At the ends of your toes are an evolutionary artifact of humans’ primate history, hiding in plain sight. Instead of sharp, curved hooks or stomping hooves, we have flat scales of double-layered keratin covering just the tops of our digits. Yet for most other vertebrate animals, that’s not the case. Only primates (and a handful of marsupial mimics) have nails, says Matthew Borths, a primate paleontologist and curator of fossils at Duke University’s Lemur Center. “It’s clear from the fossil record that the common ancestor of all primates had nails,” says Borths. There’s not one single, certain answer for why that is but “there’s lots of speculation,” he adds.
From the present, human perspective, it’s not hard to understand the value of fingernails: We might use our upper nails to pop a soda tab, pick up a coin, or scratch an itch. But toenails, which are generally overlooked unless it’s time to clip them, make less intuitive sense. So why do we have toenails? Here’s what the experts think.
Getting a grip
Toenails, like everything about our physiology, are the product of evolution. Most theories tie the emergence of nails over claws to primates’ particular climbing lifestyle. Unlike the clawed animals that also climb, primates rely on opposable toes and thumbs to grasp with whole feet and hands. Claws, in contrast, work by grappling over or hooking into trunks and branches. With claws, you need to be picky about your climbing surface, says Borths, but with a whole-appendage grip, primates can more easily grasp limbs of different texture, shape, and size.
Claws might have disappeared from primate anatomy because they somehow impeded our ability to safely grip (imagine a long, sharp claw digging into your palms every time you tried to grab a narrow object). Though this isn’t universally true and many arboreal clawed animals can grasp, notes Stephanie Maiolino, an assistant professor of anatomical sciences at Stony Brook University–probably, she says, other factors were also at play.
Mammalian claws may have unfurled into flattened nails because there was some upside to the change. Our flat nails allow for wider toe- and fingertips, and at the same time provide a counter-pressure surface for our soft pads to push back against. With nails, the ends of our digits spread wider when we apply pressure, increasing the contact surface area, friction, and thus grip, explains Borths.
Though humans have mostly lost our flexible, prehensile feet as we’ve become bipedal, opposable toes actually show up earlier in the fossil record than opposable thumbs, says Borths. Our ancestors were gripping with their feet before their hands, and so toenails would have played an early role in primate climbing grip. To this day, all other contemporary primates have retained their gripping feet, he adds.
Another theory is that what we were grabbing was of primary importance in shaping primate evolution, and ultimately our nails. One hypothesis is that gripping and subsequently nails emerged specifically to help our predecessors forage fruits and nuts at the end of tree branches, says Doug Boyer, an associate professor of evolutionary anthropology at Duke University. Navigating thin branches and collecting ripe fruits may be easier with nails over claws, Boyer suggests. “If most of your survival probability is dependent on foraging out at the ends of branches, there might be a selective pressure for getting rid of claws,” he adds. Yet so far, he notes there aren’t biophysics studies clearly proving this hypothesis.
Our clawless fingers and feet also make precision-grasping much simpler. Delicately plucking an insect off a leaf, for instance, is probably easier and with our soft, nailed digits over unwieldy claws. “Maybe the claws got in the way of trying to quickly reach out and grab small things,” Boyer says. One hypothesis is that nocturnal, vision-dependent insect feeding influenced primate evolution and claw loss, he notes.
Sensitivity and sociality
The same fine motor skills that aid in picking tasty bug snacks off of leaves also apply to picking bugs and other parasites off of others. Whether or not our hands and feet (and thus nails) evolved for this purpose or not, many primates use these nailed appendates to groom their peers. “If we had claws, but everything else was the same, would we still groom each other? Probably, but it’s hard to say,” says Borths.
A lack of claws makes self-grooming more challenging in some instances, because it’s tougher to scratch those hard-to-reach spots, notes Boyer. It’s possible that social grooming emerged, in part, because of nails, he says. “Once nails evolved, there could have been more of a selective pressure for allogrooming because they were less able to do it themselves,” Boyer posits. In some primate species, grooming claws have re-evolved on certain toes and fingers. Most primates with grooming claws do seem to be less socially complex and more solitary, Boyer says, adding evidence to this idea.
Most other social mammals groom by licking, but primates are unique among tetrapod vertebrates in our reliance on hands and feet for tasks many other taxa use their faces for, says Borths. When you live on the ground, smelling and whiskers are critically important. But a life in the trees swinging from branch to branch, makes odor trails less useful and touch more safely outsourced to limbs.
[ Related: Why do we have earlobes? They make no evolutionary sense. ]
Nails may have evolved, in part, as an aspect of this distal sensory system, says Maiolino. Our wide digital tips provide more surface area for sensitive touch, and nails have a particular benefit here, too. Humans have mechanoreceptors in our fingertips that get “squished against the sides of the nails and help us determine the directions [and] positions of how we are touching an object,” she explains–it’s unknown, though possible, that the same system is present in other primates’ hands and feet. And, even now, it’s possible our toenails help us balance, redistribute force, and sense direction through our feet, she adds.
Protection or purposeless?
Our human lifestyles are different from that of other primates in many ways. We’re terrestrial, upright, and cursorial (i.e. adapted for running). Though we’re no longer climbing, it’s possible we’ve held onto nails instead of re-gaining claws on our feet because they still offer some benefit.
Nails are made up of the same types of cells as our skin, which has a clear protective purpose, says Grace Torres-Hodges, a board certified podiatrist and food surgeon based in Pensacola, Florida. In her line of work, she’s often asked by patients about the purpose of toenails. Though she doesn’t have any single, definite answer for them, she thinks they help to protect against cuts, infection, and other damage to the toes. There is a lot of vascular and nerve structure in our toes, Torres-Hodges says, which is potentially fragile–not to mention the small bones.
And, as a bonus, assessing toenails can offer clues to whole-body health, she says. Peculiarities in color, texture, and shape can hint at chronic health issues like respiratory, thyroid, and liver disease she says.
But it’s also possible that, at this point in our species’ trajectory, toenails are little more than a souvenir of our tree-climbing past. It can be a fallacy “to assume a function where there may not be a function,” says Tracy Thomson, a paleontologist at the University of California, Davis. “There are lots of structures that may not be functional or have lost their function over evolutionary time,” he adds.
“Everything about the human body is a work in progress, because our way of moving is so new and something that hasn’t really been tried before,” says Borths. As tail-less, big-headed, bipeds “we’re just kind of the first weirdos like us,” in the fossil record, he adds. “So we’ll see what happens.” Maybe, in the long-run, we’ll lose our toenails too.
This story is part of Popular Science’s Ask Us Anything series, where we answer your most outlandish, mind-burning questions, from the ordinary to the off-the-wall. Have something you’ve always wanted to know? Ask us.
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