Vestigial remnant

In the context of human evolution, human vestigiality involves those traits (such as organs or behaviors) occurring in humans that have lost all or most of their original function through evolution. Although structures called vestigial often appear functionless, a vestigial structure may retain lesser functions or develop minor new ones. In some cases, structures once identified as vestigial simply had an unrecognized function. In the context of human evolution, human vestigiality involves those traits (such as organs or behaviors) occurring in humans that have lost all or most of their original function through evolution. Although structures called vestigial often appear functionless, a vestigial structure may retain lesser functions or develop minor new ones. In some cases, structures once identified as vestigial simply had an unrecognized function. The examples of human vestigiality are numerous, including the anatomical (such as the human tailbone, wisdom teeth, upper lip, and inside corner of the eye), the behavioral (goose bumps and palmar grasp reflex), and molecular (pseudogenes). Many human characteristics are also vestigial in other primates and related animals. Charles Darwin listed a number of putative human vestigial features, which he termed rudimentary, in The Descent of Man (1890). These included the muscles of the ear; wisdom teeth; the appendix; the tail bone; body hair; and the semilunar fold, in the corner of the eye. Darwin also commented on the sporadic nature of many vestigial features, particularly musculature. Making reference to the work of the anatomist William Turner, Darwin highlighted a number of sporadic muscles which he identified as vestigial remnants of the panniculus carnosus, particularly the sternalis muscle. In 1893, Robert Wiedersheim published The Structure of Man, a book on human anatomy and its relevance to man's evolutionary history. This book contained a list of 86 human organs that he considered vestigial, or as Wiedersheim himself explained: 'Organs having become wholly or in part functionless, some appearing in the Embryo alone, others present during Life constantly or inconstantly. For the greater part Organs which may be rightly termed Vestigial.' His list of supposedly vestigial organs included many of the examples on this page as well as others then mistakenly believed to be purely vestigial, such as the pineal gland, the thymus gland, and the pituitary gland. Some of these organs that had lost their obvious, original functions later turned out to have retained functions that had gone unrecognized before the discovery of hormones or many of the functions and tissues of the immune system. Examples included: Historically, there was a trend not only to dismiss the vermiform appendix as being uselessly vestigial, but an anatomical hazard, a liability to dangerous inflammation. As late as the mid-20th century, many reputable authorities conceded it no beneficial function. This was a view supported, or perhaps inspired, by Darwin himself in the 1874 edition of his book The Descent of Man, and Selection in Relation to Sex. The organ's patent liability to appendicitis and its poorly understood role left the appendix open to blame for a number of possibly unrelated conditions. For example, in 1916, a surgeon claimed that removal of the appendix had cured several cases of trifacial neuralgia and other nerve pain about the head and face, even though he stated that the evidence for appendicitis in those patients was inconclusive. The discovery of hormones and hormonal principles, notably by Bayliss and Starling, argued against these views, but in the early twentieth century, there remained a great deal of fundamental research to be done on the functions of large parts of the digestive tract. In 1916, an author found it necessary to argue against the idea that the colon had no important function and that 'the ultimate disappearance of the appendix is a coordinate action and not necessarily associated with such frequent inflammations as we are witnessing in the human'. There had been a long history of doubt about such dismissive views. Around 1920, the prominent surgeon Kenelm Hutchinson Digby documented previous observations, going back more than thirty years, that suggested lymphatic tissues, such as the tonsils and appendix, may have substantial immunological functions. In modern humans, the 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. 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. 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.