Vestigial response

Last updated October 17, 2022

A vestigial response or vestigial reflex in a species is a response that has lost its original function. In humans, vestigial responses include ear perking, goose bumps and the hypnic jerk.

In humans

Ear perking

It has been observed that some people have slight protrusions on the outer ear (also known as the auricle). These protrusions tend towards the top of the auricle. This has been tagged and coined Darwin's tubercle of the auricle. This phenomenon agrees with the accepted scientific explanation: the incidence of tubercles of the auricle among humans, are vestigial structures testifying to our evolutionary past. They are a throwback to the pointed ears of many mammals and just one more vestigial trace of human evolutionary history.

The focus on this part of the human anatomy has finally been followed by a much later observation testifying to our evolutionary past. The subsequent observation concerns an automatic ear-perking response seen, for example, in dogs when startled by a sudden noise. This response, though faint, fleeting and hardly discernible in humans nonetheless clearly manifests itself.^[1] This phenomenon is an automatic-response mechanism that activates even before a human becomes consciously aware that a startling, unexpected or unknown sound has been "heard".^[2]

That this vestigial response occurs even before becoming consciously aware of a startling noise would explain why the function of ear-perking had evolved in animals. The mechanism serves to give a split-second advantage to a startled animal – possibly an animal being stalked and hunted. The evolutionary advantage of the ear-perking response could spell the difference between life and death. The perking response serves to gather and focus that much more audible information that is fed into the brain and on its way to being analyzed even before the animal actually becomes aware of the sound. This fraction-of-a-second advantage would explain the evolutionary selection for this response.

Goose bumps

The pilomotor reflex, more commonly known as goose bumps, was originally a reflex that assured the raising of fur for additional insulation against cold. When scared, this response also made the frightened animal seem bigger and a more formidable enemy.^[3]

Hypnic jerk

The sudden startled arm-jerking response sometimes experienced when on the verge of sleeping is known as the hypnic jerk.

The evolutionary explanation for the existence of the hypnic jerk is unclear, but a possibility is that it is a vestigial reflex humans evolved when they usually slept in trees. Experiencing a hypnic jerk prior to falling asleep may have been selected so that the individual would be able to readjust their sleeping position in the tree with a branch-grabbing response to avoid falling,^[4] much as orangutans grasp upper branches of trees while sleeping.

Related Research Articles

Tickling is the act of touching a part of a body in a way that causes involuntary twitching movements or laughter. The word "tickle" (help·info) evolved from the Middle English tikelen, perhaps frequentative of ticken, to touch lightly.

<span class="mw-page-title-main">Vestigiality</span> Evolution keeping organs no longer needed

Vestigiality is the retention, during the process of evolution, of genetically determined structures or attributes that have lost some or all of the ancestral function in a given species. Assessment of the vestigiality must generally rely on comparison with homologous features in related species. The emergence of vestigiality occurs by normal evolutionary processes, typically by loss of function of a feature that is no longer subject to positive selection pressures when it loses its value in a changing environment. The feature may be selected against more urgently when its function becomes definitively harmful, but if the lack of the feature provides no advantage, and its presence provides no disadvantage, the feature may not be phased out by natural selection and persist across species.

A hypnic jerk, hypnagogic jerk, sleep start, sleep twitch, myoclonic jerk, or night start is a brief and sudden involuntary contraction of the muscles of the body which occurs when a person is beginning to fall asleep, often causing the person to jump and awaken suddenly for a moment. Hypnic jerks are one form of involuntary muscle twitches called myoclonus.

<span class="mw-page-title-main">Myoclonus</span> Medical condition

Myoclonus is a brief, involuntary, irregular twitching of a muscle or a group of muscles, different from clonus, which is rhythmic or regular. Myoclonus describes a medical sign and, generally, is not a diagnosis of a disease. These myoclonic twitches, jerks, or seizures are usually caused by sudden muscle contractions or brief lapses of contraction. The most common circumstance under which they occur is while falling asleep. Myoclonic jerks occur in healthy people and are experienced occasionally by everyone. However, when they appear with more persistence and become more widespread they can be a sign of various neurological disorders. Hiccups are a kind of myoclonic jerk specifically affecting the diaphragm. When a spasm is caused by another person it is known as a provoked spasm. Shuddering attacks in babies fall in this category.

Goose bumps, goosebumps or goose-pimples are the bumps on a person's skin at the base of body hairs which may involuntarily develop when a person is tickled, cold or experiencing strong emotions such as fear, euphoria or sexual arousal.

In anatomy, a tubercle is any round nodule, small eminence, or warty outgrowth found on external or internal organs of a plant or an animal.

<span class="mw-page-title-main">Auricle (anatomy)</span> Visible part of the ear that is outside the head

The auricle or auricula is the visible part of the ear that is outside the head. It is also called the pinna, a term that is used more in zoology.

The acoustic reflex is an involuntary muscle contraction that occurs in the middle ear in response to loud sound stimuli or when the person starts to vocalize.

