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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 tailbone, 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.
- 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.