The Appendix: Don’t close the book on it yet!
● The vermiform appendix is the poster child of vestigial organs leading to the joke that, “Its major importance would appear to be financial support of the surgical profession.” Like the wings of an ostrich or the eyes of the blind cave-dwelling catfish, the appendix no longer supports the function that it was designed to do: digest tough cell walls of plants. In herbivores, this function resides in the caecum, an off shoot of the large intestine, that houses symbiotic bacteria, producing enzymes (cellulases) by fermentation. Did you know that in the koala, the caecum is longer than the animal itself?! But, as Darwin noted, in hominids -apes and humans, the switch from a leafy to predominantly fruit diet made the caecum redundant and eventually, it degenerated into the finger-like appendix. Although we still eat plants, our vestigial organ does not house enough cellulase-secreting bacteria to digest more than a few grams of cellulose per day.
● So why do we still have an appendix? It is notoriously prone to infection, commonly in children 8-13 years old. Before modern surgical methods, acute appendicitis was often fatal. What a poor design! But there is evidence that the appendix has useful functions. Like the tonsils, the appendix houses lymphoid tissue, or white cells, important for immunity. It has been compared to a “safe house”, lodging beneficial bacteria that can repopulate our gut after an infection wipes out existing microbial flora.
● A new study by a group at Duke University has concluded that the appendix has arisen independently more than 30 times in the evolution of mammals. By plotting diet on the evolutionary tree of mammals, researchers found that the appearance of the appendix did not correlate with a change away from herbivorous diets. Species with an appendix were scattered so widely on the evolutionary tree that they concluded that the appendix evolved separately along distinct branches. Also, they found that the larger the caecum, the larger the appendix: opposite to what one would expect for a vestigial remnant of the caecum. But naysayers argue, if it is so useful, why don’t all mammals have an appendix? We’ll have to wait until Science adds another Chapter to the Appendix!
Ref: Multiple independent appearances of the cecal appendix in mammalian evolution and an investigation of related ecological and anatomical factors. Smith et al. (2013) http://goo.gl/zJyviw
Counterpoint: The vestigiality of the human vermiform appendix. A modern reappraisal. http://goo.gl/v9Qvm0