What the appendix is good for

Some body parts seem pointless but in fact have purpose.


The appendix is a small organ that looks like a little worm (circled in red on the lower left of image). It doesn't lead anywhere, but may serve as a haven for good bacteria.


It was a Saturday morning in 1991 when 12-year old Heather Smith woke up feeling sick to her stomach. Spring break was just beginning. The next dayher parents were planning to take her skiing in Flagstaff, Ariz.  It was  two hours from their home in Tempe.

A stomachache was not how Smith wanted to start vacation. “I was hoping I would get better,” she says, “So I could go ski.”

As the day went on, things got worse. A sharp pain developed in her lower right side. She couldn’t swallow the soup her sister warmed up for her lunch. By the time she saw a doctor later that afternoon, she was hunched over in pain.

Her doctor said her appendix was infected. The girl didn’t have much time to be afraid. She was rushed into surgery. The next morning, her appendix was gone.

“It was a little scary because it happened so quickly,” says Smith, now an adult. She works as an evolutionary biologist at the Arizona College of Osteopathic Medicine in Glendale, Ariz. But she has never missed her long-lost organ. In fact, the emergency left her with a lifelong fascination for a body part she no longer has.

“I have always been interested in the appendix and trying to figure out why we have one,” Smith says. “There’s been this idea for so long that it didn’t do anything.”

Appendices have long been considered “vestigial structures.” That means we don’t actually need them. The brain, heart, skin and most other organs are essential for survival. But you can live a long life without an appendix. The same goes for tonsils, wisdom teeth, body hair and other vestigial structures.

At best, some biologists have argued, vestigial structures just take up space. At worst, they can get infected and cause all sorts of trouble. So why do we have then?

Growing evidence suggests that we have them because they aren’t actually unnecessary at all. Their function probably depends on where you live (and perhaps when you lived). In some parts of the world, people still need vestigial body parts. Studying where and when these features are or were useful is helping scientists make new advances in modern medicine. The work is also providing clues to the history of humankind — telling scientists new things about our ancestors.

“It may be the case with a lot of unnecessary body parts that they may have had a function in the past but we don’t necessarily need that function anymore,” says Smith. “That can give us insights.”

The hidden point

Consider your body, and you’ll notice a hodgepodge of random features that might seem silly when you stop to think about them. What’s the point of fingernails, for example? Why is there hair on your toes? And what’s the deal with muscles in your ears? Do we really need muscles in our ears?

Throughout history, scientists, too, have wondered about structures that don’t seem to do anything useful. The appendix is a common example. This little, worm-shaped pouch is about four inches long and less than half an inch wide.

The organ grows near where the long intestine meets the short intestine. The intestines are essential for digestion, but the appendix appears to just sit there.

“It’s a dead-end sack,” says William Parker. “it doesn’t go anywhere.” Parker is an immunologist at Duke University in Durham, N.C.

Parker didn’t start out intending to study the appendix. His specialty is the immune system — a collection of organs, cells and molecules that our bodies use to stay healthy. 

Parker knew that the human body is full of tiny bacteria. These microbes can overwhelm the immune system, cause infections and make someone sick. But he also knew that some bacteria are good for human health. Among their benefits, these “good” bacteria help people digest food and fight off “bad” bacteria that cause disease.

The immune system doesn’t just benefit from good bacteria, though. In the 1990s, Parker and his colleagues began to figure out that the immune system also helps good bacteria flourish. These bacteria appear in thin layers called biofilms. These goos grow on the side of the gut near and inside the appendix. Biofilms, the researchers learned, provide a barrier that keeps out bad bacteria.

“Once we figured that out, it should have been obvious to us what the appendix did,” says Parker. After all, his team found that the appendix has a vert robust biofilm. “It’s in the perfect spot to harbor bacteria — out of the flow and with a thin, narrow opening. And there’s a large amount of immune tissue associated with it.”

