They are the structures that apparently have no apparent function in a particular organism.
They appear to be a residual part of a past ancestor. For example, as in snakes, they have pelvic bones although they do not have legs because they are descendants of reptiles that have legs.
The human vermiform appendix is another example of vestigial organs and this is the most cited example to support human evolution . These unused structures present in an organism are called vestigial structures.
Before the Darwinian theory of evolution, the reason for the existence of vestigial structures was speculated for a long time. In the 4th century BC, Aristotle explained in his history of animals about the vestigial organs of moles (eyes), which lack pattern recognition.
This is due to a layer of skin that completely envelops them. Despite this deep-rooted curiosity, it is only in the last few centuries that anatomical remains have become a subject of serious study.
In 1798, Étienne Geoffroy Saint-Hilaire hypothesized that vestigial structures exist because nature leaves vestiges of an organ whenever it plays an important role in species of the same family.
Another naturalist, Jean-Baptiste Lamarck, also investigated vestigial structures, naming some of them in his 1809 book, Philosophie Zoologique.
Lamarck speculated that Guillaume-Antoine Olivier’s blind rat Spalax had lost its sight because it rarely came in contact with daylight, leaving behind the vestiges of unnecessary organs and passing these traits on to its offspring.
Therefore, he advocated the once popular concept of the inheritance of acquired traits, or that the individual efforts of an organism during its life were the main mechanism that propels species to adaptation.
Several decades later, the inheritance of acquired traits was greatly diminished by Charles Darwin’s theory of natural selection, which used presumably vestigial structures.
These structures were: the ear muscles, the wisdom teeth, the appendix, the tail bone, the body hair and the semilunar crease in the corner of the human eye as evidence of his theory.
He also made the important distinction in The Origin of Species (1859), that if a structure had lost its primary function, but still retained secondary anatomical functions, it could still be described as vestigial.
The human vermiform appendix was identified by Charles Darwin as a vestigial-based structure that has no known function.
Scientists later discovered that the appendix plays a role in the immune system and identifying the appendix as vestigial has become a point of contention.
Robert Wiedersheim (late 19th century) released a list of 86 human organs that he claimed had lost their original function. Then he classified them as vestiges, theorizing that they were vestiges of evolution.
Later versions of Wiedersheim’s list were expanded to up to 180 human “vestigial organs,” which were later mentioned in the Scopes Monkey Trial study by the zoologist Newman, as evidence of evolution.
Currently, the term “vestigial organ” is used in the media to refer to a completely useless organ, while retaining Darwin’s functional distinction in evolutionary biology.
The “theory of natural selection” offers a proposed mechanism by which vestigial organs can arise.
In many cases, although the structure may not be directly damaging, all structures require additional energy to build and maintain, providing some selectivity.
Also this energy provides pressure for the elimination of parts that do not contribute to the fitness of an organism.
It is theorized that a structure that is not harmful takes longer to be eliminated by natural selection than one that is.
Darwin’s theory of descent with modification remains the most widely accepted scientific theory of why such organs exist, with Darwin’s theory of natural selection the most widely accepted scientific theory of how such organs arose.
However, technically, vestigial organs only strongly support the theory of descent with modification (the non-causal “pattern” of evolution between ancestral and descendant taxa), not the theory of natural selection (the “process” by which the evolution took place to get in the pattern).
It is often claimed that vestigial versions of a structure can be compared to the original version of the structure in other species in order to determine the homology of a rudimentary structure.
In turn, homologous structures indicate common ancestors with those organisms that have a functional version of the structure. However, technically, the presence of homology, as currently defined, does not provide evidence of common descent.
This is because homology is commonly defined as any similarity between structures that is attributed to their shared ancestry.
Anatomical structures that are similar in different biological taxa (species, genera, etc.) would be called homologous if they evolved from the same structure in some ancestor.
Vestigial structures and evolution
Vestigial structures in one species are homologous to other similar functional structures in other species.
Therefore, these structures can be considered evidence for evolution. The existence of vestigial structures can be credited to change in the environment and behavior patterns of the particular organism.
But if the presence of the trait is not more useful for the organism, it is most likely that it will not be inherited in future generations or it will gradually decrease.
