Human vestigiality

In the context of human evolution, vestigiality involves those traits occurring in humans that have lost all or most of their original function through evolution. Although structures called vestigial often appear functionless, they may retain lesser functions or develop minor new ones. In some cases, structures once identified as vestigial simply had an unrecognized function. Vestigial organs are sometimes called rudimentary organs. Many human characteristics are also vestigial in other primates and related animals.

History

listed a number of putative human vestigial features, which he termed rudimentary, in The Descent of Man. 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 that 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 evolutionary history. This book contains a list of 86 human organs he considered vestigial, which he called "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:

the role of the pineal in the regulation of the circadian rhythm ;
discovery of the role of the thymus in the immune system lay many decades in the future; it remained a mystery until the mid-20th century;
the pituitary and hypothalamus, with their many and varied hormones, were far from understood, let alone the complexity of their interrelationships.

Historically, there was a trend not only to dismiss the appendix as being uselessly vestigial, but an anatomical hazard liable 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 poorly understood role left it 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 said 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 20th century, a great deal of fundamental research remained 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 surgeon Kenelm Hutchinson Digby documented previous observations, going back more than 30 years, that suggested lymphatic tissues, such as the tonsils and appendix, might have substantial immunological functions.

Anatomical

Appendix

The appendix was once believed to be 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. This view has changed in recent decades, with research suggesting that the appendix may serve an important purpose. In particular, it may serve as a reservoir for beneficial gut bacteria, possibly to allow the bacteria to reestablish in the colon during recovery from diarrhea or other illnesses.
Some herbivorous animals, such as rabbits, have a terminal vermiform appendix and cecum that apparently bear patches of tissue with immune functions and that may also be important in maintaining the composition of intestinal flora. It does not 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, 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 have appendices too, but few have more than vestigial caeca. In line with the possibility that vestigial organs develop 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 reestablishment 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 refutes the idea of an inverse relationship between cecum size and appendix size and presence. It is widely present in Euarchontoglires and has also evolved independently in the diprotodont marsupials and 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: when the gut is affected by diarrhea or another illness that cleans out the intestines, the good bacteria in the appendix can repopulate the digestive system and keep the person healthy.

Coccyx

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, 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.
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 these cases, the spine and skull were determined to be entirely normal. The only abnormality was that of a tail approximately 12 centimeters long. These tails, though of no deleterious effect, were almost always surgically removed.

Wisdom teeth

are vestigial third molars that human ancestors used to help in grinding down plant tissue. The common postulation is that their skulls 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, but the third molars, or "wisdom teeth", still commonly develop in human mouths.
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.

Vomeronasal organ

In some animals, the vomeronasal organ is part of a second, completely separate sense of smell, known as the accessory olfactory system. Many studies have been performed to find if there is an actual presence of a VNO in adult human beings. Trotier et al. estimate that around 92% of their subjects who had not had septal surgery had at least one intact VNO. Kjaer and Fisher Hansen, on the other hand, found that the VNO structure disappeared during fetal development as it does for some primates. Smith and Bhatnagar asserted that Kjaer and Fisher Hansen simply missed the structure in older fetuses. Won found evidence of a VNO in 13 of his 22 cadavers and in 22 of his 78 living patients. Given these findings, some scientists have argued that there is a VNO in adult human beings. Most have sought to identify the opening of the vomeronasal organ in humans, rather than identify the tubular epithelial structure itself. Thus it has been argued that such studies, employing macroscopic observational methods, have sometimes missed or even misidentified the vomeronasal organ.
Among studies that use microanatomical methods, there is no reported evidence that human beings have active sensory neurons like those in other animals' working vomeronasal systems. Furthermore, no evidence suggests there are nerve and axon connections between any existing sensory receptor cells in the adult human VNO and the brain. Likewise, there is no evidence of any accessory olfactory bulb in adult human beings, and the key genes involved in other mammals' VNO function have become pseudogenes in human beings. Therefore, while the presence of a structure in adult human beings is debated, a review of the scientific literature by Tristram Wyatt concluded, "most in the field... are sceptical about the likelihood of a functional VNO in adult human beings on current evidence."

Ear

The ears of a macaque monkey and most other monkeys have far more developed muscles than those of humans, and therefore have the capability to move their ears to better hear potential threats. Humans and other primates such as the orangutan and chimpanzee however have ear muscles that are minimally developed and non-functional, yet still large enough to be identifiable. A muscle attached to the ear that cannot move the ear, for whatever reason, can no longer be said to have any biological function. In humans there is variability in these muscles, such that some people are able to move their ears in various directions, and it can be possible for others to gain such movement by repeated trials. In such primates, the inability to move the ear is compensated mainly by the ability to turn the head on a horizontal plane, an ability which is not common to most monkeys—a function once provided by one structure is now replaced by another.
The outer structure of the ear also shows some vestigial features, such as the node or point on the helix of the ear known as Darwin's tubercle which is found in around 10% of the population.