7 Vestigial Features Of The Body: In the context of human evolution, human vestigiality involves those traits (such as organs or behaviors) 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. The examples of human vestigiality are numerous, including the anatomical (such as the human tailbone, wisdom teeth, and inside corner of the eye), the behavioral (goosebumps and palmar grasp reflex), and molecular (pseudogenes). Many human characteristics are also vestigial in other primates and related animals.
History Vestigial Features Of The Body
Charles Darwin listed a number of putative human vestigial features, which he termed rudimentary, in The Descent of Man (1871). These included the muscles of the ear; wisdom teeth; the appendix; the tail bone; body hair; and the semilunar fold in the corner of the eye.
Darwin also commented on the sporadic nature of many vestigial features, particularly musculature. Making reference to the work of the anatomist William Turner, Darwin highlighted a number of sporadic muscles which he identified as vestigial remnants of the panniculus carnosus, particularly the sternalis muscle.
In 1893, Robert Wiedersheim published The Structure of Man, 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.”
His list of supposedly vestigial organs included many of the examples on this page as well as others then mistakenly believed to be purely vestigial, such as the pineal gland, the thymus gland, and the pituitary gland. Some of these organs that had lost their obvious, original functions later turned out to have retained functions that had gone unrecognized before the discovery of hormones or many of the functions and tissues of the immune system.
- the role of the pineal in the regulation of the circadian rhythm
- discovery of the role of the thymus in the immune system lay many decades in the future; it remained a mystery organ until after the mid-20th century;
- the pituitary and hypothalamus with their many and varied hormones were far from understood, let alone the complexity of their interrelationships.
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.
The organ’s patent liability to appendicitis and its poorly understood role left the appendix open to blame for a number of possibly unrelated conditions. For example, in 1916, a surgeon claimed that removal of the appendix had cured several cases of trifacial neuralgia and other nerve pain about the head and face, even though he stated that the evidence for appendicitis in those patients was inconclusive.
The discovery of hormones and hormonal principles, notably by Bayliss and Starling, argued against these views, but in the early twentieth century, there remained a great deal of fundamental research to be done on the functions of large parts of the digestive tract. In 1916, an author found it necessary to argue against the idea that the colon had no important function and that “the ultimate disappearance of the appendix is a coordinate action and not necessarily associated with such frequent inflammations as we are witnessing in the human”.
There had been a long history of doubt about such dismissive views. Around 1920, the prominent surgeon Kenelm Hutchinson Digby documented previous observations, going back more than thirty years, that suggested lymphatic tissues, such as the tonsils and appendix, may have substantial immunological functions.
7 Vestigial Features Of The Body
Vestiges are remnants of evolutionary history—“footprints” or “tracks,” as translated from the Latin vestigial. All species possess vestigial features, which range in type from anatomical to physiological to behavioral. More than 100 vestigial anomalies occur in humans.
The following list explores 7 of them:
1. Palmar Grasp Reflex
The palmar grasp reflex is a characteristic behavior of human infants, developing as early as 16 weeks gestational age when the fetus begins to grasp the umbilical cord in the mother’s womb. Early research found that human newborns, relying on their grasp reflex, could hold their own weight for at least 10 seconds when hanging by their hands from a horizontal rod. By comparison, monkey infants, which possess a similar involuntary grasping behavior, were able to hang from one hand for more than half an hour.
The reflex is essential for monkey infants, enabling them to cling to the mother’s body fur. But humans, who evolved out of an arboreal existence and lost the covering of fur over the body, presumably no longer require that powerful grasp. Human infants typically begin to lose the reflex around three months of age. Despite its diminished strength and loss in early infancy, some researchers think that the grasp reflex may retain important functions in humans.
2. Wisdom Teeth
As the human species migrated out of Africa, it came to populate a variety of habitats, and eventually, human civilizations developed. Coincident with those events was a shift in the human diet toward the consumption of soft and processed foods, which gradually eliminated the need for large, powerful jaws.
With a reduction in human jaw size, molars—particularly the third molars, or wisdom teeth—became highly prone to impaction. Increasingly, wisdom teeth are congenitally absent. As a consequence, they are now considered a vestigial feature of the human body.
3. Nictitating Membrane
The plica semilunaris is a fold of conjunctiva at the inner corner of the human eye. Its likeness to the nictitating membrane, or third eyelid, of other animals, led to the idea that it might be the vestige of such a structure, which is still part of the eye in some primates, including gorillas. In the chimpanzee, however—one of the human species’ closest relatives—the plica semilunaris also appears to be vestigial.
The function of the nictitating membrane in many animals is protective—for example, keeping the eye clean and moist or concealing the iris from predators. In some species, the membrane is sufficiently transparent so as to enable vision when underground or underwater. Though the reason for the loss of a nictitating membrane in humans is unclear, changes in habitat and eye physiology may have rendered the tissue unnecessary.
In modern humans, the appendix is sometimes believed to be a vestige of a redundant organ that an ancestral species had digestive functions, much as it still does in extant species in which intestinal flora hydrolyze cellulose and similar indigestible plant materials. This view has changed over the past decades, with research suggesting that the appendix may serve an important purpose. In particular, it may serve as a reservoir for beneficial gut bacteria.
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 intestinal flora. It does not however seem to have much digestive function, if any, and is not present in all herbivores, even those with large caeca. As shown in the accompanying pictures, however, the human appendix typically is about comparable to that of the rabbit’s in size, though the caecum is reduced to a single bulge where the ileum empties into the colon.
Some carnivorous animals may have appendices too, but seldom have more than vestigial caeca. In line with the possibility of vestigial organs developing new functions, some research suggests that the appendix may guard against the loss of symbiotic bacteria that aid in digestion, though that is unlikely to be a novel function, given the presence of vermiform appendices in many herbivores. Intestinal bacterial populations entrenched in the appendix may support quick re-establishment of the flora of the large intestine after an illness, poisoning, or after an antibiotic treatment depletes or otherwise causes harmful changes to the bacterial population of the colon.
In the sixth week of gestation, the human embryo possesses a tail, complete with several vertebrae. In the next couple weeks of development, however, the tail disappears, and over time the vertebrae fuse to form the coccyx, or tailbone, in the adult. Humans and their ape relatives are distinguish from other groups of primates in part by their taillessness, though it is unclear why apes lost their tails.
On rare occasions, a human infant is born with a vestigial tail. In modern medical literature, such tails lack vertebrae and typically are harmless, though some are associate with spina bifida (failure of the vertebrae to completely enclose the spinal cord). Tails in human infants typically are removed through surgery without complication.
The ears of a macaque monkey and most other monkeys have far more developed muscles than those of humans and therefore have the 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 can be possible for others to gain such movement by repeated trials. In such primates, the inability to move the ear is compensate mainly by the ability to turn the head on a horizontal plane, an ability which is not common to most monkeys—a function once provide by one structure is now replace by another.
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.
The formation of goosebumps in humans under stress is a vestigial reflex; a possible function in the distant evolutionary ancestors of humanity was to raise the body’s hair, making the ancestor appear larger and scaring off predators.
Raising the hair is also use to trap an extra layer of air, keeping an animal warm. Due to the diminished amount of hair in humans, the reflex formation of goosebumps when cold is also vestigial.
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