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Vestigiality refers to the existence of organs, tissues, or cells in a body that no longer serve their original function. Vestigial structures, also known as vestigia or rudimentary structures, are fully developed and functional in earlier species but have little to no use for the current organism. They are remnants of evolutionary history, like footprints or tracks derived from the Latin vestigial. Vestigial muscles, such as those in the human ear, are examples of structures that have lost their original function but may have gained a secondary purpose or become functionless.
| Characteristics | Values |
|---|---|
| Definition | Vestigial structures are non-functional features that were once fully developed and functional in earlier species but now serve little to no purpose for an organism. |
| Synonyms | Vestigial structures are also referred to as "vestigia", "rudimentary structures", or simply "remnants". |
| Origin | Vestigial structures arise when a species experiences behavioural or environmental changes that reduce the usefulness of an organ or body part. Over time, the structure may degenerate or take on a new, often minor, function. |
| Examples in Humans | The coccyx or tailbone, the plica semilunaris (a fold of tissue in the corner of the eye), goosebumps, the appendix, wisdom teeth, ear muscles, and the palmaris longus muscle. |
| Examples in Other Species | Vestigial wings in flightless birds, non-functional eyes in cave fishes, pelvic bones in whales, and rear legs in some snakes. |
| Broader Context | Vestigial features are not limited to anatomical structures but can also be physiological or behavioural. |
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What You'll Learn
Vestigial muscles illustrate evolution and adaptation
Vestigiality refers to non-functional features that were once fully developed and served a purpose in earlier species. Vestigial structures are remnants of evolutionary history, like "footprints" or "tracks", with the term translating from the Latin "vestigial". All species possess vestigial features, which can be anatomical, physiological, or behavioural. Vestigial muscles, therefore, illustrate evolution and adaptation.
Vestigial muscles are those that have no apparent function and are considered residual parts from past ancestors. They can arise due to gene mutations causing changes in proteins, which then result in the formation of vestigial structures. Vestigial muscles can also be understood as those that are greatly reduced in size compared to homologous muscles in other species, have become principally tendonous, or are highly variable in their frequency within or between populations.
In humans, the muscles of the ear are considered vestigial. While many mammals use these muscles for sound localisation and the expression of emotion, humans have instead learned to capture sounds by positioning their heads to receive them. This compensates for the loss of function in the auricular muscles. However, a 2015 study found that the vestigial muscles of the ear may still be activated by certain intense or exciting sounds. Similarly, the orbitalis muscle in the human eye, also known as Müller's muscle, forms an important part of the lateral orbital wall in some animals, but in humans, it serves no significant function.
The palmaris longus muscle, which provided wrist flexion and grip strength, is absent in approximately 10-14% of humans. With the development of the thumb and its associated thenar muscles, the palmaris longus lost its function and is now commonly used for tendon grafts. This illustrates how vestigial muscles can be repurposed for other uses, or simply become useless.
In addition, the human coccyx or tailbone is a vestige of a tail. When human ancestors learned to walk upright, the tail was no longer needed for balance and mobility, and it slowly disappeared. The coccyx now serves secondary functions, such as providing an attachment site for muscles and ligaments of the pelvic floor.
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Vestigial muscles can be useless or repurposed
Vestigiality refers to the remnants of evolutionary history, with vestigial structures being non-functional features that were once fully developed and functional in earlier species. Vestigial muscles, therefore, can refer to muscles that are either useless or have been repurposed.
Useless Vestigial Muscles
Some muscles in the human body are considered vestigial because they serve no apparent function. For example, the orbitalis muscle, or "Müller's muscle", is a vestigial or rudimentary nonstriated muscle of the eye that has no known significant function in humans. Similarly, the muscles of the human ear, including the anterior auricular muscle, the superior auricular muscle, and the posterior auricular muscle, are considered nonfunctional. While these muscles play a role in sound localization and the expression of emotion in many mammals, humans have compensated for their loss by positioning the head to receive sounds.
Repurposed Vestigial Muscles
On the other hand, some vestigial muscles have been repurposed and serve secondary functions. For instance, the coccyx or tailbone is a vestige of a tail that served a purpose in maintaining balance and mobility in human ancestors. While the tail itself is now useless, the coccyx serves as a site of attachment for muscles and ligaments of the pelvic floor. Similarly, the plica semilunaris, a small fold of tissue on the inside corner of the eye, is a vestigial remnant of the nictitating membrane or third eyelid found in other animals. However, in humans, it helps maintain tear drainage and flushing of debris, illustrating how a once-functional structure can evolve into a vestigial one with a new purpose.
Variable Vestigial Muscles
The presence and functionality of certain vestigial muscles can vary among individuals and populations. For example, the palmaris longus muscle, which is absent in about 10-14% of humans, originally provided grip strength but now has no impact on grip strength whether it is present or absent. In modern times, this muscle is commonly used for tendon grafting. Additionally, while most humans cannot move their ears, a small percentage of people can wiggle their ears, and others may gain this ability through repeated trials.
