Science Questions with Surprising Answers
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Dr. Christopher S. Baird

Why do humans have an appendix even though it is unnecessary?

Category: Biology      Published: October 2, 2013

By: Christopher S. Baird, author of The Top 50 Science Questions with Surprising Answers and Associate Professor of Physics at West Texas A&M University

digestive system
Public Domain Image, source: NIH.

The human cecal appendix is not completely unnecessary. In humans, the appendix is a small, dead-end tube that connects to the colon near where the small intestines feeds into the large intestines. Being a dead-end tube, the appendix can obviously not transport food and waste through the intestinal tract. Furthermore, many people have had their appendix removed and yet seem to suffer no symptoms at all from having no appendix. This fact may seem to indicate that the appendix is useless; an abandoned evolutionary relict of some ancient predecessor. But the appendix turns out to not be completely useless.

According to a study performed by a team lead by J. H. Grendell of Winthrop University Hospital, people without an appendix were found to experience recurring intestinal C. diff. infections 2.5 times as much as people with an intact appendix. They state, "The CDI recurrence rate for patients with an appendix was 18%, compared with 45% in those without an appendix." Additionally, the tissue of the appendix has been found to be rich in active lymphatic follicles similar to those found throughout the intestinal tract. Such lymphatic tissue plays several important biochemical roles in the immune system. In fact, the immune tissue of the appendix has a higher density of immunoglobulin (Ig)A- and IgG-producing immunocytes than the colon. The appendix therefore appears to play a role in the gut's defense against infection.

While the details are not yet fully confirmed by experiment as to how this all works, the line of thinking goes like this: There are typically two kinds of bacteria in the human colon. The bad bacteria destroys tissue and therefore causes infection and disease. The good bacteria does not harm the colon, and in fact benefits humans by keeping the bad bacteria scarce. They do this by beating the bad bacteria in a competition for resources and space. When a person takes antibiotics or experiences an illness that gets rid of the bacteria in the colon, the bad bacteria finds it has the space to get the upper hand. According to this line of thinking, the role of the appendix is to store good bacteria when the colon is being flushed out, so that it can reintroduce the good bacteria into the colon before the bad bacteria takes control. The appendix therefore helps maintain healthy gut flora and is not vestigial. In fact, recent research into the evolution of the appendix has revealed a complex winding path through history that does not fit the old vestigial-structure storyline. H. F. Smith and various collaborators at the University of Arizona have stated,

"A recently improved understanding of gut immunity has merged with current thinking in biological and medical science, pointing to an apparent function of the mammalian cecal appendix as a safe-house for symbiotic gut microbes, preserving the flora during times of gastrointestinal infection in societies without modern medicine...analyses indicate that the appendix has evolved independently at least twice (at least once in diprotodont marsupials and at least once in Euarchontoglires), shows a highly significant (P < 0.0001) phylogenetic signal in its distribution, and has been maintained in mammalian evolution for 80 million years or longer."

Note that the fact that the appendix has a purpose in no way disproves evolution or the concept of vestigial structures. There are plenty of other vestigial structures in animals to give these concepts a sound footing. Swimming whales still have leg bones, flightless birds still have wings, various blind cave-dwellers still have eyes, and many slithering snakes still have a pelvis.

Topics: appendix, evolution, gut, vestigial