Human vestigiality

In the context of human evolution, human vestigiality involves those characters (such as organs or behaviors) occurring in the human species that are considered vestigial - in other words having lost all or most of their original function through evolution. Although structures usually called "vestigial" are largely or entirely functionless, a vestigial structure may retain lesser functions or develop minor new ones.^[1]

Vestigial characters occur throughout nature, one example being the vestigial hind limbs of whales and snakes. Many human cases are also vestigial in other primates and related animals. The following characters have been or still are considered vestigial in humans:

Anatomical

Appendix

The vermiform appendix is a vestige of the cecum, an organ that would have been used to digest cellulose by humans' herbivorous ancestors.^[2] Analogous organs in other animals similar to humans continue to perform that function, whereas other meat-eating animals may have similarly diminished appendices. The modern functionality of the appendix is still controversial in the field of human physiology, although most scientists and physicians believe that it has little or no function. Recent research by Duke University suggests that the appendix may guard against the loss of symbiotic bacteria that aid in digestion. ^[3]

Coccyx

The coccyx, or tailbone, is the remnant of a lost tail. All mammals have a tail at one point in their development; in humans, it is present for a period of 4 weeks, during stages 14 to 22 of human embryogenesis.^[4] This tail is most prominent in human embryos 31-35 days old.^[5] 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. In rare cases a short tail can persist after birth, with 23 human babies possessing tails having been reported in the medical literature since 1884.^[6]^[7]

Wisdom teeth

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 diet changed, a smaller jaw was selected by evolution, yet the third molars, or "wisdom teeth", still commonly develop in human mouths.^[8]

However, other findings suggest that a given culture's diet is a larger factor than genetics in the development of jaw size (and, consequently, the space available for wisdom teeth):

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.^[10] Humans and other primates such as the orangutan and chimpanzee however have ear muscles that are minimally developed and non-functional.^[2] A muscle 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 has been said that it may be possible for others to gain such movement by repeated trials.^[2] 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.^[11]

Eye

Sensory

Although the sense of smell or olfaction is highly important for many animals in avoiding predators, finding food, or both, it is far less essential to human survival, having for the most part no predators, and obtaining food mostly by agriculture. There is great variation in olfactory sensitivity from person to person, which is common in vestigial characteristics. It has been observed that native South Americans, American Indians, and African peoples have a highly developed sense of smell, such that they may be able to identify others in the dark by their odor alone. ^[2] This does not mean that having any olfactory ability at all is vestigial, for example it may save a person from inhaling toxic fumes; however, a highly developed olfactory system seems to be of little survival value. It should be noted that a characteristic may deteriorate despite being of some use so long as there is very little or no selection pressure on the genes associated with it. In other words, having a good sense of smell may be something a person would desire, but unless those without such abilities have a lower reproductive success or fitness, there is no barrier to it degenerating.

Behavioral

Humans also bear some vestigial behaviors and reflexes. For example, the formation of goose bumps in humans under stress is a vestigial reflex; a possible function in human evolutionary ancestors was to raise the body's hair, making the ancestor appear larger and scaring off predators. Raising the hair is also used to trap an extra layer of air, keeping an animal warm. This reflex formation of goosebumps when cold therefore has a useful function in humans with thick body hair, but the reflex to form them under stress is vestigial.^[14]

Infants are able to support their own weight from a rod, although there is no way they can cling to their mother.^[15] However, an ancestral primate would have had sufficient body hair for an infant to cling to, allowing its mother to escape from danger, such as climbing up a tree in the presence of a predator.

Molecular

There are also vestigial molecular structures in humans, which are no longer in use but may indicate common ancestry with other species. One example of this is L-gulonolactone oxidase, a gene that is functional in most other mammals and produces an enzyme that synthesizes Vitamin C.^[16] In humans and other primates, a mutation disabled the gene and made it unable to produce the enzyme. However, the remains of the gene are still present in the human genome as a vestigial genetic sequence called a pseudogene.^[17]

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