Probing Question
Why do humans have appendices?
Courtesy A.D.A.M.
Sometimes an inheritance can be a thrill and sometimes it can be a pain in the gut.
The appendix, that little pouch at the top of your large intestine, has a useful function in some mammals, but not in primates, the group of mammals to which humans belong, says Nina Jablonski, professor of anthropology.
In some mammals, the appendix is a special compartment of the large intestine used for digesting cellulose, fibrous plant material, Jablonski says. In these animals, the first part of the large intestine houses and enlarged appendix and cecum which serve as fermentation chambers for the breakdown of cellulose. A grass-eating animal like a rabbit has a very large cecum and appendix, where specialized bacteria can have ample time to break down this fibrous material.
By contrast, "Ancient primates, as far as we can tell from studying their fossils, were mostly fruit and insect eating animals," Jablonski says. With this significant shift in diet, they had no real need for cellulose-digesting equipment.
"At some point in the evolution of the ancestors of primates, the appendix lost its ability to function and lost its large size and anatomical differentiation," Jablonski explains. To this day, however, humans have retained a miniaturized version of the cecum-appendix structure.
Although some recent research suggests that the appendix may play a role in the human immune system, "In most primates it doesn't do anything except occasionally cause trouble," Jablonski says. Appendicitis occurs when material becomes lodged in the tiny opening to the appendix and causes it to become inflamed, Jablonski explained.
The chances that the appendix will disappear completely through evolution over the next million years are slim, she adds. "People with appendicitis are taken to hospitals and live, so the size of their appendix does not adversely affect their own reproductive success." With natural selection thus pre-empted, the trait of the useless appendix is passed on in our genes.
Nor is the retention of the appendix a particularly unusual evolutionary phenomenon, Jablonski says. "There are lots of things in our bodies that are just there. They're not the best design, but they're not causing any particular problems, so we just hang on to them."
Nina Jablonski, Ph. D., is professor and head of anthropology in the College of the Liberal Arts and is author of the book Skin: A Natural History. She can be reached at ngj2@psu.edu.