<P> In modern humans, the vermiform appendix is a vestige of a redundant organ that in ancestral species had digestive functions, much as it still does in extant species in which intestinal flora hydrolyze cellulose and similar indigestible plant materials . Some herbivorous animals, such as rabbits, have a terminal vermiform appendix and cecum that apparently bear patches of tissue with immune functions and may also be important in maintaining the composition of intestinal flora . It does not however seem to have much digestive function, if any, and is not present in all herbivores, even those with large caeca . As shown in the accompanying pictures however, the human appendix typically is about comparable to that of the rabbit's in size, though the caecum is reduced to a single bulge where the ileum empties into the colon . Some carnivorous animals may have appendices too, but seldom have more than vestigial caeca . In line with the possibility of vestigial organs developing new functions, some research suggests that the appendix may guard against the loss of symbiotic bacteria that aid in digestion, though that is unlikely to be a novel function, given the presence of vermiform appendices in many herbivores . Intestinal bacterial populations entrenched in the appendix may support quick re-establishment of the flora of the large intestine after an illness, poisoning, or after an antibiotic treatment depletes or otherwise causes harmful changes to the bacterial population of the colon . A 2013 study, however, refutes the idea of an inverse relationship between cecum size and appendix size and presence . It is widely present in euarchontoglires (a superorder of mammals that includes rodents and primates) and has also evolved independently in the diprotodont marsupials, monotremes, and is highly diverse in size and shape which could suggest it is not vestigial . Researchers deduce that the appendix has the ability to protect good bacteria in the gut . That way, when the gut is affected by a bout of diarrhea or other illness that cleans out the intestines, the good bacteria in the appendix can repopulate the digestive system and keep the person healthy . </P> <P> The coccyx, or tailbone, is the remnant of a lost tail . All mammals have a tail at some point in their development; in humans, it is present for a period of 4 weeks, during stages 14 to 22 of human embryogenesis . This tail is most prominent in human embryos 31--35 days old . The tailbone, located at the end of the spine, has lost its original function in assisting balance and mobility, though it still serves some secondary functions, such as being an attachment point for muscles, which explains why it has not degraded further . The coccyx serves as an attachment site for tendons, ligaments, and muscles . It also functions as an insertion point of some of the muscles of the pelvic floor . In rare cases, congenital defect results in a short tail - like structure being present at birth . Twenty - three cases of human babies born with such a structure have been reported in the medical literature since 1884 . In rare cases such as these, the spine and skull were determined to be entirely normal . The only abnormality was that of a tail approximately twelve centimeters long . These tails were able to be surgically removed, and the individuals have resumed normal lives . </P> <P> Wisdom teeth are vestigial third molars that human ancestors used to help in grinding down plant tissue . The common postulation is that the skulls of human ancestors had larger jaws with more teeth, which were possibly used to help chew down foliage to compensate for a lack of ability to efficiently digest the cellulose that makes up a plant cell wall . As human diets changed, smaller jaws were naturally selected, yet the third molars, or "wisdom teeth", still commonly develop in human mouths . Currently, wisdom teeth have become useless and even harmful to the extent where surgical procedures are often performed to remove them . </P> <P> Agenesis of wisdom teeth in human populations ranges from zero in Tasmanian Aboriginals to nearly 100% in indigenous Mexicans . The difference is related to the PAX9 gene (and perhaps other genes). </P>

What has a head but no other organs