Wings on a flightless bird, eyes on a blind fish, and sexual organs on a flower that reproduces asexuallyâ€”the casual observer might ask, whatâ€™s the point? But these vestigial organs and structures, once useful in an ancestor and now diminished in size, complexity, and/or utility, carry important information and give us clues to our evolutionary past.
Though humans often think of vestigial organs as useless little fixtures that sometimes, as in the case of the appendix, cause us extreme anguish, we wouldnâ€™t know nearly as much about macroevolution as we do now without their presence. In On the Origin of Species, Charles Darwin used vestigial organs as evidence for evolution, and their presence has helped define and shape our phylogenetic trees.
Why the Leftovers?
Contrary to what most think, vestigial doesnâ€™t necessarily mean useless; in some cases, we may just not yet know exactly how the organ is used in its current incarnation. (The human thymus was once thought to be vestigial). Because these structures can be traced back through the ancestors, they essentially serve as a marker of evolution; no organism can have a vestigial organ that hasnâ€™t been found in its forefathers. For this reason, you wonâ€™t ever find feathers on a mammal or gills on a primate.
One striking example of an atavism is the human coccyx, or tailbone, which is a relic of the mammalian tail. Useful for mammals that use tails for balance, species-to-species signaling, and support, the tail is missing in apes and in humans. However, all human embryos initially have a tail. Normally, they regress into four to five fused vertebrae (the coccyx). However, there have been numerous case studies of human children being born with an extended coccyxâ€”a tailâ€”that was removed without incident. Ranging from one inch to five, the gene that normally stops vertebrae elongation is decreased and the human tail remains at birth.
When we get goose bumps, itâ€™s the action of muscle fibers called erector pili, which cause the hairs in follicles to stand to attention. In animals, such as a cat, this causes a larger appearance and can be used to thwart an attacker, as well as trap air between feathers and fur for insulation. However, humans, with our minimal coating of fur, donâ€™t really need the raised hair; we use jackets instead. It is therefore thought that goose bumps donâ€™t really serve much of a purpose. However, the small expenditure of energy used to contract the muscles could, perhaps, cause a tiny release of heat. Or, because goose bumps are associated not only with cold, but emotional responses as well (listening to a good song, seeing a scary movie) they could now serve as a form of communication with others.
More at Divine Caroline
Ah, the tailbone. What a pain in the ass…literally. I fell down my cellar stairs a couple of years ago and fractured my coccyx. It still hurts if I sit wrong. Forget hard benches.