Rodents don’t exactly have the best reputation. They chew through wires, sneak into kitchens, and sometimes make their homes in places we’d rather they didn’t. Yet despite being seen as pests, these small creatures have managed something extraordinary: they’ve conquered almost every corner of the planet.
From bustling cities to remote forests, from mountain slopes to sun-scorched deserts, rodents thrive. Today, they make up the largest group of mammals on Earth, accounting for more than 40 percent of all mammal species. You can find them on every continent except Antarctica. But here’s the surprising twist—scientists now believe one of the reasons for this global success story lies in something almost invisible: their thumbnails.
Yes, that tiny patch of keratin on their thumbs may have been one of the secret evolutionary tools that helped rodents rise to dominance.
A Small Feature With a Big Impact
When you think about survival traits, you might picture sharp claws, big teeth, or camouflaged fur. But researchers argue that something as modest as a thumbnail could have been just as powerful.
According to a study published in Science, thumbnails gave rodents more dexterity—that is, better control over how they handled food and objects. For small creatures, this is a game-changer. Imagine being able to hold, peel, and crack food in ways your competitors can’t. Suddenly, you have access to resources that others can’t touch—like tough nuts loaded with energy.
And when you’re tiny, every calorie counts.
The study also suggests that thumbnails may have helped with climbing. With a nail instead of a claw, rodents could grip branches or surfaces differently, giving them more mobility and access to tree-dwelling food sources. That combination—skillful hands and versatile climbing—might have paved the way for rodents to spread into new environments all over the world.
Read more: Scientists Say the Universe Is Set Up With Impossible Precision, And It Doesn’t Make Any Sense
The Old Assumption: “Useless Thumbs”
Before this study, scientists largely believed rodent thumbs were evolutionary leftovers—quirky little stubs with no real purpose. Many animals have body parts that are reduced versions of once-useful traits, like the tiny pelvic bones in whales or the wings of flightless birds. Rodent thumbs were thought to fall into this category: just a remnant of something their ancestors used but they no longer needed.
But as it turns out, this assumption underestimated rodents. Far from being pointless, their thumbs—and more specifically, their thumbnails—may have been critical to their evolutionary journey.
“This is a nice way to rethink what we consider a human characteristic,” says Anderson Feijó, a mammalogist at the Field Museum in Chicago. “We’re showing that rodents actually have very good handling behavior, and their thumbnail may have played a critical role in that.”
Humans and Rodents: An Unexpected Connection
At first glance, thumbnails feel like a distinctly human trait. After all, they’re the tools we use to peel stickers, scratch an itch, or pick up tiny objects. But surprisingly, humans aren’t alone in having them.
Among mammals, only two groups have thumbnails: primates and rodents. That’s it. Most other mammals either have claws or lack a defined thumb altogether.
Even more interesting, scientists believe that primates and rodents didn’t inherit thumbnails from a common ancestor. Instead, they developed them independently—a fascinating case of “convergent evolution,” where two very different creatures arrive at the same solution to a problem.
For humans, thumbnails help with fine motor skills and tool use. For rodents, they might have been the key to cracking open nature’s toughest snacks.
Building a Rodent Family Tree
To test their ideas, Feijó and his team analyzed rodent specimens from museum collections, looking at both species with thumbnails and those without. In total, they examined 433 rodent groups, which represents the vast majority of the more than 530 known groups.
The results were striking: 86 percent of the groups studied had species with thumbnails.
Next, the researchers built a rodent “family tree” that mapped out which species used their hands to manipulate food and which relied only on their mouths. They supplemented this data with journal articles, textbooks, and even photos submitted by everyday people on the citizen science app iNaturalist.
A clear pattern emerged. Rodents without thumbnails—like guinea pigs and capybaras—rarely used their hands to eat. They usually just grabbed food with their mouths. By contrast, rodents with thumbnails showed a remarkable ability to pick up, hold, and nibble food with precision.
Read more: The Fourth Dimension Could Be Hiding Right in Front of Us, Scientists Say
Fossil Clues: A 50-Million-Year-Old Trait
So where did this feature come from?
The researchers also looked into the fossil record. Some of the earliest known rodents, dating back around 50 million years, have thumb bones that are short and wide. That bone shape is much more consistent with supporting a nail rather than a claw.
This suggests that thumbnails were not a recent development but an ancient adaptation, one that could have shaped rodent evolution from the very beginning.
Why Nuts Changed Everything
It may sound funny, but nuts could be the hidden star of this story. Nuts are high in fat, protein, and calories—the perfect survival food. But they’re also difficult to open without the right tools.
“Nuts are a very high-energy resource, but opening and eating them requires good manual dexterity,” says Feijó. “Rodents’ thumbnails may have allowed them to exploit this resource when other animals couldn’t.”
In other words, thumbnails may have given rodents a buffet that their competitors had to walk past. Access to such food could have helped them survive environmental shifts, spread to new territories, and diversify into thousands of species.
The Best of Both Worlds: Nails and Claws
One of the coolest findings in the study is that rodents don’t have to choose between claws or nails—they get both.
Claws are excellent for digging, climbing rough surfaces, and defense. Nails, on the other hand, provide better grip for delicate tasks. Together, they give rodents a dual-purpose toolkit.
Think of it like having a Swiss Army knife for hands. The claws handle the heavy-duty jobs, while the thumbnail takes care of the precision work.
The researchers also noticed that rodents with thumbnails are more likely to live in trees, where agile hands are crucial for survival. After all, if you’re cracking a nut while perched on a branch, you don’t want to drop your lunch—or yourself.
Rodents: Almost Human, Almost Alien
This study adds to a growing list of surprising rodent traits that echo our own.
David Thybert, a computational biologist at the University of East Anglia who was not involved in the research, puts it this way: “Rodents have such fantastic traits that are sometimes similar to humans. They’re occasionally almost extraterrestrial.”
From using their hands in eerily human-like ways to their ability to adapt to nearly any environment, rodents seem to embody a blend of the familiar and the strange.
What Comes Next
The researchers are far from done. To push the idea further, Feijó wants to use high-speed cameras to capture rodents in action. Watching them handle food frame by frame could provide the clearest proof yet that thumbnails give them superior dexterity.
It may sound like a small detail, but science often finds big answers in tiny places. By focusing on something as humble as a thumbnail, researchers are uncovering clues to one of the most successful evolutionary stories of all time.
Read more: Research Paper Suggests Earth Could Have Been Terraformed by Aliens
Small Nail, Global Impact
So, what do we make of all this? A thumbnail may not seem impressive, but in the world of rodents, it could have made all the difference.
This little nail likely helped early rodents grab hold of tough food, explore new environments, and outcompete rivals. Over millions of years, that advantage snowballed, helping them diversify into the thousands of species we see today—from house mice to porcupines to beavers.
Next time you see a squirrel cracking open a nut, pause for a moment. That tiny flick of its thumbnail is part of an ancient evolutionary legacy, one that helped rodents spread across the globe and carve out a permanent place in nature’s story.
It’s proof that sometimes, the smallest features can leave the biggest marks on history.
Image: Freepik.
