In 1871, Charles Darwin published The Descent of Man, a book in which he applied his theory of evolution to human form and psychology. He explores a number of topics in this book, and while some of his conclusions on the social habits and structures of humanity are rather outdated, he displayed groundbreaking insight in other areas.

One place where Darwin had it right is vestigial structures. A vestigial structure is a body part or organ that was useful in an ancestor, but is diminished in size, complexity, or utility in their descendant. It has been a long time since the ancestors of dolphins or whales set foot on dry land, but occasionally dolphins are born with rudimentary hind limbs. These hind limbs are vestigial, and don’t seem to provide any benefit.

There are a wide variety of vestigial structures in humans as well, several of which Darwin noted in his book. Some of these organs appear completely useless while others maintain some functionality. Either way, vestigial structures provide an interesting glimpse into the past and give us unique insight into the lives of our ancestors.

The Appendix

The appendix is the vestigial structure that is generally most familiar to people. We all have them, except for the one in fifteen that have theirs removed after a bout of appendicitis. The appendix is a small organ attached to the large intestine. People show no apparent adverse effects when it is removed, but it does appear to have a small role to play.

The appendix is currently thought to be a reservoir for beneficial gut bacteria. Bacteria in the large intestine aid digestion but can become depleted after illness. The colonies of bacteria in the appendix may serve as an emergency back-up for cases like this and can repopulate the gut once the coast is clear. Some animals have much larger appendixes, thought to assist in the digestion of high-cellulose diets.

Ears

Darwin's tubercle. Image by Luis Fernandez.

Some of my favorite evolutionary relics are found in the ear. If you are a fan of cats like I am, you’ll have noticed how a cat’s lovely pointed ear can swivel around to track sounds. By looking at our vestigial features, we can tell that our ancestors must have had swiveling pointed ears as well.

About 10% of the human population has a structure called Darwin’s tubercle, named after Darwin described it in The Descent of Man. Darwin’s tubercle presents itself as a subtle thickening in the outer rim of the ear. It represents what was once the tip of our ancestor's pointy ears. Some populations are much more prone to Darwin’s tubercle than others, and it just takes a quick glance in the mirror to tell if you have this feature.

My maternal grandmother could wiggle her ears, and I can too. I discovered this talent in middle school when I was bored in class and began to move my glasses back and forth on the bridge of my nose.

In our ear muscles we can find more traces of our evolutionary history. For the most part, human ears are fairly static and unmoving. However, we still have the muscles that twitched them around in the distant past. These days, only a select few people can wiggle their ears. Even for them, it isn’t exactly a useful trait, except for impressing the occasional friend or coworker.

Interestingly, even though they aren’t powerful enough to move the ear most of the time, human ear muscles still respond to stimuli. A surprising sound from behind will activate the ear muscles on that side, as if your muscles are still trying to pull your ear into position to listen to it.

The Mystery Tendon

Darwin’s tubercle is a fairly uncommon vestigial feature, but there’s one that almost everyone has. 85-90% of people have an extra tendon, and there’s an easy way to check for it. Hold out your arm and touch your thumb to your pinky or middle finger. Next, experiment with tilting your hand back at different angles. If you are in the 85%, you will see a raised tendon running down the middle of your wrist, like the picture to the right. It is often much more subtle than pictured, however.

This tendon connects to the palmaris longus muscle. Though most people have this muscle, it isn’t doing us much good anymore. People with this muscle don’t have better grip or arm strength than those without. It’s considered so superfluous that reconstructive surgeons often remove it entirely and repurpose it for use elsewhere. The palmaris longus muscle is far more prominent in other mammal species, especially those that use their forearms for locomotion, like lemurs and monkeys.

Vestigial Behavior

Evolutionary remnants appear in our body, but they also can affect our behavior. I wrote an entire blog post dedicated to goosebumps and how they’re a relic from a time when our ancestors had fur that fluffed up in response to cold or fear. Goosebumps aren’t terribly useful now that we are sparsely-furred humans, and this isn’t the only vestigial behavior our species is holding onto.

Taking candy from a baby is the definition of easy, but it might be tougher than you think. Infants have surprisingly strong grip strength, using something called the Palmar grasp reflex. When you put a finger or another object into the palm of an infant, it will reflexively grip with strength seemingly disproportionate to its size.

In the days before ethics boards oversaw experiments, scientists tested the Palmar grasp reflex and found that 37% of babies can support their own weight when holding onto a rod (Please don’t try this at home!). What purpose could there possibly be for such beefy babies? Like goosebumps, it all comes down to fur.

Ancestral babies had a strong grip because they were holding onto their furry mother, leaving her arms free to climb or forage. Babies holding onto their mother is very common across primate species, and can also be found in other mammals, like anteaters. A baby’s toes often curl too, in a manner highly reminiscent of chimpanzees. When studying vestigial behaviors, babies are a treasure trove of information.

The Others

There are numerous other features that provide evidence of our ancestry. Human embryos form tails during development which later regress into the 4-5 fused vertebrae of the tailbone. Like dolphins born with hind limbs, sometimes tail regression is incomplete and babies are born with tails still intact.

Wisdom teeth are another obsolete feature, one that comes from a time when our ancestors needed extra teeth to process a diet high in foliage and rough plant matter. Some lucky human populations don’t develop wisdom teeth anymore. Most of the rest of us have to have them removed when they start crowding the rest of the teeth in our small jaws.

You can also see evolution in our eyes. At the very corner of your eye, there is a small fold of tissue that is the last remnant of what was a nictitating membrane in our ancestors, a third eyelid that could close horizontally over the eye to protect it.

Our DNA holds even more evolutionary secrets, like the gene that allowed our ancestors to synthesize vitamin C. We still have this gene, but it no longer functions. These days, we have to get vitamin C from our food or risk getting scurvy.

From inside to out, the history of our species is written throughout our bodies. By understanding our form and its function today, we can better understand where we came from. From our primitive ancestors to Darwin to today, vestigial structures are here to stay!

- Kate Dzikiewicz, Paul Griswold Howes Fellow