Vestigial Structure

Dr J K Sarkar

Darwin’s Tubecle_arrow

Introduction

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" 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.

Examples of human vestigiality are numerous

Anatomical (such as human appendix, tailbone, wisdom teeth,& inner corner of eye) Behavioral (goose bumps & palmar grasp

reflex), sensory (decreased olfaction), and molecular (junk DNA). Many human characteristics are also vestigial

in other primates and related animals.

History

In 1893, Robert Wiedersheim published a book on human anatomy and its relevance to man's evolutionary history.

This book contained a list of 86 human organs that he considered vestigial, or as Wiedersheim himself explained: "Organs having become wholly or in part functionless, some appearing in the Embryo alone, others present during Life constantly or inconstantly. For the greater part Organs which may be rightly termed Vestigial”

History Contd.

Historically there was a trend not only to dismiss the vermiform appendix as being uselessly vestigial, but an anatomical hazard, a liability to dangerous inflammation.

As late as the mid 20th century many reputable authorities conceded it no beneficial function.

This was a view supported, or perhaps inspired, by Darwin himself in the 1874 edition of his book The Descent of Man, and Selection in Relation to Sex.

Appendix In modern humans, the vermiform appendix is a

vestigial organ that in ancestral species had digestive functions.

Some herbivorous animals, such as rabbits, have a terminal vermiform appendix and cecum that apparently bear patches of tissue with immune functions and may also be important in maintaining the composition of gut flora

• Some carnivorous animals may have appendices too, but seldom have more than vestigial caeca

Ileum-Caecum-Colon_of_rabbit As shown in accompanying pictures however, human appendix typically is about comparable to that of rabbit in size, though caecum is reduced to a single bulge where ileum empties into colon

Appendix

• The human vermiform appendix on the vestigial caecum

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. This tail is most prominent in human embryos 31–35 days

old. 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 congenital defect results in a short tail-like structure being present at birth. Twenty-three cases of human babies born with such a structure have been reported in the medical literature since 1884.

Wisdom teeth Wisdom teeth are vestigial 3rd 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 diets changed, smaller jaws were selected by evolution, yet the 3rd molars, or "wisdom teeth," still commonly develop in human mouths.

Currently, wisdom teeth have become useless and even harmful to the extent where surgical procedures are often done to remove them.

Agenesis of wisdom teeth in human populations ranges from zero in Tasmanian Aboriginals to nearly 100% in indigenous Mexicans.

The difference is related to the PAX9 gene (and perhaps other genes).

Vomeronasal organ (VNO) In some animals VNO is part of a 2nd, completely separate sense of

smell, known as the accessory olfactory system. Many studies have been performed to find if there is an actual

presence of a VNO in adult human beings. Trotier et al. estimated that around 92% of their subjects that had no septal surgery had at least one intact VNO. Kjaer and Fisher Hansen, on the other hand, stated that VNO structure disappeared during fetal development as it does for some primates.However, Smith and Bhatnagar (2000) asserted that Kjaer and Fisher Hansen simply missed the structure in older fetuses. Won (2000) found evidence of a VNO in 13 of his 22 cadavers (59.1%) and in 22 of his 78 living patients (28.2%).

Given these findings, some scientists have argued that there is a VNO in adult human beings.However, most investigators have sought to identify the opening of the vomeronasal organ in humans, rather than identify the tubular epithelial structure itself.

Thus it has been argued that such studies, employing macroscopic observational methods, have sometimes missed or even misidentified the vomeronasal organ.

VNO Contd. Among studies that use microanatomical methods, there is

no reported evidence that human beings have active sensory neurons like those in working vomeronasal systems of other animals.

Furthermore, there is no evidence to date that suggests there are nerve and axon connections between any existing sensory receptor cells that may be in the adult human VNO and the brain.

Likewise, there is no evidence for any accessory olfactory bulb in adult human beings, and the key genes involved in VNO function in other mammals have become pseudogenes in human beings.

Therefore, while the presence of a structure in adult human beings is debated, a review of the scientific literature by Tristram Wyatt concluded, "most in the field ... are sceptical about the likelihood of a functional VNO in adult human beings on current evidence."[

Ear The ears of a Macaque monkey and most other monkeys have far

more developed muscles than those of humans, and therefore have capability to move their ears to better hear potential threats.

Humans and other primates such as the orangutan and chimpanzee however have ear muscles that are minimally developed and non-functional, yet still large enough to be identifiable.

A muscle attached to the ear 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.

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.

Ear The outer structure of the ear also shows some vestigial features, such as the node or point on the helix of the ear known as Darwin's tubercle which is found in around 10% of the population, this feature is labelled (arrow) in the accompanying figure.

Darwin's tubercle

Eye: plica semilunaris • The plica semilunaris is a small fold of tissue on

the inside corner of the eye. • It is the vestigial remnant of the nictitating

membrane, an organ that is fully functional in some other species of mammals.

• Its associated muscles are also vestigial. • The plica semilunaris of Africans and Indigenous

Australians are slightly larger than in other peoples.

