellyfish and comb jellies, have weathered all five. And that may be thanks to the weird thing they have in common: their jelliness.

The main difference between gelatinous animals and the rest of us is water content. Most creatures, including humans, are 60 to 70 percent water. But animals that are considered “gelatinous” are 95 percent water or more.

And there's no doubt that being that gooey has its perks. After all, gelatinous bodies have independently evolved in almost every major phylum of animals— from protists to chordates. And gooeyness might explain why species like jellies survived mass extinctions when most other creatures didn't.

For example, during the late Devonian around 360 million years ago, roughly three-quarters of Earth's species disappeared. Many experts think pulses of low oxygen — or hypoxic events — were a big part of that, especially in oceans. Which makes sense, because oxygen is kind of important — especially for animals.

For the past 800 million years or so, we've been pretty dependent on the stuff. But gelatinous animals can tolerate lower oxygen levels than us firmer creatures. In part, that's because they literally don't breathe.

They just soak up oxygen from their surroundings. Because they're overwhelmingly made of water, O2 can diffuse directly from the water around them into the water inside them. So there's no need for dedicated respiratory structures like gills, lungs, or noses.

They don't even need blood to shuttle oxygen around. And that means they need less oxygen overall, because those complex tissues require a bunch of it to function well. Jellies are also able to store oxygen inside their structural goo, or mesoglea.

So basically, they have a reserve tank that can help them make it through temporary oxygen outages. Of course, hypoxic events aren't the only thing animals have to survive during an extinction. Sudden rises in seawater acidity have also been to blame for a lot of mass death.

These tend to be tied to pulses of volcanic activity, because when volcanoes erupt, they spew carbon dioxide — a lot of which ends up in the water. And dissolved CO2 reacts with seawater to make it more acidic. Acidification can also happen without volcanoes.

Like, that big asteroid that wiped out non-avian dinosaurs about 66 million years ago? Researchers think it vaporized a bunch of sulfur-rich rocks. That led to acidic rains, which caused the ocean's pH to plummet.

But whatever the cause, acidification is a huge problem for lots of marine animals. See, the chemical reactions that drive acidification also reduce the number of carbonate ions in seawater — ions that many species need to build their shells or skeletons. So in acidic waters, carbonate shells and skeletons dissolve or cost more to construct.

And that makes creatures like corals and clams extremely sensitive to even the smallest changes in pH. Even animals that don't build carbonate body parts —like fish— can struggle as seawater pH drops, because the extra acid messes with their blood chemistry. But acidification isn't such a big deal if you don't have blood or carbonate parts.

Some jellies, for instance, can grow and reproduce in extremely acidic conditions. The low pH doesn't seem to bother them. Finally, just in case acidification and hypoxia weren't hard enough to survive, extinction events also tend to mean less food is available.

Because, you know, lots of things die. That's kind of the definition of a mass extinction. And that means anything still living has less to eat.

It turns out even that is not as big a problem if you're gooey. One of the main reasons animals eat is to obtain carbon: an essential molecular building block. Gelatinous bodies contain very little carbon to begin with, so their food demands are much lower.

Plus, when you're gooey, you can really maximize your food intake, because every little morsel in the water sticks to your mucusy feeding parts! Also, thanks to being mostly water, gelatinous creatures are close to neutrally buoyant — so, they don't really sink or float. And that means they don't need to exert much energy to swim.

So basically, it seems like, if you're made of goo, nothing can take you down. Which might explain why gelatinous organisms have outlasted everything from trilobites to T. rex. And they might make it through the next extinction crisis, too — the one happening right now because of us.

We're already seeing signs of extreme biodiversity loss, especially in the ocean. So, there's less food to go around. On top of that, the nutrients in our sewage and spilling out from our farmlands are driving massive blooms of algae which use up the available oxygen in their waters — leading to huge, hypoxic dead zones.

And at least one quarter of the carbon dioxide that we put into the atmosphere from burning fossil fuels is absorbed by our oceans. In fact, in the past 200 years, our oceans have become 30 percent more acidic! So basically: less food, oxygen shortages, and acidification are becoming the new normal in our seas.

And that means being made of goo might be more useful than ever. [Optional: I, for one, welcome our gelatinous overlords.] Thanks for watching this episode of SciShow, which was sponsored by Brilliant. Brilliant offers courses in science, engineering, computer science and math— so, you know, all those topics that you, as a SciShow viewer, might love. And they're really fun and hands on, too.

For example, their Introduction to Neural Networks course explains how we've taught computers to teach themselves— like, how they can learn to identify letters and numbers. And you can take it and all their engaging courses with a Premium subscription. If that sounds like something you'd enjoy, the first 200 people to sign up at Brilliant.org/SciShow will get 20% off the annual price! [ outro ].