When most people think of evolution, they imagine that famous "monkey to man" image, where an organism evolves to be more complex as time goes on. But that's not always the case, or even the rule. Animals are just as likely to lose features through evolution as they are to gain them.

Take the whale, for example. Its ancestors grew legs and emerged from the water, only to return and lose them again. But some traits keep coming back: the eye, wings, and legs, for example (unless you're a lizard—over and over, different lizards have lost their limbs to become more snake-like). The fact that those features evolved multiple times tells scientists that they're pretty useful, evolutionarily speaking. When an organism loses a trait for it never to be seen again, that suggests that it didn't provide much of a benefit.

That's the approach Midwestern University researchers took for the 2017 study they published in the French scientific journal Comptes Rendus Palevol. They examined the evolution of 533 mammal species over 11 million years to find points where the appendix had emerged as a new trait or disappeared entirely. To support the common belief that the tiny organ is just a vestigial feature with no real benefit, the researchers would expect to see it evolve only a handful of times and disappear pretty regularly. But that's not what they found. Instead, the appendix appears to have evolved between 29 and 41 times, but only disappeared 12 times. It's clear that the appendix serves some purpose. But what is it?

The leading theory about the appendix is that it supports the immune system. The study backs this up: Where they found an appendix, they also found lymph tissue, which is an essential part of the immune system and can aid in the growth of healthy gut bacteria. Other studies have shown that people without an appendix are more likely to suffer from bacterial infections than people with theirs intact.

Of course, that mysterious organ does its fair share of damage, too. A quarter-million people come down with the painful and sometimes life-threatening condition of appendicitis every year, and the standard treatment is to remove the appendix completely. Most people live a long, healthy life after an appendectomy, however, which could mean that other parts of the immune system pick up the pace to make up for its absence.

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