Many have speculated that it exists to keep surgeons in business. Leonardo da Vinci thought it might be an outlet for “excessive wind” to prevent the intestines from bursting. The great artist and anatomist was not entirely off base in that the human appendix does appear to have originated at a time when primates ate plants exclusively, and all that fiber was tougher to digest.
The intestinal offshoot formally known as the vermiform appendix is a long, slender cavity, closed at its tip. It branches off the cecum, which is itself a big pouch at the beginning of the large intestine that receives partly digested food emptying from the small intestine. While food stalls in the cul de sac of the cecum, friendly gut microbes help to break it down further. Some of today’s herbivorous animals, such as rabbits and koalas, have a large appendix, filled with specialized cellulose-digesting bacteria for the same purpose. Yet plenty of plant-eating mammals, including some monkeys, have no appendix at all, relying on an enlarged cecum to break down plants. Because the appendix seems to be optional even among primates, biologists cannot simply infer that ours is a shrunken legacy from a common ancestor with the bunny. Rather the primate appendix and the appendices of other herbivorous mammals appear to have evolved independently as extensions of the cecum—perhaps for the same digestive purpose—but the human appendix has long since lost that function.