Vestigial organs are the organs which are now “useless” or “non-functional”. Some biologists say that these vestigial organs are proof of our evolution. It was first mentioned by Charles Darwin in his book – “The Descent of Man (1890)”. However, many biologists now say that there are no vestigial organs in human body and those so-called vestigial organs functions are not yet known to humans. Of course, modern research is gradually proving that the organs that were once thought to be vestigial have some role to play in our body. This article on human vestigial organs and functions lists 15 such organs and functions that were (or are still) considered vestigial. Recent findings on those organs are also provided.
Human Vestigial Organs and Functions: 1-5
1. Appendix – The Vermiform Appendix is a vestigial organ in our digestive system which was functional in our ancestors and had some digestive functions. It is present in some mammals like rodents, primates (superorder: eurachontoglires) and help in digesting cellulose and other such indigestible components of plants.
It is also seen that the appendix supplies the good bacteria to large intestine when needed (when the colon is infected, or the animal is ill and good bacteria is washed out). In humans, appendix was once thought to be useless and was often surgically removed even though the appendix was healthy. Recent studies however show that appendix has some important functions to play in human body. According to Scientific American, appendix plays the following important roles:
Function 1: In human fetus, endocrine cells start appearing in appendix in 11th week. These endocrine cells start producing various peptide hormones and biogenic amines and several other compounds that help in homeostatic mechanisms (biological control mechanisms in human body).
Function 2: In adult humans, appendix performs immune functions. Lymphoid tissue gets accumulated in the appendix within a short time after birth. The accumulation of lymphoid tissue reaches its peak somewhere between 20 and 30 years of age and then it starts falling rapidly only to completely disappear after 60 years of age is attained.
Function 3: Researchers have also found that in humans during early development years, this organ actually works as a lymphoid organ and helps in B lymphocyte maturation. B lymphocytes are types of white blood cells. During those development years, it also helps in production of IgA antibodies (immunoglobulin antibodies). Researchers have also figured out that the appendix is also involved in production of various molecules that are responsible for lymphocytes movement to various parts of human body.
Scientists now believe that appendix actually exposes WBCs (White Blood Cells) to various foreign substances or antigens that are present in gastrointestinal tract, thereby suppressing potentially destructive lymph-borne and blood-borne (together known as humoral) antibody responses.
As of now, healthy appendix is not removed from the body and is left for use in reconstructive surgery when urinary bladder is removed from our body surgically. When the defected urinary bladder is removed, a portion of the intestine is taken to form a replacement bladder and the appendix is taken to form sphincter muscle so that the person can retain urine (that is, remain continent). Also, a healthy appendix can be used to fashion new ureter when a diseased ureter has to be replaced surgically. So, for various important surgeries, appendix remains as a backup organ.
2. The Coccyx – It is also known as tail bone. The tail is present in all mammals at some stage of the development. For humans, it is present during the embryonic stage when the embryo is 31-35 days old. Through evolution, coccyx has lost its function as a tail. However, it is of some use. It helps maintain balance while in a sitting position.
Its secondary function is to attach the tendons, ligaments etc. and for insertion for some muscles in the pelvic joint. Source
3. Wisdom Teeth – These are the third molars which were present in our ancestors. Even now wisdom teeth are seen in most of the people.
Ancient humans had bigger and stronger jaws to chew on plants and somewhat raw meat but as the humans developed, smaller jaws were naturally chosen over bigger jaws because there is no need to chew the food that hard! Now the wisdom teeth are completely useless and in some cases, they must be surgically removed.
4. External Ear – Orangutan, human and other primate species have ears which are externally identifiable but they serve no biological function whereas animals like deer, cats, macaque monkey have externally identifiable ears which help them to hear better and they move too. The inability to move our ears is compensated with the head moving in a horizontal plane.5. Nictitating Membrane – Many animals have a third eyelid called as nictitating membrane. Its function is to protect and keep the eyes moist and at the same time aid in vision. In humans’ plica semilunaris replaces the nictitating membrane.
