Great question, Rebecca. Evolution shapes organisms into efficient beings, not unlike factories strive to be efficient. Suppose your business made cars. If you added parts to the car that served no function, that would add unnecessary cost. As a result, your company would lose customers, and slowly you'd go out of business. Organisms generally lack "useless" parts, for the same reason cars do. Useless parts put an organism at a competitive disadvantage, making that organism less likely to produce offspring compared to its more efficient cousins.

Good question. There aren’t many parts we can lose without it causing some problems, but there are some. For example, “wisdom teeth” are molars that we often need to have removed because we don’t have enough room in our mouths for them. But before tooth brushing and dental visits, people probably lost more of their permanent teeth, so they may have actually been useful before not too many generations ago.

Tonsils are an example of something that is useful, but we’re sometimes better off without. Tonsils are part of our immune system. They help protect us from the germs that are constantly entering through our nose and throat. But when they are swollen and painful too often, they can be removed and other parts of the immune system can take over their job pretty well.

The appendix is a small tube that comes off of the place where our large and small intestines connect. It is a dead end tube off of a dead end tube, so sometimes things get stuck in it. It can also get infected without anything trapped in it. Sometimes it has to be removed when that happens, but these days, they may just give a person antibiotics to clear up the infection. The appendix really has no function in us, but in our ancestors, it was probably larger and was important in breaking down plant material. The appendix is very large in some animals that eat only plants.

Some people believe that these evolutionary leftovers (also called “vestigial parts”) will just disappear over the course of evolution. This only happens if making or having those parts is costly enough to make people with those parts less likely to reproduce. But having tonsils, or wisdom teeth, or an appendix rarely kills anyone, and probably doesn’t make them less likely to have kids. So future generations will probably have them too.

Can you think of any other parts of the body that seem useless?

Your question is touching on evolution, or the change in genetic traits of a biological population across numerous generations. As organisms evolve, certain features or body structures may be lose purpose when it is no longer necessary for successful continuation of that organism. The loss of function may occur due to a change in environment, such as the development of impermeable eggs for organisms that first began laying eggs on land (positive selection), or due to a structure becoming harmful, such as a mutation that causes infertility (negative selection).

Sometimes a feature may be neither helpful nor harmful, therefore it may not be phased out through natural selection as its presence persists across the generations. The retention of these features that have lost some or all of their original function through evolution is called vestigiality. Humans maintain several vestigial features: the tailbone (or coccyx), wisdom teeth (or third molars), the inner corner of the eye (or plica semilunaris), the appendix, goose bumps (arrector pili), and tonsils. Since these features are not advantageous, it is alright if they are removed.

Body parts are expensive: they take energy to make. Because it is more efficient energy-wise to have only the body parts that you need, natural selection favors those animals that have only the needed body parts. Any animal that has unneeded body parts will evolve to lose them.

Why would we have useful parts in our body that can be removed with no effect? If we had too many parts like that, we would be wasting energy building them and keeping them alive, and people without those body parts might evolve and win out over the people who were wasting all that energy on those body parts that aren't needed.

On the other hand, things don't just disappear from plants and animals because they're not used. They only disappear if the plants or animals without those things win out over the plants and animals that have those useless things.

I think we actually do have some body parts that can be removed with no effect. On part is the appendix. The appendix can be removed with no effect. The appendix is a tiny piece of the intestine in humans that is much bigger in herbivorous animals like the guinea pig, where it is called the caecum ['SEE-cum']; and it has microbes that digest all the cellulose the guinea pigs eat in the grass they eat.

We used to think our appendix didn't do anything useful, but now some scientists think the appendix stores useful microbes that go into our intestines after we have diarrhea:

Appendix (Anatomy): Appendix Picture, Location, Definition, Function ... here

Another body part that can be removed is the kidney, because people can donate a kidney to someone else.

And our wisdom teeth can be removed without problems.

Hello! You’ve asked a really interesting question that plays into the mechanisms of evolution.

Evolution is the process by which organisms change through generations in order to be better suited for their environment. The way this works is that organisms with traits that help them survive better will be more likely to have children and pass these traits onto the next generation.

Imagine an especially long-necked giraffe that is able to reach more leaves. It will be healthier than the other giraffes because it is able to eat more. Since it is healthier, it is more likely to have more children and pass on its long-necked traits to its children. An important idea to note is that traits will only develop if they ultimately help the organism produce more offspring.

This preliminary explanation may have seemed unrelated, but let’s get back to your original question. These body parts that are not useful to us anymore (called vestigial structures) were at one point useful in our ancient ancestors.

Our appendix used to aid in digestion when our diets were different, but now it is no longer useful. Having an appendix didn’t hurt people, it just did nothing. Since people who were born without an appendix aren’t any healthier (and by extension, can produce more children) than people who have their useless appendixes, there’s no evolutionary drive to remove the appendix. This goes for any other vestigial structures; there’s no reason to remove them!

As a fun fact, I’d like to add that many other animals have vestigial structures. One of the most bizarre is the pelvic (hip) bone in whales. Whales evolved from land mammals so they used to have legs. At some point they returned to the ocean and their legs shrank through generations. At some point they disappeared completely but it was never advantageous to remove the tiny pelvic bone so they still have it. Thank you for your question!

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Please read the latest news about the whales' pelvic bones here