But if there is no selection pressure that can decrease the fitness of the organism, it will persist in future generations until it is eliminated by genetic drift or other similar events.
Although in most cases the vestigial structures do not cause direct damage to the organism, but require additional energy for their development and maintenance.
There is also some risk of disease, such as cancer . Therefore, selection pressure forces those parts to be eliminated.
Some vestigial structures persist in organisms due to some limitations. In some cases, the complete removal of the organ would cause a drastic change in the development pattern of the organism that could present seriously negative results.
Vestigial structures in an organism can be compared to original and functional structures in other organisms to determine the homology of that particular structure.
Homologous structures present in organisms indicate that those organisms share a common ancestor.
15 human vestigial organs and functions
Vestigial organs are organs that are now “useless” or “non-functional.” Many biologists now say that there are no rudimentary organs in the human body and that the so-called functions of vestigial organs are not yet known to humans.
Of course, modern research is gradually showing that organs that were once considered vestigial have some role to play in our bodies.
Listed below are 15 remnant human organs and functions that were (or still are) considered vestigial. Recent findings on these organs are also provided.
1. The Appendix
The vermiform appendix is a vestigial organ in our digestive system that was functional in our ancestors and had some digestive functions.
It is present in some mammals such as rodents, primates (superorder: eurachontoglires) and helps in the digestion of cellulose and other indigestible components of plants.
It is also observed that the appendix supplies the good bacteria to the large intestine when needed (when the colon is infected, or the animal is sick and the good bacteria are washed out).
In the case of humans, the appendix was once thought to be useless and was often surgically removed even though the appendix was healthy. However, recent studies show that the appendix has some important functions to play in the human body.
According to some important studies the appendix plays the following important roles:
Function 1: In human fetus, endocrine cells begin to appear in the appendix at week 11. These endocrine cells begin to produce various peptide hormones and biogenic amines and various other compounds that aid in homeostatic mechanisms (biological control mechanisms in the body).
Function 2: In adult humans, the appendix performs immune functions. Lymphoid tissue accumulates in the appendix shortly after birth.
The accumulation of lymphoid tissue peaks between 20 and 30 years of age and then begins to fall rapidly only to disappear completely after reaching 60 years of age.
Function 3: Researchers have also found that in humans during the early years of development, this organ actually functions as a lymphoid organ and helps in the maturation of B lymphocytes.
B lymphocytes are types of white blood cells. During those years of development, it also helps in the production of IgA antibodies (immunoglobulin antibodies).
Researchers have also discovered that the appendix is also involved in the production of various molecules that are responsible for the movement of lymphocytes to various parts of the human body.
Scientists now believe that the appendix actually exposes white blood cells to various foreign substances or antigens that are present in the gastrointestinal tract, thereby suppressing potentially destructive lymph-borne and blood-borne antibody responses.
As of now, the healthy appendix is not removed from the body and is left for use in reconstructive surgery when the urinary bladder is removed from our body surgically.
When the defective urinary bladder is removed, a portion of the intestine is taken to form a replacement bladder, and the appendix is taken to form the sphincter muscle so that the person can retain urine (i.e. stay on the continent).
Also, a healthy appendix can be used to form a new ureter when a diseased ureter has to be surgically replaced. So, for several major surgeries, the appendix remains as a supporting organ.
2. The coccyx
It is also known as the tail bone. The tail is present in all mammals at some point in development. For humans, it is present during the embryonic stage when the embryo is 31-35 days old.
Through evolution, the coccyx has lost its tail function. However, it is of some use. Helps maintain balance while sitting.
Its secondary function is to unite the tendons, ligaments, or others, for the insertion of some muscles in the pelvic joint.
3. Wisdom teeth (wisdom teeth)
These are the third molars that were present in our ancestors. Even now wisdom teeth are seen in most people.
Ancient humans had larger, stronger jaws for chewing on plants and some raw meat, but as humans developed, smaller jaws were naturally chosen over larger jaws because there was no need to chew food as hard. .
Now wisdom teeth are completely useless and in some cases, they must be surgically removed.
4. Outer ear
Orangutans, humans, and other primate species have ears that are externally identifiable but serve no biological function, while animals such as deer, cats, and macaque monkeys have externally identifiable ears that help them hear better and move as well.