This plica semilunaris is a small fold of tissue which is present inside corner of the eye. All primates except for Calabar angwantibo have plica semilunaris. Calabar angwantibo has nictitating membrane.
Human Vestigial Organs and Functions: 6-10
6. Hymen – Hymen is a layer of tissue which either partially or fully covers the vaginal orifice. According to some scientists the hymen present in young girls protects the vagina and reproductive system from infection starting from embryonic stage till the time when they first mate.
7. Hirsuties Coronae Glandis – These are small outgrowths which protrude from the ridge of glans of the penis. They are sometimes mistaken with HPV infection. They are also called as hirustoid papillomas or pearly penile papules.
They are remnants of penile spines which are present in the males of other species of animals. The penile spines which are completely functional help in sexual stimulation and it also aids in quicker orgasms.
8. Extra Nipples or Breasts – Some women (very rare) have extra nipples and/or extra breasts. This is unusual occurrence is because our mammal ancestors and the present mammal relatives have more than two breasts and this feature sometimes occur in humans too.
9. Palmaris Longus Muscle – This muscle runs from wrist to elbow. It functioned in grip especially for hanging. The muscle is absent in 10% of the human population and in the rest 90% too, it serves no purpose.
10. Tonsils – Tonsils are said to be in the first line of defense organs where they protect the body from the harmful microorganisms which are either inhaled or ingested.
However, these tonsils have become a cause of concern lately because they get infected and inflamed and in some cases to such an extent that they have to be surgically removed. They remain as vestigial organs in human body.
Human Vestigial Organs and Functions: 11-15
11. Male Nipples – Males or females start their journey of life as females in the womb. The nipples are formed in the fetus by the time the Y chromosome starts its action.
When the Y chromosome produces and releases testosterone and other male hormones, the nipples on male fetus are completely formed and remain so for the entire life. This one vestigial organ (if we call so) is the result of a developmental or embryological event but not evolutionary one. Now the question is, ‘do male nipples have function?’
Yes! They do! They help in sexual arousal when stimulated properly. They too have nerve endings and in reality, the nerve network is far more concentrated in males than females. So, the sensory response for nipple stimulation in males is more discreet compared to women. Source
12. Arrector Pili – These are smooth muscles (smooth muscles are the muscles which act involuntarily) which are present under our skin and these muscles are responsible for the goose bumps we get when we feel cold or stressed. It is a vestigial behavioral reflex. This reflex had a couple of very important functions in our ancestors.
Our ancestors had their bodies covered with hair. In case of a predatory threat, the hair used to stand erect, making them look bigger than usual and thereby warding off predators. Also, erect hair helped to trap body heat from escaping and thereby helped them to stay warm. But now, the hair on human body is scanty and the goose bumps are of no real function now.
13. The Palmar Grasp Reflex – The Palmar Grasp Reflex is shown by infants. The ability of babies to hold onto any object is Palmar Grasp Reflex. It is seen in hands and feet.
For the animal babies, they can hold onto to their mothers’ body hair and the mother can run away from the predator. However, with no to minimal hair present on the human body, this reflex’s main purpose is lost in humans.
14. Hiccups – According to old theory, “Hiccups are remnant of amphibian respiration”. Tadpoles take in air and water through their gills by a motor reflex which is similar to the reflex seen when we get a hiccup. The motor pathways of hiccupping during fetal development later transform to the motor pathways which help in respiration through lungs.
Modern research on the other hand is yet to provide a reason for the hiccups. However, several theories have been put forward as to what causes hiccups. Those theories can be found here.
15. L- Gulonolactone Oxidase – It is a gene which was responsible for production of an enzyme which synthesizes vitamin C. It is present in most of the animals and some mammals too. But in animals of the suborder Haplorrhini (which includes human species too), this gene is disabled because of a mutation. Now this gene is non-functional and remains as a pseudogene in human DNA.