Some human body parts have become useless over the past few million years.
These include wisdom teeth, the tail bone, and the muscle fibers that produce goose bumps.
Take a look at nine body parts that remained in the human body despite having no function for millions of years.
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Some human body parts serve no purpose despite once having a specific function among our ancestors.
For example, some of them can be removed, without lowering people’s quality of life.
Insider spoke with Dorsa Amir in 2019, who was a developmental psychologist at Boston College at the time, to learn more about what she calls “evolutionary leftovers.”
Amir said that if a trait becomes no longer useful but remains harmless to humans, then it may tag along for the evolutionary ride.
Take a look at nine body parts that remained in the human body despite having little to no function for millions of years.
The appendix may be the most commonly known organ that’s lost its main function in humans.
Many years ago, the appendix may have helped people digest plants that were rich in cellulose, according to a 2016 study in the journal Clinical and experimental immunology.
While plant-eating vertebrates still rely on their appendix to help process plants, the organ is not necessarily an essential part of the human digestive system. Though some research suggests it may act as storage for good bacteria.
“As we started switching to a more diverse diet and targeting meat, we didn’t need super long and complicated intestinal tracts anymore,” Amir told Insider.
The palmaris longus muscle runs from the wrist to the elbow. About 10% of humans do not have it.
Athletico Physical Therapy/ YouTube
If you rest the back of your wrist on a table and connect your thumb to your pinky, you may see a band of muscle pop up on your wrist.
That is a vestigial muscle called the palmaris longus. Amir said this muscle is there because it helped our ancestors climb trees.
According to the Encyclopedia Britannica, the muscle likely also helped early humans with their grip, possibly while hanging. But we started walking on two feet about 3.2 million years ago, eventually making the muscle more-or-less useless.
“It’s been a while since it was useful,” Amir said.
Now, however, someone’s grip strength is the same whether or not they have the muscle. “Natural selection is not a system geared toward perfect efficiency,” Amir said.
Wisdom teeth used to help our ancestors work through tough food out in the wild, but now, they often don’t even fit in our jaws.
Humans no longer need very powerful jaws because our diets have shifted toward soft foods and cooked grains. Our jaws are also smaller, leaving less space for all our wisdom teeth, according to the Cleveland Clinic.
“Given that we eat pretty soft food now, and molars are usually used for grinding, we don’t really need them anymore,” Amir said.
Often times, if wisdom teeth do come in, they impact in odd angles, hitting the jaw bone or sticking in the gums, in an effort to fit into our mouths. When this happens, it’s best to just remove them, since they’re more a nuisance than a benefit at that point.
Arrector pili are muscle fibers that produce goose bumps when they contract. Our ancestors, who had a lot more body hair, used these fibers to their advantage, but we don’t have a use for them anymore.
For animals with thick fur, arrector pili can help provide insulation. When the hairs stand on end, it traps warm air in between individual strands, helping to keep the animal warm.
The fibers can also make animals look bigger — a porcupine benefits from this phenomenon.
Human embryos develop a tail between five and eight weeks after conception. The tail vanishes by the time humans are born, and the remaining vertebrae merge to form the coccyx, or tailbone.
Tailbones helped our ancestors with mobility and balance, but the tail began disappearing around 20 million years ago. The coccyx now serves no purpose in humans.
“Our ancestors who had mutations that got rid of the tail seemed to fare better, and thus our tails went away over many generations,” Amir told Insider.
Human infants are rarely born with a vestigial tail, though it does happen, according to Britannica. Doctors can remove the tail through surgery without major issues
Auricular muscles control the visible part of the ear, but humans have lost the ability to use them. Other mammals use these muscles to detect prey and predators.
The auricular muscles of the ear help other mammals localize sound and express emotion, according to Britannica. Unlike humans, animals such as cats move their ears to hear well. But Amir said that since we have flexible necks, we no longer have the need to move our ears toward sounds.
Some humans can wiggle their ears, but that’s the best we can do.
The pyramidalis muscle, which is located in the lower abdomen, is shaped like a triangle. Some people lack these muscles completely. But even with people that have them, the muscles don’t really seem to help with anything.
According to Britannica, the pyramidalis muscle can help contract the linea alba, which is a line of connective tissue that you might think of as the vertical line in the middle of six-pack abs. But it’s not relevant to the function of one’s abdominal muscles, according to Brittanica.
About 20% of humans do not have any pyramidalis muscles.
Male and female fetuses initially develop the same way, and testosterone triggers the formation of male sex organs later on. Before these hormones kick in, however, nipples have already begun to develop.
Nipples are typically a vehicle for milk when new mothers feed their babies. But men, who also have nipples, can’t lactate under natural circumstances.
That said, a high level of prolactin, the hormone that helps produce milk, can create this effect. For example, taking digoxin, a heart medication, can increase prolactin levels and in turn cause men to lactate, Scientific American reported.
While many male mammals could lactate under extreme scenarios, only the Dayak fruit bat, found in Southeast Asia, lactates spontaneously, according to Scientific American.
The plica semilunaris, or third eyelid, is a fold of tissue found on the inside corner of the eye. It resembles membranes that some animals use to protect their eyes.
Birds, reptiles, and some mammals can pull these membranes across their eyes to keep them moist and free of debris.
Our plica semilunaris is a remnant of these membranes, though humans are now unable to control the tissue, Britannica reported.
“It’s not exactly clear why humans don’t have these anymore,” Amir said, “but they’re actually rare among primates so we must have lost them a long time ago.”
This post has been updated. This story was originally published in January 2019. It was last updated on May 11, 2023.
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