Thumbnails are super important, guys.

And I’m not talking about the ones that entice you to click on our videos. I’m talking about these thumbnails.

Because when you’re an animal with thumbs, sometimes claws just won’t do. And some animals only have nails on their thumbs, because they’re just that helpful. But exactly how you and I  started growing thumbnails, or any nails really, is still a mystery.

So let’s dive into what  scientists have uncovered so far. [INTRO] Let’s start with a bit of Nail 101. The Latin name for a nail is an ungula. Not to be confused with unguis, the general term for any kind of  thing protruding out of the digits: claw, hoof, nail, et cetera.

And claws, nails, and hooves  do all share some basic parts. For example, they all contain  a protein called keratin, which gives them their toughness. But nails are unique because they feature a flat  plate of keratinized skin that sits on top of a digit and  then kind of juts out from there.

Hooves are a little special in their own way, because most of the animal’s  weight is bearing down on them. Because of that, the plate grows at an angle and wraps around most of the tip of the digit. It also means the keratinized  bit has to be thicker.

And claws? Well, if you’ve ever been unfortunate enough to be at the receiving end of a cat swipe, you’ll know they’re longer,  and more compressed than nails. For the purpose of this video, we’re going to stick to just nails and claws… which thanks to their different forms,  are better suited for different tasks.

For example, since they’re pointy, claws are great for grabbing a thing and helping that thing rest  in the hand…paw…whatever. If you’re a predator, that “thing” might be prey. If you’re something like a sloth, it could be a fat, bark-covered tree branch.

But nails might be a better  option if you want to be stealthy. Why let claws loudly scratch up that tree branch… risking your prey hearing you before  you’re close enough to strike… when you can evolve much quieter nails? And speaking of branches, if the ones you keep reaching for are so slim you can wrap your digits around them, then claws are a pain in the butt  because they’ll dig into your palm.

Or should I say pain in the leg,  because Hank tells me butt is legs… Whether or not what I just said has  got you curling your own fingers into your palm right now, try touching something with your fingertip. Our nails can also help make  our fingertips more sensitive, because they give the skin under  the nail something to push against. This extra sensitivity also makes handling  and manipulating small objects easier.

But let’s get back to these guys . You probably already have an  inkling that having a thumb digit… also known as the pollex…is super handy. Especially an opposable one But it turns out some animals might get similar  benefits from having just the thumb nail.

In 2025, a team of scientists retraced the  evolution of thumbnails in one very particular, but also very massive, animal group: rodents. If it surprises you to learn that rodents  have thumbnails, you wouldn’t be alone. Even other scientists have  kind of dismissed the rodent   thumbnail as a vestigial or non-functional unguis.

And that’s partly because the  rest of their digits end in claws. It’d be weird to have one  unguis for all your fingers and a different unguis for  just your thumbs, right? Well, this research team dove into the Field  Museum’s massive collection of rodent skins, and looked at 433 different  genus groups to catalogue which rodents actually had thumbnails.

They also noted where all these  rodents preferred to live… like in trees or underground… as well as whether they used their  mouths or hands to handle food. From there, the team could  reconstruct a family tree to see how and when thumbnails  popped up in evolutionary history. It turns out a whopping 86 percent  of rodents have thumbnails… at least of the ones sampled.

But they all likely shared a  thumbnail-wielding common ancestor. As the descendants of that ancestor evolved into the many different species we see today, sometimes that thumbnail turned into a claw… and sometimes it was just lost altogether! After all, sometimes thumbnails  might be less helpful than a couple more claws.

For example, most rodents with  thumbclaws live below ground where they’d need to dig. But there are a ton of rodent species, from squirrels, to beavers, to  everyone’s favorite, the capybara. They’re the most abundant kind of mammal, found in all sorts of habitats around the world.

And according to this research team, thumbnails may have had a part to play in that. Specifically, because they provided  access to a super exclusive food source. Nuts are chock full of energy, but  getting at that energy is a challenge.

Opening and eating them requires  a good amount of dexterity. So if you evolve yourself some  thumbnails to improve your dexterity, you might be able to access  nutrients your neighbors can’t. And if you’re hankering for nutrients right now, it’s a great time to grab some, because it’s time for an ad break.

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Just use the QR code on screen  or go to joindeleteme.com/SCISHOW and use promo code SCISHOW at checkout. You’ll get 20% off DeleteMe US consumer plans. Alright, so rodents probably started out with  digits rocking mostly claws and two thumbnails.

But what about primates? What about me? Did humanity also start out with  just one thumbnail on each hand, and then somewhere along the line  swap everything else to match?

Well, it’s complicated. Like many of our rodent friends, many living primates also  have an odd-one-out digit. Although in our case, that  digit is on our hind feet… which for primates with actual  hands, are just called feet.

That arrangement might sound weird to a human, because all our digits have nails. But some New World Monkeys only  have nails on each big toe, a.k.a. the hallux. The rest are claws.

Meanwhile, Madagascan lemurs are  rocking kinda the opposite setup. Most digits on their hind feet end in a nail, but the toe next to each  hallux has a specialized claw. They use it for cleaning themselves, so humans call it a grooming claw.

So what did the ancestor of all primates have? And how did humanity’s  “oops all nails” come about? We don’t know.

One hypothesis says primates  started out with mostly claws, which came in a variety of shapes and helped us scurry up thick tree branches. Then, as they started to use their palms more… for moving objects around  and eventually using tools… those claws broadened and flattened into nails. However, in a paper published back in 1999, one research group concluded  the ancestor to modern primates had already lost all its claws  except for the grooming claw.

They argued that early primates  probably lost the deepest, hardest layer of their claws, which meant the claws were less  strong and the tips could be worn away as primates moved around more on the ground. Then in 2018, a paper building off that work  tackled the evolution of that grooming claw. The study looked at animals called omomyoids, a group of early primates  that kinda looked like a mix between lemurs and tarsiers.

Researchers think that as very early primates evolved more developed big toes… specifically, big toes that could  help them climb more easily… their second toes were freed up for other uses. And since keeping parasites off  yourself is a good way to stay healthy, grooming claws might have been one of those uses. So around 56 million years ago, our ancestors were probably  rocking nails and one claw.

Then, as some primates became more  social and could groom each other, they swapped that claw for another nail. And if you’re something like a titi monkey, you became too solitary and had to  re-evolve a grooming claw to DIY. So scientists have these sort of snapshots  of what primate claws or nails looked like at different times.

But it’s still not clear exactly how… or even really when… we went from a diversity of claws,  to potentially just a grooming claw, to the nails we have today. For now, the best idea we have is that  somehow claws became a disadvantage, either because our ancestors  weren’t climbing thick tree trunks, or were spending more time on the ground. We might not know exactly  how our thumbnails evolved.

But if you want to check out the  evolution of SciShow’s thumbnails, you can go explore that for yourself. [ OUTRO ]