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Vestigial Structures in Humans

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One of the most often cited evidences for human evolution is the existence of vestigial structures. Vestigial structures are body parts that seemingly have no purpose or function. 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 a new function. The following are a few of the structures that seem to be left over from an earlier version of humans and now have no necessary function.

1. The Appendix

Olek Remesz

The appendix is a small projection off the side of the large intestine near the cecum. It looks kind of like a tail and is found near where the small and large intestines meet. No one knows the actual original function of the appendix, but Charles Darwin proposed it was once used by primates to digest leaves. Now, the appendix in humans seems to be a depository of sorts for bacteria that is used in the colon to aid in digestion and absorption. These bacteria, along with others, may cause appendicitis and, if left untreated, can be fatal if the appendix ruptures and the infections spreads.

2. The Tail Bone

Grays Anatomy of the Human Body, 20th ed

Attached to the bottom of the sacrum is the coccyx, or tail bone. 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. The coccyx would be where the tail was attached to the skeleton. Since tails on humans have been selected against in nature, the coccyx is unnecessary in modern day humans. Yet, it is still a part of the human skeleton.

3. The Plica Luminaris

C hamiltonkolb

Have you ever noticed that little flap of skin that covers the outside corner of your eyeball? That's called the plica luminaris, and it is a vestigial structure. It doesn't really have a purpose, but it is still there from our ancestors. It is believed to have once been part of a nictitating membrane. Nictitating membranes are like third eyelids that move across the eye to protect it or to moisten it as needed. Most animals have fully functioning nictitating membranes, although the plica luminaris is now a vestigial structure in some mammals.

4. The Arrector Pili

Grays Anatomy of the Human Body, 20th ed.

When humans become cold, or sometimes scared, they get goose bumps. Goose bumps are caused by the arrector pili muscle in the skin contracting and pulling the hair shaft upwards. This whole process is vestigial for humans because we do not 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 to threats that have scared them. Humans still have the response of the arrector pili muscle pulling up the hair shaft, but lack enough fur or hair for the response to actually work.

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