The discovery that ancient rodents evolved a thumbnail in place of a claw helps explain why they are the most successful mammalian order on the planet. That small anatomical change opened up a whole new range of ecological niches, triggering an explosive radiation of new rodent species.
This fact alone should worry creationists who cling to a child-like understanding of science. Their favourite avoidance tactic—when pressed for an example of evolution—is to retreat hastily down their rabbit hole with the familiar cry: “Ah! But that’s not real, ‘macro’-evolution. That’s just variation within a ‘kind’.”
Of course, creationists are consistently reticent about defining what they mean by “macro-evolution,” or explaining how the processes that supposedly produce it differ from those of normal evolution. In scientific terms, evolution is simply a change in allele frequencies in a population over time. There is no separate mechanism for “macro” versus “micro.”
So here’s the awkward question for them: was the evolution of the thumbnail from a claw a case of “macro-evolution” or not?
According to the new research, led by Rafaela Missagi of the University of São Paulo, Brazil, with collaborators from the Field Museum of Natural History (Chicago, USA), Northwestern University (Chicago, USA), and the Natural History Museum (London, UK), this change was pivotal. It allowed rodents to diversify into countless species—just as the elongation of bat fingers into wings enabled bats to radiate into hundreds of species. Crucially, in both cases no “new structures” were created from nothing; existing ones were repurposed.
This sort of question usually sends creationists scurrying for cover, chanting Bible verses as they go.
Unlike creationist dogma, which collapses under this kind of scrutiny, the new findings provide yet another vindication of evolutionary theory. Evolution predicts that when a new function arises, it can open up new ecological opportunities, leading to rapid diversification. Not because there is a plan, but because natural selection now has something new to work on.
The Rodent Evolutionary Tree & Timeline.The new research is published in Science and in a Field Museum news item released through EurekAlert!
- Origins: Rodents belong to the order Rodentia, which first appeared in the Palaeocene (~60 million years ago), shortly after the extinction of the dinosaurs.
- Key Adaptation: All rodents share a distinctive pair of continuously growing incisors in both jaws, specialised for gnawing.
- Diversification: By the Eocene (~50 million years ago), rodents had already radiated into multiple families across the Northern Hemisphere.
- Global Spread:
- Reached South America ~40–35 million years ago, later exchanging species with North America after the Isthmus of Panama formed (~3 million years ago).
- Spread to Africa early and gave rise to species like mole-rats and cane rats.
- Colonised Australia only in the last 5 million years, likely via rafting from Southeast Asia.
- Major Groups Today (~2,500 species; ~40% of all mammals):
- Squirrels & relatives (Sciuroidea).
- Beavers, pocket gophers, kangaroo rats (Geomyoidea).
- Mice, rats, voles, hamsters, lemmings (Muroidea) — by far the most speciose group.
- Porcupines, cavies, capybaras, agoutis (Caviomorpha).
- Ecological Success: Rodents now occupy nearly every terrestrial habitat on Earth, from deserts to rainforests to tundra, and even human cities.
Timeline snapshot
- 66 Ma — Dinosaur extinction.
- 60 Ma — First rodents appear.
- 50 Ma — Rapid radiation in Eurasia & North America.
- 40–35 Ma — Arrival in South America.
- 5 Ma — Colonisation of Australia.
- Today — ~2,500 species, from mice to capybaras.
Most rodents have thumbnails instead of claws. It might help explain how they took over the world.
Picture a squirrel eating an acorn. It’s holding the nut in its front paws. More specifically, squirrels handle their food with their thumbs. And instead of the thin, curved claws on the rest of the squirrel’s fingers, their thumbs have smooth, flat nails.
For hundreds of years, scientists have noticed that some rodents, including squirrels, have thumbnails instead of claws. But no one went through the whole rodent family tree and documented which ones have nails and which ones have claws. In a new paper published in Science, researchers examined hundreds of rodents in museum collections to see where thumbnails crop up. This study could help explain how rodents evolved into so many different species that thrive all over the world.
When I talk with people about this research, I always start by asking, ‘Did you know rodents have thumbnails?’ Most people don’t. I didn’t. I had studied rodents for years, and I didn’t know anything about their nails until I started working on this project at the Field Museum.
Assistant Professor Rafaela Missagia, first author
Department of Zoology
University of São Paulo
São Paulo, Brazil.
The Field Museum in Chicago is home to one of the world’s largest collections of mammals, and as rodents make up about 40% of all known mammal species, there are lots and lots of rodent specimens at the Field. Gordon Shepherd, a professor of neuroscience at Northwestern University, heard about how some rodents have thumbnails and some have claws, and he decided to use the Field Museum’s collections to find a bigger pattern.
Before we did the research, we knew that some had nails, some had claws, and some had no thumbs at all. There were hints that the rodents that have thumbnails also use their thumbs to hold their food.
Professor Gordon Shepherd, co-corresponding author
Department of Neuroscience
Northwestern University
Chicago, IL, USA.
The research team set about examining the preserved skins of rodents in the Field’s collections, looking at the animals’ thumbs (or lack thereof).
