Monday, August 24, 2009

A fully functional vestigial organ?

Creationists get pretty excited about vestigial organs. A search for vestigial in CELD gives 28 hits, with articles about horse muscles, male nipples, and several about the appendix, that little dangly thing on your large intestine (provided you still have one). It's not surprising that this would be the case, since I've always thought the evolutionary case for vestigial organs has been overstated. Early on, zealous evolutionary biologists made up lists of vestigial organs that, for the most part, were merely organs of unknown function. They just ignored the standard canon, "Absence of evidence is not evidence of absence." Such mistakes made for easy - and justifiable - targets for creationist arguments.

A more subtle mistake is the insistence that vestigial organs have no function, that they are true evolutionary leftovers that are just waiting to be eliminated from our bodies. This leads to the equally fallacious response that by demonstrating a function - any function - the vestigial argument is nullified. In response to this reasoning, Steve Matheson made a great analogy to the function of a 1989 Yugo: Just because you can turn a Yugo into something useful, like a mailbox or port-a-potty, doesn't make it any less a vestigial car. In fact, it makes its car-ness all the stranger, since its automotive attributes have nothing to do with its function as a mailbox or whatnot. See my point? Having any function does not mean something can't be a vestige. I personally prefer to use the old term rudimentary structures for what we now call vestigial organs.

All that is just a preface to a new article on the human appendix that warrants a few comments. Smith et al. did a great study of appendices in many mammal species and argue that the appendix is neither nonfunctional nor a rudimentary cecum. Now before you get excited and say "I told you so!" remember just what kind of work this is. First, the argument they make is an explicitly evolutionary argument. They use a comparative methodology to conclude that the appendix evolved twice. This is not an explicit study of functionality in one species but rather a study of the occurrence of appendices in many species in order to reinforce theories of appendix functionality.

Second, Smith et al. published their work in the Journal of Evolutionary Biology. I think that's significant for those conspiracy-minded folks who want to say that evolutionary theory stifles dissenting theories. These aren't some outsiders arguing against an evolutionary mainstream. They've made a good argument against the vestigiality and nonfunctionality of the appendix, and it was published by an evolutionary biology journal. I'll leave the conclusions of that line of thinking to your own imagination.

In conclusion, I'll just remind you that the functionality of vestigial organs and noncoding DNA sequences is entirely incidental to the argument for their origin by evolution. Please make a note of that.

Smith et al. 2009. Comparative anatomy and phylogenetic distribution of the mammalian cecal appendix. Journal of Evolutionary Biology doi 10.1111/j.1420-9101.2009.01809.x.