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A vestigial structure is an organ or limb that has lost all or most of its original function due to evolution. Vestigiality is a special case of homology, where corresponding parts in different organisms have greater structural resemblance than what is necessary for their functions.
Examples run the gamut from being occasionally harmful to the organism (i.e. the human appendix) to having no discernible effect (i.e. the wings on an emu), and even to having other functions. Vestigial features can be either persistent, like the human coccyx or the Palmaris longus, or transient, like the human embryonic tail, which briefly grows then gets resorbed. Many creationists argue that vestigial implies functionless, and that many vestigial features are really functional and thus not vestigial, but vestigiality is from structural, not functional, resemblance.
Charles Darwin himself had noted in Origin of Species the presence of numerous "rudimentary, atrophied, or aborted organs" in organisms, and he criticized then-common beliefs about them like that they had been created "for the sake of symmetry" or "to complete the scheme of nature". According to such pre-Darwinian beliefs, human embryos briefly grow tails, with the coccyx remaining, because the creator thought that we would be incomplete without tails.
Creationists often like to point to Robert Wiedersheim's 1893 list of 86 human vestigial organs, claiming that most of them have since been discredited. A comparison of his list to more recent lists like it is nevertheless revealing. His "vestigial features" fall into several categories:
- Persistent features still considered vestigial, like the coccyx and wisdom teeth.
- Transient embryonic organs, like the embryonic tail.
- Misidentified as vestigial, like endocrine glands.
One reason an organ can still be "vestigial" while retaining function is that the point in time at which the organ might be examined by human biologists is at an arbitrary place in that organ's evolution. Like other evolutionary processes, vestigiality doesn't occur in a single generation, but over a very long period of time. Eventually, a "fully vestigial" organ can be expected to shrink or otherwise disappear so much that hardly anything is left except perhaps the few parts which are essential. (The human tailbone may be considered an example of this.) Evolution doesn't try to hurry up and "vestigialize" something before humans have a chance to talk about it. The point isn't functionality, but the structure's clues to its past.
Like turkey sandwiches made after American Thanksgiving, vestigial structures are leftovers. Leftovers in general are edible (or "functional" as food), but it would be silly to assume that all the turkey sandwiches made after Thanksgiving actually use turkey bought for the sake of the sandwich, or in general to think that "edible" means "not leftovers" by definition. It would be even sillier to suppose that the subsequent dirty dishes (being structures with almost no remaining edibility/functionality) were actually made dirty in the dish factory, possibly for aesthetic reasons, and have no subsequent history of use for dining.
Creationist interpretation of vestigial structures, and obsession with finding function in them as if this would "disprove" evolution, is ultimately a misidentification of the point of evolutionary biology, which is not to kick God out of everything. If it were, perhaps scientists would focus myopically on "this structure is useless, so no creator could have made it, done", as creationists may pretend.
Scientists don't do this, because what they're interested in is the structure's actual evolutionary history -- what function did it once have, and how did it go from that function to its current one? Creationists can claim that vestigial structures are the way they are because of some definite purpose, but as with all other evidence for the nested hierarchy, they can't explain why the particular structures happen to fit the tree of life so well. Why make a human vestigial tail in embryonic development, rather than vestigial wings?
- The Origin of Species: Chapter 13, section "Rudimentary, atrophied, or aborted organs"
- See the Wikipedia article on Vestigiality., from which some of the text of this article was shamelessly purloined.
- See the Wikipedia article on Robert Wiedersheim. He had proposed 86 human vestigial features in 1893.
- See the Wikipedia article on Human vestigiality. That article lists features that are nowadays considered vestigial.