Evolutionary Role of the Mammalian Appendix Revealed

A new study points to an apparent function of the appendix as a safe-house for symbiotic gut microbes, preserving the flora during times of gastrointestinal infection in societies without modern medicine.

Researchers from Duke University Medical Center (Duke, Durham, NC, USA), the University of Arizona (Tucson, USA), and Arizona State University (Tempe, USA) used a modern approach to evolutionary biology called cladistics, which utilizes genetic information in combination with a variety of other data to evaluate biological relationships that emerge over the ages. Their study revealed that that the appendix has three apparent mammalian morphotypes, as well as appendix-like structures in some species that lack a true cecal appendix. The cladistic analyses indicated that the appendix has evolved independently at least twice, shows a highly significant phylogenetic signal in its distribution, and has been maintained in mammalian evolution for 80 million years or longer. According to the researchers, the "safe house” function is potentially a selective force for the evolution and maintenance of the appendix, and provides an impetus for reassessment of its evolution. The study was published online on August 12, 2009, in the Journal of Evolutionary Biology.

"If Darwin had been aware of the species that have an appendix attached to a large cecum, and if he had known about the widespread nature of the appendix, he probably would not have thought of the appendix as a vestige of evolution,” said senior author William Parker, Ph.D., an assistant professor of surgical sciences at Duke, who also claims that Darwin was also unaware that appendicitis is not due to a faulty appendix, but rather due to cultural changes associated with industrialized society and improved sanitation.

"Those changes left our immune systems with too little work and too much time their hands--a recipe for trouble,” added Professor Parker. "We didn't really have a good understanding of that principle until the mid 1980s. Even more importantly, Darwin had no way of knowing that the function of the appendix could be rendered obsolete by cultural changes that included widespread use of sewer systems and clean drinking water.”

The researchers suggested that now that they can determine the normal function of the appendix, scientists can devise ways to challenge immune systems in much the same manner that they were challenged back in the Stone Age. If this could be achieved, they reported, far fewer cases of allergies, autoimmune disease, and appendicitis should occur.

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Duke University Medical Center
University of Arizona
Arizona State University