In animals, including humans, the startle response is a largely unconscious defensive response to sudden or threatening stimuli, such as sudden noise or sharp movement, and is associated with negative affect. Usually the onset of the startle response is a startle reflex reaction. The startle reflex is a brainstem reflectory reaction (reflex) that serves to protect vulnerable parts, such as the back of the neck and the eyes (eyeblink) and facilitates escape from sudden stimuli. It is found across many different species, throughout all stages of life. A variety of responses may occur depending on the affected individual's emotional state, body posture, preparation for execution of a motor task, or other activities. The startle response is implicated in the formation of specific phobias.

<span class="mw-page-title-main">Tensor tympani muscle</span>

The tensor tympani is a muscle within the middle ear, located in the bony canal above the bony part of the auditory tube, and connects to the malleus bone. Its role is to dampen loud sounds, such as those produced from chewing, shouting, or thunder. Because its reaction time is not fast enough, the muscle cannot protect against hearing damage caused by sudden loud sounds, like explosions or gunshots.

Prepulse inhibition (PPI) is a neurological phenomenon in which a weaker prestimulus (prepulse) inhibits the reaction of an organism to a subsequent strong reflex-eliciting stimulus (pulse), often using the startle reflex. The stimuli are usually acoustic, but tactile stimuli and light stimuli are also used. When prepulse inhibition is high, the corresponding one-time startle response is reduced.

<span class="mw-page-title-main">Human vestigiality</span> Human traits which lost their original function through evolution

In the context of human evolution, human 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, a vestigial structure 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.

Animal consciousness, or animal awareness, is the quality or state of self-awareness within a non-human animal, or of being aware of an external object or something within itself. In humans, consciousness has been defined as: sentience, awareness, subjectivity, qualia, the ability to experience or to feel, wakefulness, having a sense of selfhood, and the executive control system of the mind. Despite the difficulty in definition, many philosophers believe there is a broadly shared underlying intuition about what consciousness is.

<span class="mw-page-title-main">Posterior auricular muscle</span> Muscle that pulls the ear upward and backward

The posterior auricular muscle is a muscle behind the auricle of the outer ear. It arises from the mastoid part of the temporal bone, and inserts into the lower part of the cranial surface of the auricle of the outer ear. It draws the auricle backwards, usually a very slight effect.

The neural correlates of consciousness (NCC) constitute the minimal set of neuronal events and mechanisms sufficient for a specific conscious percept. Neuroscientists use empirical approaches to discover neural correlates of subjective phenomena; that is, neural changes which necessarily and regularly correlate with a specific experience. The set should be minimal because, under the assumption that the brain is sufficient to give rise to any given conscious experience, the question is which of its components is necessary to produce it.

A hiccup is an involuntary contraction of the diaphragm that may repeat several times per minute. The hiccup is an involuntary action involving a reflex arc. Once triggered, the reflex causes a strong contraction of the diaphragm followed about a quarter of a second later by closure of the vocal cords, which results in the "hic" sound.

The Lazarus sign or Lazarus reflex is a reflex movement in brain-dead or brainstem failure patients, which causes them to briefly raise their arms and drop them crossed on their chests. The phenomenon is named after the Biblical figure Lazarus of Bethany, whom Jesus raised from the dead according to the Gospel of John.

Secondary consciousness is an individual's accessibility to their history and plans. The ability allows its possessors to go beyond the limits of the remembered present of primary consciousness. Primary consciousness can be defined as simple awareness that includes perception and emotion. As such, it is ascribed to most animals. By contrast, secondary consciousness depends on and includes such features as self-reflective awareness, abstract thinking, volition and metacognition. The term was coined by Gerald Edelman.

Saccular acoustic sensitivity is a measurement of the ear's affectability to sound. The saccule's normal function is to keep the body balanced, but it is believed to have some hearing function for special frequencies and tones. Saccular acoustic sensitivity is considered to be simply an extension of the sense of hearing through the use of the saccule.

References

↑ Hackley, Steven A. (2015). "Evidence for a vestigial pinna-orienting system in humans". Psychophysiology. 52 (10): 1263–1270. doi:10.1111/psyp.12501. PMID 26211937.
↑ Hankins, Peter (2005-06-02). "Libet's short delay". Archived from the original on 2014-08-18. Retrieved 2014-12-16.
↑ Darwin, Charles (1872). "The Expression of the Emotions in Man and Animals". London: John Murray. pp. 101–103.
↑ Coolidge & Wynn (2009). The Rise of Homo sapiens: The Evolution of Modern Thinking. Wiley-Blackwell.

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[1] Hackley, Steven A. (2015). "Evidence for a vestigial pinna-orienting system in humans". Psychophysiology. 52 (10): 1263–1270. doi:10.1111/psyp.12501. PMID 26211937.

[Hankins2005-2] Hankins, Peter (2005-06-02). "Libet's short delay". Archived from the original on 2014-08-18. Retrieved 2014-12-16.

[Darwin1872-3] Darwin, Charles (1872). "The Expression of the Emotions in Man and Animals". London: John Murray. pp. 101–103.

[4] Coolidge & Wynn (2009). The Rise of Homo sapiens: The Evolution of Modern Thinking. Wiley-Blackwell.

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