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It's commonly thought that the appendix is useless, only in our bodies to cause pain and be removed. That idea stemmed from the work of Charles Darwin himself. He theorized that human ancestors subsisted primarily on plants, which required a large cecum, an organ that breaks down some of the tough parts of plants. As humans introduced more meat into our diets, the cecum shrank, becoming smaller and smaller. In contemporary humans, the appendix, which is placed on the now tiny cecum, is one of its former folds, he said, and now is entirely useless.

However, that may not appear to be the case. According to Science Now, the appendix houses a certain type of tissue that is important to the function of the lymphatic system, which holds the white blood cells that humans need to fight infections. This tissue also appears to play an instrumental role in the production of gut bacteria. Other research has found that the appendix appears in a variety of mammals, not just humans and primates: koalas, beavers and porcupines also have the same organ.

In a study conducted by researchers at Arizona State University, Duke University Medical Center in the United States and the University of Stellenbosch in South Africa, they found that it appears that the appendix appears in 50 species. These species are so unrelated - beyond the fact that they are mammals - that researchers believe that the appendix evolved as many as 38 times, or as little as 18, if researchers just look at the clear-cut examples.

Even more interestingly, researchers found that the appendix's evolution had very little to do with change in diet. In most cases, except for apes, the appearance of the appendix had nothing to do with a dietary shift.

Because the appendix has independently evolved so many times, researchers are certain that the organ has a purpose. However, they are not quite sure what that purpose actually is. They believe that the appendix may play an important role for the immune system, providing a safe house for good bacteria if bad bacteria colonize and overtake the gut. That theory would make sense, due to the appendix's out-of-the-way location in the body.

However, that theory would lend the question why all mammals do not have the appendix, instead of just 50 out of the 361 species studied. That may mean that the mystery of the appendix has not yet been solved.

The study was published in the journal Comptes Rendus Palevol.