Vestigiality

Vestigiality refers to the evolutionary retention of genetic structures or functions that have lost all or some of their ancestral function over time. Bodily organs, structures, and functions that no longer serve any purpose are typically said to be vestigial. Scientists believe that such features become vestigial over a long period, as evolution gradually eliminates organisms' dependence on them for survival. Some examples of vestigial human body parts and functions include the appendix, the plica semilunaris, and wisdom teeth. Vestigiality occurs in nonhuman animal species as well. The sightless eyes of cave-dwelling fishes and the apparently useless wings of flightless birds are among the most common examples of nonhuman vestigiality. Many experts point to vestigial organs and functions as proof of evolution. Although this connection is not universally accepted, the concept of vestigiality nonetheless offers some important clues about how evolution works and what causes living things to change over time.

Background

The idea of vestigiality first came to the public's attention through the writings of French naturalists Étienne Geoffroy Saint-Hilaire and Jean-Baptiste Lamarck in the late eighteenth and early nineteenth centuries. While Saint-Hilaire and Lamarck were able to describe what vestigial organs were, they offered no explanation as to how they became vestigial or what their original purpose might have been. Their writings laid the necessary groundwork for English naturalist Charles Darwin to eventually address these questions later in the nineteenth century.

Darwin essentially proposed that vestigiality was a product of common descent, which is the key evolutionary tenet that all organisms started with a common ancestor but gradually diverged and evolved in different ways as select traits proved necessary for survival. When he wrote The Descent of Man in 1871, Darwin included a list of about twelve vestigial organs and structures that he argued were functionally useless because humans did not appear to use them in the way that other animals do. Further, he suggested that the existence of vestigial organs was evidence of evolution, noting how the vestigial organs in one species were similar to functioning organs in other species. Some critics disputed this suggestion, however. Religious proponents of creationism, or the belief that all forms of life were made by a creator deity, argued that all of the body's organs had a purpose even if scientists did not understand what that purpose was. Regardless, Darwin's evolutionary theory and his explanation of vestigiality largely came to be accepted throughout most of the scientific community in the years that followed.

The term vestigial was initially coined by German anatomist Robert Wiedersheim in his landmark 1893 book The Structure of Man: An Index to His Past History. In this work, Wiedersheim listed eighty-six organs he believed were the vestiges of human evolution. Although some of the organs Wiedersheim listed were later proved to be functional, most are still among those classified as vestigial by modern science.

Overview

Humans have a number of prominent vestigial organs, structures, and functions. Perhaps the most well known of these is the appendix. A small, tail-like organ that extends out from the intestinal tract near where the small and large intestines meet, the appendix has no definitively known function. Darwin suggested that the appendix was an evolutionary holdover that may have once helped primates digest leaves. Research has revealed that the appendix is not entirely vestigial, however. It appears the appendix is home to various types of bacteria that aid digestion and absorption in the colon. These same bacteria are also partially responsible for causing appendicitis, a dangerous infection that can be fatal if the appendix ruptures. As a result, people who develop appendicitis usually have to have their appendix surgically removed. Similarly, the tonsils are a pair of vestigial organs found in the throat that can also become inflamed and infected and are often surgically removed.

The plica semilunaris, which is the small flap of skin that covers the outside corner of the eyeball, is another example of a vestigial structure. Like many birds, reptiles, and fish, humans once had a nictitating membrane, or a sort of translucent third eyelid that protected and moistened the eyeball. Humans lost the need for a nictitating membrane at some point in their development, but they kept a small piece of it that came to be known as the plica semilunaris. Some of the surrounding muscles that were once used to control the nictitating membrane also still remain and are themselves considered vestigial.

Some bodily functions, such as the palmar grasp reflex, are also classified as vestigial. When a person puts a finger in the palm of a baby's hand, the baby will instinctively grip the finger. This instinctive grip is known as the palmar grasp reflex and is seen in babies' hands and feet until around six months of age.

Some muscles are also effectively vestigial. Among these are the erector pili, the small muscle fibers that cause goosebumps and force hairs to stand on end. The reaction to pain, pleasure, or other stimuli was originally used to make primitive humans appear larger to predators or add insulation from cold by raising body hair to create an extra layer of warmth. In modern humans, goosebumps are little more than a visual sign of stimulation.

Wisdom teeth are another bodily feature now largely considered vestigial. Although wisdom teeth likely served a function at some point, they eventually became extraneous, as the human jaw grew smaller over time. In modern humans, wisdom teeth usually have to be removed because there is simply no room for them in the average-size mouth.

There is also a portion of the human genome that is thought to be vestigial. Some scientists believe that a significant part of human DNA is actually nothing more than the remnants of past purposes that are no longer being served. Many DNA sequences include traces of fragments known as pseudogenes and transposons. These fragments indicate a defect in the strand that may have been caused by a virus or mutation that occurred at some point during human evolutionary development. Because they do not cause any harm, these fragments remain in human DNA even though they serve no purpose.

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