Thanks to the Monterey Bay Aquarium for partnering with us on this episode of SciShow.

They hope you find it fin-teresting! [♪ INTRO] There’s a funny story about beavers. In the 1700s, a Canadian bishop asked  the Catholic church if it was okay to eat beaver during Lent, a period of time  when Catholics abstain from eating meat.

But eating fish was okay and,  look, beavers spend so much time in the water they should totally count. And, remarkably, the church agreed. Beavers, at least as far as it  mattered during Lent, were fish.

Well, in most cases, people  would probably say that beavers obviously should not count as fish. They have fur and live babies  and breathe air, for instance. As for what science says.

Oh boy. Uh, see, the thing is, the  answer is surprisingly complicated. Because beavers are not fish, but that  might be because either we’re all fish, or fish kind of don’t exist.

In the end, this is going to be  a story about different ways of grouping animals together, why we  use language the way that we do, and how evolution has made things kind of funny. For a long time, people grouped animals together by some common body plan or lifestyle. Like “fish”, “reptile”, or “bird”.

This is good because it  means we can talk about many different organisms at once without  needing to name each one individually. This lets us make useful general statements,  like “It’s fish tacos for lunch today”. And this is generally how science worked too.

But over the years, as we  learned more about evolution, it became clear that this old way  of talking wouldn’t work as well. There were too many exceptions. Too many groups of things that seemed similar but weren’t when you looked under the hood.

Instead, we developed a  new, more scientific way of grouping animals together called cladistics. In this approach, we sort living  things into groups, or clades, that reflect their evolutionary history  and how closely related they are. There’s a couple of different  ways to do this precisely, but a good example might be to say that,  rather than trying to define primates by things like opposable thumbs or  big brains, instead we would say: “The group of animals known as  primates includes all species that are descended from the most recent common  ancestor of humans, apes, and lemurs.” That’s a bit of a mouthful,  but defining things this way is actually really useful for biologists.

It gives us a way to make  predictions, for instance. Like if two different species are  grouped very closely together, we can predict that their genomes  must be relatively similar. This runs into funny situations, though.

Like, again, the question of whether  beavers should count as fish. Let’s, take, for example, the evolutionary history of a very-definitely-a-fish fish, like a goldfish. Its early ancestors were some  of the first vertebrates.

Unlike their close more  worm-like cousins lancelets, they had a backbone, which  let them swim more powerfully. As time went on, this group split. Some, which would become the  ancestors of hagfish and lamprey, stayed roughly how they were.

But the group that led to goldfish  would go on to develop powerful jaws. Over time, more groups split off  as more characteristics evolved. Sharks and rays split off, while the  goldfish’s ancestors got bony skeletons.

In another split, lobe-finned fishes diverged. These are the ancestors of fish with thick, fleshy paired fins for limbs,  like coelacanths and lungfish. Meanwhile, our goldfish’s  ancestors went on to develop fins made of bony spines connected by skin.

Eventually, we arrive at today,  and the familiar goldfish. So far so good. And everything we mentioned so far would be in the category of what most people would call a fish.

But, it turns out, the beaver's  evolutionary story is largely the same. Beavers share the same  vertebrate ancestor as goldfish, and their ancestors also watched  hagfish and sharks split off. And as lobed-fin fish split off, some  of them, a group called tetrapods, used their fleshy limbs to  haul themselves onto land, eventually giving rise to early mammals,  and, eventually, to our beaver friends.

And, of course, we humans are in there too! In the tetrapod group. Along with every other tetrapod, like frogs, and alligators, snakes, birds, and horses.

If you zoom out, the tetrapod  branch is just one nestled among a bunch of different fishy clades. So, back to the question: are beavers fish? Remember that in the cladistic  approach, for a definition to be useful, we want to include everything that is  descended from a common ancestor so that we can make predictions about who is  related to whom on an evolutionary scale.

So this means that, if we want to  be able to say that sharks, hagfish, and goldfish are all fish, we  kind of have three options: One, animals like humans and beavers are fish. Two, you have to carve out some  ridiculous definition, like: “Fish would be any organism that  is descended from the most recent common ancestor of both hagfish  and goldfish EXCEPT for the subset of that group that is descended from the most recent common ancestor of frogs and beavers.” Which is awkward, but it also kind  of defeats the purpose of cladistics. Or three, fish, as a scientific  category, don’t exist.

So in the end, are we fish? Well, we are all vertebrates. And there are  smaller categories like “ray-finned fish” or “sharks and rays”, but there is no single  “fish” category in the cladistic approach!

If that sounds like scientists being  unnecessarily pedantic, they do it for a reason. Despite the foibles, the cladistic  approach has been a huge success. It helps scientists talk about the evolution of life on Earth in a way that makes sense.

And if there are some weird edge cases,  it’s not the fault of the system, but an opportunity to marvel at  the history of life on Earth. And it also means that we use  language in different ways based on what is useful for that conversation. That means it’s still okay  to call a fish a fish, and, if it’s Lent, maybe even to  include beavers in that category.

Thanks again to the Monterey Bay Aquarium for collaborating with us on this episode! They think it would beavery cool if you checked out their social channels  or montereybayaquarium.org. The people at the Monterey Bay  Aquarium are o/fish/ial sea enthusiasts who love to share the wonders  of the underwater world, so they have curated in-person  and online exhibits that anyone with internet access can enjoy.

They range from live exhibit webcams to crafts, like puppets and coloring pages, to  educational videos in English and Spanish! And if you want to see what  the Aquarium calls fishes, you can surf their webpage  called “Animals from A to Z.” There, they give you the option to filter  your environment and just see fishes, complete with cool facts and  conservation practices for each one. [♪ OUTRO]