Vermiform musings

Well, maybe the human appendix has a function after all. Long one of the textbook examples of a “vestigial organ,” a recent publication now reinterprets the appendix as serving a critical role in helping deal with contamination of food. Here is a Science Buzz article summarizing the research. Both detailed (i.e., microscopic) studies of the appendix and comparisons of the equivalent organ in other mammals provide evidence that this structure serves as a refuge where a reserve force of “good” bacteria can be maintained in the event of an influx into the intestines of toxic food-borne bacteria. The paper also suggests that appendicitis can be intepreted as a pathological response to a modern diet–a diet which the appendix was not evolved to handle.

Drawing of human small and large intestines, with appendix at lower left (4).

Drawing of human small and large intestines, with appendix at lower left (4)

From http://www.gutenberg.org/files/18467/18467-h/images/advise028.png

So what? Well, the thing is that the “vermiform appendage” (as Darwin called it) has long played a role in arguments about the history of life, and especially the history of the human lineage. The interpretation of the appendix as a vestigial structure figured in first chapter of Darwin’s The Descent of Man. The chapter was entitled “The evidence of the descent of man from some lower form” (no beating around the bush there). In the popular imagination, this interpretation has hardened into dogma for all practical purposes, and continues to play a part in the “argumentarium” of defenders of an evolutionary world view. Vestigial structures are especially favored by critics of so-called “Intelligent Design creationism. The writers here and here make the standard argument: what kind of intelligence is it that produces a structure that is useless at best, and can even malfunction with lethal consequences on occasion? Gotcha!

So what if the appendix actually does have a function, and what if it actually is rather well configured for that function? Does this give solace to creationists? I’m sure it will, but it shouldn’t. Nature is replete with biological structures whose function was initially unknown or misunderstood before deeper studies reveal their contribution to the organisms survival. For example, a few pages after his discussion of the appendix, Darwin himself posits that the external ear of humans “together with the various folds and prominences…which in the lower animals strengthen and support the ear when erect…is of no distinct use.” Now it is thought that the folded structures of the outer ears reflect sound waves in a manner that helps the auditory system more accurately pinpoint the source of sound in space.

So, it is neither surprising nor unprecedented to discover a previously unknown function for an apparently useless structure. At the same time, such discoveries do not invalidate the more general idea that evolution does sometimes leave in modern species useless vestiges of a formerly complex structure. See this list of the “Top 10” vestigial organs for examples (a list which includes one vestigial example–the human appendix–but several examples that are true vestiges of structures that disappeared during evolution). Some creationists work strenuously to prove that apparent vestiges actually have functions specified by The Designer (you can see the fruits of their labors in creationist web sites such as Answers in Genesis), and will be greatly reassured by the new paper on the function of the appendix (the same prediction is made here). However, the phenomenon of vestigial structures remains a very conspicuous challenge to a creationist world view, and yet one very easily accommodated by Darwinian theory.

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Updated to correct broken link: http://www.livescience.com/11317-top-10-useless-limbs-vestigial-organs.html

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About ethologist

Professor in the Zoology Department at Michigan State University
This entry was posted in Evolution. Bookmark the permalink.

2 Responses to Vermiform musings

  1. David Benn says:

    Thanks for your post. You cited my earlier post as advocating the standard argument. I just wanted to point out that in a follow-up to one of the responses to my post I said this: “…I’m willing to accept that the Human Appendix may not have been the best choice to make my point. The 17 May 2008 New Scientist article “The old curiosity shop” makes a similar point to Evan’s reference re: a credible role for the Human Appendix. It then goes on to point out other more likely vestigial human organs, such as wisdom teeth, the tail bone (coccyx, a vestige of the mammalian tail), Darwin’s point (a cartilaginous bump on the outer ear), goose bumps.”

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