After stumbling on a possible link between the immune system and the appendix, however, the scientists still had some clues to compile before being sure of the organ’s purpose.

Hangout for good bacteria

In 2007, Parker’s team put together all the evidence they had gathered. This suggested a conclusion: The appendix serves as a “safe house,” Parker says, a storage bin for good bacteria. If bad bacteria attack, good bacteria can emerge from the appendix and come to the rescue.

Having a safe space for good bacteria should be especially useful in parts of the world that are poor and undeveloped. These are places where people are starving, where medicine can be hard to come by, where clean water is scarce and where diarrhea can kill. In those places, Parker says, the appendix probably helps keep people alive, especially young children.

In fact, people in the developing world rarely get infected appendixes, like Smith’s. Most cases of appendicitis, in fact, occur in the United States and other developed countries, where water is purified, hospitals are sterilized and medical care is easier to get.

Those trends suggest that the appendix evolved in our ancestors to maintain health in a bacteria-filled world. Today, places such as the United States might be too sterile for the appendix. When the organ has nothing do, the immune system can turn on itself, sending people to the emergency room, Parker says. Other problems, such as allergies and immune diseases, might have similar roots.

Even in ultra-clean societies, then, the appendix and other vestigial organs might be unrecognized heroes.

“Just because body parts don’t seem to have any usefulness here doesn’t mean you wouldn’t need them if you were suddenly thrown in the middle of the woods somewhere and had to drink from whatever mud hole you could find nearby and you had to run away from predators,” Parker says. “Problems we are having today with allergies and autoimmune diseases are a result of the body not really fitting in with our culture.”

Figuring out the true purpose of the appendix and other overlooked organs, Parker adds, is an important step toward solving medical mysteries.

“We want to understand how the body functions so we can work towards getting it to function normally,” he says.

To do that, it can help to take an historical view. By considering what was normal a long time ago and comparing the old normal to the new normal, researchers can see how evolution has shaped our bodies over hundreds of thousands of years. That process of change over time is called evolution.

“The best way to figure out how the body was designed to work,” Parker says, “is to look at how it was meant to work over hundreds of millions of years of evolution.”

Wise beyond our years

The appendix isn’t the only example of a body part with hidden powers. Wisdom teeth are another. This final set of molars usually grows in at around age 20. Today, most people get their wisdom teeth removed before the bulky molars can squeeze other teeth out of place or get infected.

Millions of years ago, though, human faces weren’t as flat as they are today and mouths had more room for wisdom teeth. After 20 years of life without dental care, our ancestors would have benefited from a fresh set of strong teeth that could chew and grind raw food.


Our ancestors may have found wisdom teeth more useful than we do.



As for other structures long thought to be pointless, a recent study found that the spleen stores a whole lot of immune cells. Among other roles, those cells help to repair hearts that are damaged. Tonsils, which are also removed routinely in many developed countries, probably help boost the immune system, as well, Parker says.

As they continue to find purposes for seemingly purposeless body parts, scientists are connecting our present with the past. They are also connecting the human animal with other animals on Earth.

Last year, Smith teamed up with Parker and other colleagues to look at a whole bunch of mammal species, some that lived tens of millions of years ago. The researchers found that the appendix has existed in a wide range of animals, from rodents to primates to Australian marsupials. The study also revealed that the appendix evolved more than once throughout history. Both findings suggest that the appendix has had an important purpose throughout time.

By looking closely at our body’s “pointless” parts, we can begin to imagine what our bodies used to be able to do. Recognizing the body’s lingering power could also open up a whole new future of possibilities.

“Our evolution gives our bodies a lot of resilience and strength we really don’t need very much in our society,” says Parker. “I sit around in my office and have all the food I want. My body can do so many things I never ask it to do.”

This story and other Science News for Kids features describing research in medicine and biology are supported with funding from The Lasker Foundation. The foundation and its programs are dedicated to the support of biomedical research toward conquering disease, improving human health and extending life.

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