• Only one species of primate, the Calabar Angwantibo, is known to have a functioning nictitating membrane

Platysma The platysma is vestigial remnant of panniculous carnosus of animals (which allows the horse to flick a fly off its back) & comes from the Greek work "plate"

It is a quadrangular sheet of muscle which originates on the fascia of the pectoralis major muscle and ascends to three main points of insertion.

The most anterior fibers decussate the midline at variable levels below chin and insert into the mentum.

Central fibers insert into body of mandible & the more posterior fibers turn anteriorly & blend closely with fibers of risorius muscle.

The platysma is innervated by cervical branch of facial nerve. The main lower branch exits the parotid & enters deep surface of platysma. The upper twigs of cervical branch & lower twigs of marginal branch intermingle before marginal nerve passes on to supply the depressor anguli oris and risorius.

Head : Occipitalis Minor

• The Occipitalis Minor is a muscle in the back of the head which normally joins to the auricular muscles of the ear.

• This muscle is very sporadic in frequency—always present in Malays, in 56% of Africans, 50% of Japanese, 36% of Europeans, and is nonexistent in the Khoikhoi people of southwestern Africa and in Melanesians.

• Other small muscles in the head associated with the occipital region and the post-auricular muscle complex are often variable in their frequency.

Face

In many non-human mammals the upper lip and sinus area is associated with whiskers or vibrissae which serve a sensory function.

In humans these whiskers do not exist but there are still sporadic cases where elements of the associated vibrissal capsular muscles or Sinus hair muscles can be found.

Based on histological studies of the upper lips of 20 cadavers, Tamatsu et al. found that structures resembling such muscles were present in 35% (7/20) of their specimens

Breasts

• Extra nipples or breasts sometimes appear along the mammary lines of humans, appearing as a remnant to mammalian ancestors who possessed more than two nipples or breasts.

Palmaris Longus • The palmaris longus muscle is seen

as a small tendon between flexor carpi radialis & flexor carpi ulnaris, although it is not always present.

• The muscle is absent in about 14% of population, however this varies greatly with ethnicity.

• One study has shown prevalence of palmaris longus agenesis in 500 Indian patients to be 17.2% (8% bilateral and 9.2% unilateral).

• The palmaris is a popular source of tendon material for grafts and this has prompted studies which have shown the absence of the palmaris does not have any appreciable effect on grip strength

Levator Claviculae

• The levator claviculae muscle in the posterior triangle of the neck is a supernumerary muscle present in only 2–3% of all people but nearly always present in most mammalian species, including gibbons and orangutans.

Pyramidalis The pyramidalis muscle of the abdomen is a small and triangular muscle, anterior to the rectus abdominis, and contained in the rectus sheath.

It is absent in 20% of humans and when absent the lower end of the rectus then becomes proportionately increased in size.

Anatomical studies suggest that the forces generated by the pyramidalis muscles are relatively small

Plantaris Plantaris muscle is composed of

a thin muscle belly & a long thin tendon.

The muscle belly is approx. 2–4 inches long, and is absent in 7–10% of the human population.

It has some weak functionality in moving the knee & ankle but is generally considered redundant and is often used as a source of tendon for grafts.

The long, thin tendon of the plantaris is humorously called "the freshman's nerve," as it is often mistaken for a nerve by first-year medical students.

Sensory Although the sense of smell, or olfaction, is essential for many animals in

avoiding predators, finding food, and other functions, olfaction is greatly decreased in humans as they have for the most part no predators and obtain 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, native North Americans, 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.

This does not mean that having any olfactory ability at all is vestigial, for example it may save a person from inhaling toxic fumes.

A characteristic may degenerate despite being of some use if 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 its degeneration.

Behavioral • Humans also bear some vestigial

behaviors and reflexes. E.g. the formation of goose bumps in humans under stress is a vestigial reflex; a possible function in human evolutionary ancestors was to raise body's hair, making ancestor appear larger and scaring off predators.

• Raising the hair is also used to trap an extra layer of air, keeping an animal warm.

• Due to the diminished amount of hair in humans, the reflex formation of goose bumps when cold is also vestigial.

Goose bump

Palmar grasp reflex

Palmar grasp reflex is supported to be a vestigial behavior in human infants. When placing a finger or object to the palm of an infant, it will securely grasp it. This grasp is found to be rather strong.

Some infants—37% according to a 1932 study—are able to support their own weight from a rod, although there is no way they can cling to their mother.

The grasp is also evident in the feet too. When a baby is sitting down, its prehensile feet assume a curled-in posture, much like what is observed in an adult chimp.

An ancestral primate would have had sufficient body hair to which an infant could cling unlike modern humans, thus allowing its mother to escape from danger, such as climbing up a tree in presence of a predator without having to occupy her hands holding her baby.

Epoophoron_Paroophoron_Gartner’s Cyst

Only parts remaining from the mesonephric system are the epoophoron, paroophoron, and Gartner's cyst

Genital Ducts in Male • Excretory tubules along the caudal pole of the testis, the paragenital tubules, do not join the cords of the rete testis. Their vestiges are collectively known as paradidymis.

• The paramesonephric ducts in male degenerate except for a small portion at their cranial ends, appendix testis.

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.

In humans and other members of the suborder Haplorrhini, 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

ANY

QUESTION?