Vestigial Structures

A "vestigial structure" or "vestigial organ" is an anatomical feature or behavior that no longer seems to have a purpose in the current form of an organism of the given species. Often, these vestigial structures were organs that performed some important functions in the organism at one point in the past.

However, as the population changed due to natural selection, those structures became less and less necessary until they were rendered pretty much useless. They are believed to be leftovers, only vestiges of the past.

Slow Evolutionary Process

Evolution is a slow process, with changes in species happening over hundreds or thousands if not millions of years, depending on how significant the change is. Although many of these types of structures would disappear over many generations, some keep being passed down to offspring because they do no harm—they aren't a disadvantage for the species—or they have changed function over time. Some are present or functioning only during the embryonic stage of fetal development, or maybe they just have no function as we get older.

That said, some structures that were once thought of as vestigial are now thought as useful, such as the whale pelvis or the human appendix. As with many things in science, the case isn't closed. As more knowledge is discovered, the information we know is revised and refined.

Examples of Vestigial Structures

The animal kingdom is ripe with vestigial structures in their skeletons and bodies.

• Snakes descended from lizards, with their legs growing smaller and smaller until all that was left is a small bump (leg bones buried in muscle) at the back of some of the largest snakes, such as pythons and boa constrictors.
• Blind fish and salamanders who live in caves still have eye structures. One explanation, in the case of the fish, is that mutations in the genes that increase taste buds degrade the eyes.
• Cockroaches have wings, though the ones on the females aren't developed enough for them to fly.
• The whale shark is a filter feeder and its rows of teeth couldn't bite anything if they tried.
• The Galapagos cormorant has vestigial wings that don't help it to fly or swim, though the birds still dry them off in the sun after they get wet, just as if they would if they still could use them to fly. This species diverged into a flightless bird about 2 million years ago.

Vestigial Structures in Humans

The human body contains many examples of vestigial structures and responses.

The coccyx or the tailbone: Obviously, humans no longer have visible external tails, because the current version of humans do not need tails to live in trees as earlier human ancestors did.

However, humans still have a coccyx or tailbone in their skeletons. In fetuses, any tail is absorbed during development. The coccyx currently serves as an anchor for muscles; that wasn't its original purpose, so that's why it's considered vestigial.

Male nipples: All people inherit nipples from both their parents, even males. Natural selection hasn't selected against them, even though they don't have a reproductive use in males.

Goosebumps: The pilomotor reflex, which raises the hair on your arms or neck when you feel alarmed, is vestigial in humans, but it's pretty useful for porcupines who raise their quills at a sign of danger—or birds, who fluff up when it gets cold.

Wisdom teeth: Our jaws have shrunk over time, so we no longer have room for wisdom teeth in our jawbone.

Appendix Actually Has Uses

The function of the appendix had been unknown, and it had been thought to be a useless, vestigial structure, especially because no domestic mammals have one. However, it's now known that the appendix serves a function.

"These endocrine cells of the fetal appendix have been shown to produce various biogenic amines and peptide hormones, compounds that assist with various biological control (homeostatic) mechanisms. ... The function of the appendix appears to be to expose white blood cells to the wide variety of antigens, or foreign substances, present in the gastrointestinal tract. Thus, the appendix probably helps to suppress potentially destructive humoral (blood- and lymph-borne) antibody responses while promoting local immunity."

—Professor Loren G. Martin to Scientific American