The team found that of the genera they surveyed, 86% of them included species with thumbnails. They compared this information with data about the different rodents’ feeding habits.There are more than 530 different genera of rodents, containing over 2,500 species. We looked at 433 of those genus groups from all across the rodent family tree.
Anderson Feijó, co-corresponding author
Negaunee Integrative Research Center
Field Museum of Natural History
Chicago, IL, USA.
We used the app iNaturalist to look at photos of different kinds of rodents eating, as well as textbooks and journal article. Using that information, we reconstructed the rodent family tree in terms of rodents that handle food with their hands versus ones that only use their mouths.
Professor Gordon Shepherd.
The researchers found that rodents like guinea pigs that don’t have thumbs, let alone thumbnails, generally don’t handle food with their hands. The family tree that the researchers assembled also indicated that rodents with thumbnails go way back. The data suggest that all modern rodents descend from a common ancestor that had thumbnails too. This early rodent ancestor’s thumbnails may help explain how rodents took over the world. The researchers hypothesize that slim, flat thumbnails allow for more manual dexterity than long, sharp claws— thumbnails make it easier to handle and eat a nut.
Rodents make up almost half of the mammal species on Earth, and they're found on every continent except Antarctica. Their thumbnails might help explain why rodents became so successful. Nuts are a very high-energy resource, but opening and eating them requires good manual dexterity that a lot of other animals don’t have — maybe rodents’ thumbnails allowed them to exploit this unique resource and then diversify broadly, because they were not competing with other animals for this food.
Anderson Feijó.
Animals’ nails or claws tell us about more than just how they handle their food, notes Missagia.
When I got involved with this project looking at rodents’ nails or claws, I immediately thought about their life modes— where they live, how they use their hands in ways beyond just eating. I knew that primates, which mostly have nails, are usually arboreal, they live in trees. We tested that correlation as well, and we found that rodents with nails also were likelier to live aboveground or in trees, while fossorial rodents, the ones that dig, were more likely to have claws and not nails on their thumbs.
Assistant Professor Rafaela Missagia.
Aside from rodents, primates like humans are the only mammals that have evolved nails on their thumbs as opposed to claws. However, the two lineages appear to have evolved this feature on their own, in a process called convergent evolution. This study, in addition to helping explain the evolution of rodents into the thousands of species that exist today, also underscores the value of museum collections.Museum collections are an endless source of discoveries. For all of the rodents that were used in this study, I bet none of the collectors would have imagined that someone someday would be studying those rodents’ thumbnails.
Anderson Feijó
This study was contributed to by Rafaela Missagia (University of São Paulo, Field Museum), Anderson Feijó (Field Museum, Chinese Academy of Sciences), Lauren Johnson (Field Museum), Maximilian Allen (Illinois Natural History Survey, University of Illinois), Bruce D. Patterson (Field Museum), Paulina Jenkins (Natural History Museum, London), and Gordon Shepherd (Northwestern University).
Publication:
AbstractRodents today make up around 40% of all mammalian species, a level of success owed in no small part to that seemingly minor evolutionary change from a claw to a thumbnail. This adaptation opened up fresh opportunities, setting the stage for their spectacular radiation into new niches and environments.
The unguis (hoof, claw, or nail) of the first digit (D1, also known as the thumb or pollex) of the tetrapod hand exhibits numerous functional adaptations, but its macroevolutionary association with ecological diversity is unknown. Across Rodentia, we find that most extant genera and ancestral lineages bear D1 nails. Exceptions follow structure-function associations that arose independently multiple times, specifically, the gain of D1 claws with subterranean habits and the loss of D1 ungues with oral-only feeding behavior. We hypothesize that early acquisition of D1 nails and manually dexterous food handling was crucial for rodents to adaptively leverage cranial specializations for efficient gnawing and thereby exploit hard seeds and nuts, a niche that they dominated after the extinction of multituberculates. Our study recasts ideas about rodent evolution and uncovers a previously unrecognized contributor to their successful radiation.
Rafaela V. Missagia et al.
Evolution of thumbnails across Rodentia.
Science 389, 1049-1053 (2025). DOI:10.1126/science.ads7926
© 2025 the American Association for the Advancement of Science.
Reprinted under the terms of s60 of the Copyright, Designs and Patents Act 1988.
Far from being a challenge to evolutionary biology, this discovery is yet another example of how well the theory explains the natural world. A small change in form can generate new possibilities, which natural selection then shapes into a dazzling array of species. No divine blueprint is required—just variation, inheritance, and the relentless filtering of environments.
For creationists, however, the finding only raises uncomfortable questions. If the evolution of the thumbnail isn’t “macro-evolution,” then what is? And if such a change can trigger the diversification of thousands of species, the “variation within a kind” defence collapses into meaninglessness.
Rodents gnawed their way to global dominance not because they were designed, but because evolution, through countless small steps, rewarded every trait that allowed them to exploit new opportunities. Their story is a textbook case of how life thrives and diversifies on an ever-changing planet.
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