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Appendix: Why do we still have it?

Imagine the time of a few million years ago, a time when our ancient primate ancestors were still chowing down on leafy greens.


human ancestors eating leaves
(Image credit: Nicolle Rager Fuller, National Science Foundation.)

Their teeth were built for plant munching. Tooth-wear studies have shown less than 2% evidence of meat consumption. But, the issue is this: those diets were rich in cellulose. And cellulose is a tough fiber in plants that humans can’t digest on their own.


To break it down, these early ancestors relied on the “cecum”, which is nothing but a poch inside our guts that houses microbes that can eat cellulose.

This fermentation chamber helped release energy from leaves. Over the period of years, our ancestors learned to cook and this diet changed. That also means that we didn’t need that pouch anymore.


Over long stretches of evolution, it shrunk— eventually becoming the tiny “appendix” sticking off the large intestine. Charles Darwin even described it as a remnant of that lost digestive system.


But here’s the wild evolutionary twist: the appendix didn’t just evolve once—it popped up many times independently in different mammal groups! Studies have traced its appearance at least 29 to over 30 times across species—from rodents to koalas and primates. That’s a strong hint evolution kept bringing it back because it served a useful purpose.

Now, what might that purpose be today?


Here’s what scientists are discovering.


  1. Immune hotspot: The appendix is rich in lymphoid tissue, which helps train and regulate immune responses in the gut.

  2. Safe‑house for good bacteria: When gut flora get wiped out—say during diarrhea or antibiotic treatment—the appendix may serve as a protected reservoir to repopulate your gut with healthy microbes.

  3. Protective friend in primates: Research in non-human primates shows those with an appendix face 85% fewer diarrhea episodes, especially when young, hinting at a clear evolutionary advantage.


Next time someone says the appendix is useless, remind them what your inner leaf-powered ancestors evolved it for—and how it’s gone from vestige to quite the defender.


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