I’m jealous of people with extra nipples. It’s honestly not prurient—people who can wiggle their ears also earn my envy. It’s not that I lack for party tricks. It’s because I love what these quirks reveal about our evolutionary past.
Our bodies are full of tiny remnants of who and what we once were. We share the straight-ahead gaze of a gorilla, and the thumbs of a chimpanzee. These reminders—wiggling ears, tiny tails, the membranes in our eyes, and the occasional extra nipples—tell us that our ancient family is all around us, and we only need to look at our own evolutionary remnants to see family resemblances.
There are plenty of examples in our anatomy of these evolutionary leftovers. In our eyes, the inside corner has what’s called the plica semilunaris. It’s all that’s left of our nictating membrane—a translucent film that can flash across the eye, from inside to outer corner. In some mammals, birds, amphibians and reptiles, the nictating membrane protects vision by sweeping away grit and moistening the eyes, while still letting the animal see the world (relatively) clearly. In humans the tiny vestigial remnant of this inner eyelid lubricates the eye and controls how our tears drain.
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Some people can use their evolutionary remnants as party tricks. Those who can wiggle their ears have voluntary control over the tiny muscles that used to orient our ears toward important sounds. It’s a talent that we might have lost as we transitioned from nocturnal lifestyles to daytime activity, around 25 million years ago. Instead, we’ve spent the past few million years following each other’s gazes and facial expressions.
But our aural skills aren’t totally lost, even in people who can’t ear-waggle on demand. A 2020 study showed that humans still do have an involuntary, tiny muscle response in our ears to sounds. When we focus to pick out a new sound, we still do minutely perk up our ears—a feature that links us to dogs, cats, squirrels, rabbits and many more.
Our most well-known remnant is probably the one in our butts—our tailbones. These final fused vertebrae used to be tails, which shifted our weight and kept us balanced when our ancestors moved on four feet. Our ancestors lost their tails around 20-25 million years ago through a single genetic mutation, and it could have helped them eventually walk upright (and give us consequent lower back pain).
Another leftover? Goosebumps. When we are cold, or feel a thrill or fear, tiny arrector pili muscles inside our skin lift each individual, now useless, hair all over our bodies. It’s a reminder of when we had fur coats that could be puffed out for warmth or display.
My favorite remnant is extra nipples. About 6 percent of the population boasts an extra areola—sometimes called a supernumerary teat, or supernumerary nipple. I call them “nipplets.” These nipplets lie along the milk line, which runs from the armpit in a gentle curve down either side of the torso past the belly button. This line forms from a ridge during fetal development. The ridge regresses as we develop in utero, until we have only two nubs left that are capable of forming mammary tissue—our nipples.
But in 5.6 percent of people, regression isn’t quite complete. These people will have a small extra nipple—sometimes clearly a nipple, sometimes an areola-colored birthmark—located on the milk line. These nipplets—supernumerary teats—are twice as common in people assigned male at birth.
The extra nipple isn’t what’s odd. The regression is. In most other mammals—cats, dogs, raccoons, squirrels and so on—the milk line develops into a paired series of mammary glands—multiple pairs of nipples. This works well when an animal needs to nurse multiple young. It’s only when a mammal goes all in on one or two babies at a time that the other nipples prove supernumerary. Our two nipples reflect our way of life now—our intensive investment in one or two young. Our extras, however, reflect what we were. They are things that we have stopped using, but that our bodies, under no real evolutionary pressure to do so, haven’t fully lost.
I know a few people with these tiny proto-nipples, and several who can wiggle their ears. I envy them. The party tricks and extra areolae are, to me, soothing reminders of our past. They remind us that we needed more mobile ears to hear calls in the forest. That we could nurse whole litters of pups. That we sometimes needed a tiny extra layer of eye protection against sand or water. That we could balance as we hopped from limb to limb, clambered up rocks and ran on all fours.
All of these connect us to our wider family—our animal family. Many of us have a deep desire to know our past—to meet our distant relatives. People use DNA services like Ancestry.com and 23andMe to track down siblings or cousins or to dig through family secrets—other families, adoptions, stories made up to pass as part of a dominant group. Some want clues to medical mysteries. Some are searching for their history—ripped away in the slave trade or through war or genocide. People seek out images to connect to newfound kin through similar tilts of a chin, corners of a smile, sets of others’ shoulders.
Finding our distant human families can bring us new loved ones, new communities or just new recipes and hobbies. But finding our evolutionary family links us even more. We are humans, we are mammals. We are vertebrates. Seeing ourselves in the perked-up ears of a dog, the glassed-over eyes of a crocodile, the tail of a squirrel or the nipples on a possum can bring us together, maybe a little weirded out, but still, together.
Personally, I find our evolutionary remnants an unexpected comfort when life gets overwhelming. We might be frantically trying to meet deadlines and schedule meetings with 15 people with conflicting schedules and time zones. But deep down, our evolutionary remnants tell us that we’re not so far from the little hairy things we once were, hanging out in the trees and nursing whole litters of young—for a little bit of extra perspective.
This is an opinion and analysis article, and the views expressed by the author or authors are not necessarily those of Scientific American.