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Vestigial muscles can be reduced in size or tendonous
Vestigiality refers to the existence of certain organs, tissues, or cells in a body that no longer serve the function they did in their ancestral form. Vestigial structures are often referred to as "footprints" or "tracks" of evolutionary history. They can arise due to gene mutations, changes in behaviour or environment, or the loss of function due to a lack of positive selection pressures. Vestigial muscles, specifically, can become reduced in size, primarily tendonous, or highly variable in their frequency within or between populations.
The human body has several examples of vestigial muscles that have reduced in size or become tendonous. One such example is the palmaris longus muscle, which is absent in about 10-14% of humans. This muscle was originally used for wrist flexion and providing grip strength, but with the development of the thumb and its associated muscles, it lost its function. Today, it serves no meaningful purpose and is commonly used for tendon grafts.
Another example of a vestigial muscle is the orbitalis muscle, also known as Müller's muscle. This muscle is a vestigial or rudimentary nonstriated muscle of the eye that has no known significant function in humans. However, in some animals, it forms an important part of the lateral orbital wall.
The muscles of the human ear, including the anterior auricular, superior auricular, and posterior auricular muscles, are also considered vestigial. While these muscles play a role in sound localisation and the expression of emotion in many mammals, they are non-functional in humans. Charles Darwin proposed that humans compensate for this loss by positioning their heads to capture sounds.
In addition to these examples, the muscles responsible for goosebumps and the tiny muscle that causes hair to stand upright are also considered vestigial. While these muscles served a purpose in human ancestors, such as making the body appear larger to scare off predators or providing warmth, they have lost their functionality in modern humans.
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Vestigial muscles can be highly variable in frequency
Vestigiality refers to the remnants of evolutionary history, which are also known as "footprints" or "tracks" in Latin. Vestigial structures, or vestigial organs, are non-functional features that were fully developed and functioning in earlier species but now serve little to no purpose for an organism. Vestigial structures can arise due to gene mutations that cause changes in proteins, resulting in the formation of vestigial structures.
Vestigial muscles are a type of vestigial structure that can be highly variable in frequency within and between populations. For example, the occipitalis minor, a muscle in the back of the head, is always present in Malays, present in 56% of Africans, 50% of Japanese, 36% of Europeans, and absent in the Khoikhoi people of southwestern Africa and Melanesians. Other muscles associated with the occipital region and the post-auricular muscle complex also show variability in frequency. The platysma, a quadrangular muscle in a sheet-like configuration, is another example of a vestigial muscle that varies in frequency.
The plantaris muscle is another example of a vestigial muscle that exhibits high variability in morphology and frequency across different populations. Researchers have noted that the insertion of the plantaris muscle can vary significantly, with some individuals lacking the muscle altogether. Despite its small size and limited influence on movement, the plantaris muscle has clinical significance, particularly in injuries involving the painful calf.
The palmaris longus muscle, which runs between the wrist and the elbow, is also considered vestigial by some. This muscle is absent from both arms in about 10% of humans and may have been important for grip and hanging. However, recent studies suggest that the palmaris longus may contribute more to grip strength than previously thought, as high-performance athletes in sports requiring a strong grip were more likely to possess this muscle.
In summary, vestigial muscles can indeed be highly variable in frequency within and between different populations. The occurrence and frequency of vestigial structures may increase if they provide a beneficial advantage, even if they serve no apparent function.
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Vestigial muscles can be sporadic, like the sternalis muscle
Vestigial structures are non-functional features that were fully developed and functioning in earlier species but now serve little to no purpose for an organism. They are remnants of evolutionary history, like "footprints" or "tracks", with the term translating from the Latin "vestigial". All species possess vestigial features, which can be anatomical, physiological, or behavioural.
The sternalis muscle has no known physiological function, but it does hold clinical significance. Its presence can alter electrocardiogram readings and confuse routine mammography, potentially leading to misdiagnoses. However, it can be beneficial in reconstructive surgery of the head, neck, and anterior chest wall, where it can be used as a flap.
The presence of the sternalis muscle is asymptomatic, but it may cause aesthetic concerns due to chest asymmetry or deviation of the nipple-areola complex. Its embryological origin is unknown, and there is disagreement about its innervation. Cadaveric studies have shown that it has a mean prevalence of around 7.8% in the population, with a higher incidence in females.
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Frequently asked questions
Vestigial structures are non-functional features that were once fully developed and functional in earlier species but now serve little to no purpose for an organism.
The muscles of the ear, the orbitalis muscle, and the palmaris longus muscle are all considered vestigial.
Vestigial muscles are remnants of evolutionary history, illustrating the changes and adaptations that have occurred over time. They provide evidence of evolution and can help explain adaptation.
While vestigial muscles may no longer serve their original function, they can sometimes take on new, often minor, secondary functions. For example, the orbitalis muscle forms part of the lateral orbital wall in some animals but has no known function in humans.
In some cases, vestigial structures may be removed, usually for aesthetic purposes or to prevent potential complications. For instance, the rare occurrence of a human infant being born with a vestigial tail typically results in surgical removal.
