The appendix may be the most commonly known useless organ.
Many years ago, the appendix may have helped people digest plants that were rich in cellulose, Gizmodo reported.
Plant-eating vertebrates still rely on their appendix to help process plants, but the organ is not part of the human digestive system.
Read more: Appendix removal is linked to lower risk of Parkinson's
Humans no longer need very powerful jaws because our diets have shifted toward soft foods and cooked grains. Our jaws are also smaller, so we can't all fit our wisdom teeth properly.
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. 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 shrank as humans learned to walk upright. The coccyx now serves no purpose in humans.
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 help other mammals localize sound and express emotion, according to Britannica. Unlike humans, animals like cats have to move their ears to hear well.
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. People have from zero to two of these muscles, but they don't help us.
According to Britannica, the pyramidalis muscle can help contract the linea alba, but this is not relevant to the function of one's abdominal muscles.
About 20% of humans do not have any pyramidalis muscles.
The palmaris longus muscle runs from the wrist to the elbow. About 10% of humans do not have it.
According to Brittanica, the muscle likely helped early humans with their grip. Now, however, someone's grip strength is the same whether or not they have the muscle.
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.
Men can't lactate under natural circumstances, but a high level of prolactin, the hormone that helps produce milk, can create this effect. Lactation is a possible side effect of digoxin, a heart medication, 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.
Gizmodo reported that our plica semilunaris is a remnant of these membranes, though humans are unable to control the tissue.
- Read more about useless body parts:
- 5 useless human body parts left over from evolution
- 8 body parts that might seem useless but actually have interesting functions
