It’s the dream of every little kid out there to befriend a unicorn.

Riding through a magical land astride your magical companion, the wind blowing through your hair… No? Just me?

But on the scale of mythical creatures, compared to dragons and basilisks and stuff, unicorns seem downright realistic. It’s just a horse who knows how to accessorize! You could just as easily ask, why don’t they exist?

Well, we did ask, and we found some answers. Come with me on this highly specific deep dive into the science of how animals evolve to grow stuff out of their heads. [intro] Okay, if we’re going to unpack this, we need to agree on our definition of a unicorn. I know that seems obvious, but this is science!

We’ve gotta define our assumptions. For our purposes today, a unicorn is a horse-adjacent animal with one single horn or pointy appendage growing out of the front of its head, like mine. By horse-adjacent, I mean the ungulates: four-legged mammals divided into two main groups based on how their toes work.

Yes, I know, taxonomy is like that sometimes. Horses belong to the odd-toed ungulates, or perissodactyls, but we’re also going to include the even-toed ungulates, or artiodactyls – the group that includes stuff like deer and cows. And if that’s all we need, guess what?

We’re done! We’ve got not one but two animals that fit this particular bill. The first is almost definitely going to make some of you mad, because it’s narwhals.

Hear me out! They’re cetaceans, which fall into our category of horse-adjacent. Cetaceans are a type of ungulate, and they’re so closely related to artiodactyls that some researchers even mash the two names together and call the group “cetartiodactyls,” which is… a mouthful.

So like it or not, narwhals totally count under our definition. And they’ve got a big horn right on the front of their noggins, so that’s one more tick in favor of them being our planet’s unicorns. And yes, technically they’re rocking a tooth, not a horn, but we never said what it had to be made of!

Now a few of you might have clued in to another animal that meets the description we’ve already given, and you are very smart. But save it, we’re going to get to that one later. Regardless, these real-world animals aren’t exactly going to hit the spot if you want a unicorn.

They’re like having an oatmeal raisin cookie when you really want chocolate chip - it’s just a poor substitute for the real thing. It’s also worth mentioning that humans have manipulated animals like cows and goats to have a single horn in the middle of their heads, whether it’s in the interests of science or hucksterism. But we wanna know why unicorns never happen on their own – as a species, rather than a human ploy or a one in a million accident.

So if we want to look Mother Nature in the eye and ask her how to make a unicorn, we need to look at other animals that have evolved a lot more of the classic horse-y unicorn traits, including anything bone or bone-adjacent growing out of their heads. And there are no perissodactyls with any bony head ornaments. So to look at the evolution of unicorns, we have to make a quick hop to the next branch of the tree of life, back to artiodactyls.

Specifically, we’re looking at pecorans, which are things like deer, cows, and pronghorns. All of the animals that have evolved bony headgear fall into this category, so to study how to make horns on your head, they’re the perfect animals to examine. Plus there’s a lot of old timey illustrations of unicorns with deer-like cloven hooves so I say it counts.

Now there are technically some differences even within the pecorans. For instance, not all headgear is created equal. There are anatomical differences between say, horns and antlers.

Even so, we’re pretty sure all pecoran headgear evolved from one common ancestor that had its own funky headset. Which means that the neat trick of growing these specific kinds of horns… only evolved once. In evolution, something that happens once could happen again, but it’s not super likely.

Sorry, horses. But does this mean we can have our deer-icorn, if two horns could somehow become one horn? A 2019 paper looked at how these guys’ headgear forms and attributed them to neural crest cells: cells that migrate outward from the midline of an organism during embryonic development to create new structures.

Headgear-starting cells migrate to either side of the skull, towards the back of the braincase, and then become bony sticky outy bits. And there are a few genetic changes specific to pecorans that make that happen. Like some modifications to the gene OTOP3, which makes a protein associated with biomineralization.

You know, to make bone grow where it didn’t used to. And some changes to the gene OLIG1, which is the one that tells those neural crest cells what to be, and where to go. Finally, there’s a bit of shuffling around some of the big boss genes of developmental biology.

We’re talking Slug, Twist, the Hox genes – I promise these are genes and not words I just chose at random. See, while OLIG1 makes the neural crest cells move, these guys are the ones that make sure everybody’s working together properly to make a lovely, finished head-bone. Taken together, you have a set of instructions that tell neural crest cells to go to either side of the head and build a brand new bony structure in two very specific places.

In order to get a deer-icorn, we’d need those genes to tell the neural crest cells to go to the front and center of the head, not onto the sides. And they’d have to form just one new bony protuberance, not two. Meaning re-re-writing all those headgear gene changes.

And you don’t mess with genes like that lightly, because if you do interfere with the basic developmental program, odds are you just never get a grown-up organism that can pass those changes on. It’s not impossible. It seems to have been documented, once, when a deer with a single, centered horn was spotted in Italy in 2008.

But the folks who spotted him think that that deer’s specific situation probably couldn’t be passed down to its offspring, which means he was in all likelihood, one-of-a-kind. So until we see it happen to the genes that do get passed down, we aren’t going to get any deer-icorns. And to get a horse-icorn, all that stuff that happened in pecorans would have to happen again in horses.

Which… is a stretch. Okay, so biology basically says we get two horns or nothing, at least in the even-toed ungulate group . But what if I told you that the real unicorns have been with us the whole time?

Get ready because it’s about to blow your mind - it’s rhinos. It’s a horse-adjacent animal with one horn on the center of its head! Are you not entertained?!

What do you mean, people have been confusing them since medieval times at least? Eat your oatmeal raisin cookies! Rhinos and horses are both perissodactyls, meaning that rhinos are a lot more closely related to horses than deer are.

And as for their sick headgear, I would tell you how rhino horns grow in order to compare them to hypothetical unicorns, but… I can’t. Because of the whole poaching thing, rhino horns are extremely illegal to own, and researchers don’t get to have them either. Which is valid, but it means we don’t know as much about rhino horn development as we do with deer and cows.

But we do know rhino horns are made of keratin, so they’re more like a hair-or-fingernail horn instead of a bone or a tooth. Could a horse go that route and grow a giant fingernail-horn? Well, even though they’re pretty closely related in the grand scheme of things, the two groups are still separated by 50 million years of evolution.

So it’s really hard to say just when that mutation arose in rhinos, or how many changes the horses would have to go through to get there. And again, when there’s a mutation that only happened once, that tells us how unlikely it is to happen a second time. So there you have it.

The reason we can’t have unicorns is basically a developmental biology lecture… you’re welcome. But maybe the real reason is that our world just doesn’t have enough unicorn magic. Hey, if this episode made you crave a bit of that unicorn magic in your life, we’ve got just the thing for you.

It’s not this headband though - this is mine. We do, though, sell a sticker set highlighting all the real-life unicorns of the world, including our lovely rhinos. Head over to the DFTBA store to check them out, along with the rest of our sweet, sweet merch.

That link is down below in the description. Thanks for watching! [ OUTRO ]