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Among the most cited evidence for human evolution is the existence of vestigial structures, body parts that seemingly have no purpose. Perhaps they once did, but somewhere along the way they lost their functions and are now basically useless. Many other structures in the human body are thought to have once been vestigial, but now they have new functions.
Some people argue that these structures have purposes and are not vestigial. However, if there is no need for them in terms of survival, they still are classified as vestigial structures. The following structures seem to be left over from earlier versions of humans and now have no necessary function.
AppendixMedicalRF.com / Getty Images
The appendix is a small projection off the side of the large intestine near the cecum. It looks like a tail and is found near where the small and large intestines meet. No one knows the original function of the appendix, but Charles Darwin proposed that it once was used by primates to digest leaves. Now the appendix in humans seems to be a depository for good bacteria used in the colon to aid digestion and absorption, though surgical removal of the appendix causes no observable health problems. Those bacteria, however, may contribute to appendicitis, a condition where the appendix becomes inflamed and infected. And if left untreated, the appendix might rupture and the infection can spread, which can be fatal.
Tail BoneScience Photo Library / Getty Images
Attached to the bottom of the sacrum is the coccyx, or tailbone. This small, bony projection seems to be a leftover structure of primate evolution. It is believed that human ancestors once had tails and lived in trees, and the coccyx would be where the tail was attached to the skeleton. Since nature has since selected against putting tails on humans, the coccyx is unnecessary to modern-day humans. Yet it remains part of the human skeleton.
Plica LuminarisMicky Zlimen / Wikimedia Commons / CC BY-SA 2.0
Have you ever noticed the flap of skin that covers the outside corner of your eyeball? That's called the plica luminaris, a vestigial structure that doesn't really have a purpose but is leftover from our ancestors. It is believed to have once been part of a nictitating membrane, which is like a third eyelid that moves across the eye to protect it or to moisten it. Most animals have fully functioning nictitating membranes, but the plica luminaris is now a vestigial structure in some mammals, such as humans.
US-Gov / Wikimedia Commons / Public domain
When humans become cold, or sometimes scared, we get goosebumps, which are caused by the arrector pili muscle in the skin contracting and pulling the hair shaft upward. This process is vestigial in humans because we don't have enough hair or fur to make it worthwhile. Fluffing up hair or fur creates pockets to trap air and warm the body. It also can make the animal look bigger as protection against threatening creatures. Humans still have the response of the arrector pili muscle pulling up the hair shaft, but we have no use for it, making it vestigial.