Contrary to popular belief, not everyone has to have their wisdom teeth removed (or even gets them, for that matter).

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By the time you're about 13 years old, you should have 28 permanent, or "adult," teeth. Your pearly whites may not have quite finished coming in, though; between the ages of 17 and 25, some people also have up to four more teeth making an appearance. They're so far back in the jaw that you may not have even realized it was happening until the dentist pointed it out. Known as wisdom teeth, or third molars, these teeth have the potential to cause serious issues, including pain, infection or damage to other teeth around them. If wisdom teeth are such troublemakers, then why do we have them in the first place?

Wisdom teeth are a sort of evolutionary relic known as a vestigial structure -- a part of our bodies that once served an important purpose but whose original function has either diminished or completely changed over time. Wisdom teeth are unique, however, because unlike other vestigial structures like the appendix, we don't all get them (about 35 percent of the population doesn't, in fact [source: Spinney]). For some folks, they erupt and cause no problems -- maybe other than the fact that they're so far back they can be difficult to keep clean. For others, just the phrase "wisdom teeth" uttered by their dentist seems to set off waves of pain and swelling.

There are two potential reasons for our wisdom teeth's vestigial nature. Our ancestors probably needed the additional molars to help grind tough food for easier digestion. The jaws of homo neaderthalensis, or Neanderthals, were much larger and stronger than ours; they even had a space behind these third molars called the retromolar gap. The hominid jaw shrunk with the evolution of homo sapiens (modern humans) as we began eating a softer diet, but the wisdom teeth still erupt even when there isn't space for them.

Our modern dental practices may also be to blame. Thousands of years ago, it was common for hominids to have lost some teeth to decay by the time they were teenagers, so additional pairs would've been useful. Teeth straightening also exaggerates modern humans' dental arch (the way that our side teeth splay outward in our jaw instead of being parallel like Neanderthals'), leaving even less room for wisdom teeth.

It was once practically a rite of passage for a teenager or young adult to get their wisdom teeth "cut out," but today, not all dentists support removing them immediately. Learn about the pros and cons next.