The inability to move the ears is compensated for by the head moving in a horizontal plane.
5. Membrana nictitante
Many animals have a third eyelid called the nictitating membrane. Its function is to protect and keep the eyes moist and at the same time aid in vision. In humans, the plica semilunaris replaces the nictitating membrane.
This lunate plica is a small fold of tissue found inside the corner of the eye. All primates except Calabar angwantibo have plica semilunaris. The Calabar angwantibo has a nictitating membrane.
6. The Hymen
The hymen is a layer of tissue that partially or completely covers the vaginal opening. According to some scientists, the hymen present in girls protects the vagina and the reproductive system from infection from the embryonic stage until the moment they mate for the first time.
The hymen has no other vital function in a woman’s body. However, the hymen is present in horses, elephants, chimpanzees, etc; It exists even after they mate and prevents the leakage of semen from the vagina.
7. Pearly corona glandis
These are small growths that protrude from the crest of the glans penis. They are sometimes mistaken for HPV infection. They are also called hyrustoid papillomas or pearly penile papules.
They are the remains of penile spines that are present in the males of other species of animals. The fully functional penis spines aid in sexual stimulation and also aid in faster orgasms.
8. Extra nipples or breasts
Some women (very rare) have extra nipples and / or extra breasts. This is unusual because our mammalian ancestors and current mammalian relatives have more than two breasts and this characteristic sometimes occurs in humans as well.
9. Palmaris Longus Muscle
This muscle runs from the wrist to the elbow. It worked in grip especially for hanging. Muscle is absent in 10% of the human population and in the rest 90% as well, it is useless.
The tonsils are said to be on the first line of defense organs, where they protect the body from harmful microorganisms that are inhaled or ingested.
However, lately these tonsils have become a cause for concern because they become infected and inflamed and in some cases to the point where they need to be surgically removed. They remain as vestigial organs in the human body.
11. Male nipples
Men or women begin their life journey as women in the womb. Nipples are formed in the fetus when the Y chromosome begins its action.
When the Y chromosome produces and releases testosterone and other male hormones, the nipples in the male fetus are fully formed and remain that way throughout life. This vestigial organ (if we call it that) is the result of a developmental or embryological event but not an evolutionary one.
Now the question is, ‘do male nipples have a function?
If it has a function. Nipples aid in sexual arousal when properly stimulated. They also have nerve endings, and the nerve network is actually much more concentrated in men than in women.
Therefore, the sensory response to nipple stimulation in men is more discreet compared to women.
12. The drivers’ Straw
These are smooth muscles (smooth muscles are muscles that act involuntarily) that are present under the skin and these muscles are responsible for the goose bumps that we feel when we feel cold or stressed.
It is a vestigial behavioral reflex. This reflection had a couple of very important functions in our ancestors.
Our ancestors had their bodies covered in hair. In case of predatory threat, the hair used to stand up, to make them look larger than normal and thus protect themselves from predators.
Also, the upright hair helped prevent body heat from escaping and thus helped them stay warm. But now, the hair on the human body is scarce and goose bumps no longer have a real function.
13. The Palmar Grasp Reflection
The Palmar Grasp Reflex is shown by infants. Babies’ ability to grab onto any object is the Palmar Grasp Reflex. It is seen in the hands and feet.
For baby animals, they can cling to their mothers’ body hair and the mother can run away from the predator. However, by not having a hair with a minimum presence in the human body, the main objective of this reflection is lost in humans.
According to the old theory, “Hiccups are remnants of amphibian respiration.” Tadpoles take in air and water through their gills using a motor reflex that is similar to the reflex seen when we hiccup.
The motor pathways of hiccups during fetal development are later transformed into motor pathways that aid breathing through the lungs.
Modern research, on the other hand, has yet to provide a reason for setbacks. However, various theories have been put forward as to what causes hiccups. Those theories can be found here.
15. L-Gulonolactone Oxidase
It is a gene that was responsible for the production of an enzyme that synthesizes vitamin C. It is present in most animals and some mammals as well.
But in animals of the Haplorrhini suborder (which also includes human species), this gene is turned off due to a mutation. Now this gene is not functional and remains as a pseudogene in human DNA.