[Biology Class Notes] on Vestigial Organs Pdf for Exam

Vestigial organs could be defined as organs or parts of human, plant, and animal bodies that do not have any clear function and are considered to be residual parts from their respective ancestors. Vestigial organs are proof that all living organisms have evolved over time and are also helpful in explaining adaptation. Some of the examples of vestigial organs are wisdom teeth in humans, dewclaws in dogs, wings of a female cockroach, etc. 

The Key Features of Vestigial Structures

• They have no apparent function and are residual parts from an ancestor.

• These structures can become detrimental but in most cases they are harmless.

• These organs take a long time to be phased out and eliminating them may require major alterations which can have negative effects on the body.

The Evolution of Vestigial Organs

These organs are generally homologous to organs that function normally in other species, which is why they can be considered as evidence of evolution. The existence of vestigial traits can be attributed to changes in the environment or behavior patterns of an organism. If a function of a trait is no longer beneficial for the survival of an organism, the chances of its future offspring inheriting the trait’s normal form would be lower. The transition will take place over many generations and the trait may also be eliminated through genetic drift and other random events. 

It should be mentioned that gene mutation which can result in a change in protein structures can also give rise to vestigial organs. A good example is the degraded eyes of blind fish and salamanders. Mutation in genes has increased the number of taste buds in their tongue but have made them blind.

Examples of Vestigial Organs

• The human appendix and coccyx.

• In septic weed (Cassia occidentalis) the androecium is vestigial.

• The teeth of the whale shark. It cannot be used to bite anything and is a filter feeder.

Below we will talk about different types of human, animal, and plant vestigial organs.

Vestigial Organs in the Human Body

• Sinuses: The human face consists of air pockets called sinuses. They are lined by a thin layer of mucosa but they do not have any significant functions. However, an infection can lead to sinusitis.

• Coccyx: Also known as the tailbone, the coccyx forms the last part of the vertebral column. It is the residue of the lost tailbone and is observed during human embryogenesis. When our ancestors started to walk, the tail became useless and started to disappear slowly. It has been suggested that the Coccyx or the tail helps the pelvic organs and minor muscles. However, there have been many cases where the tail has been removed by surgeries and had no adverse effects on the body. 

• Appendix: The appendix is a close-ended finger-like tube connected to the cecum from which it develops in the embryo. It sits in the lower right abdomen and is the storehouse of good bacteria which helps in the digestion of food. In our ancestors, it is believed to assist in the digestion of cellulose. Herbivores such as rabbits have appendixes that are much larger. This is because their diet includes a large amount of cellulose and the appendix helps to digest that. However, if anything blocks the opening of the appendix, then it can lead to a disease called Appendicitis. The symptoms of the disease include high fever, vomiting, headaches, constipation, and bloating. It is advised to remove the appendix by a surgery called an appendectomy. 

• Wisdom Tooth: Wisdom tooth helps in the formation of the third set of molars in our buccal cavity. In their prime ancestors, they helped in chewing raw and rough food, but in the present, they have no function. They can cause pain infection and are generally extracted.  The reason behind this is that the ancestors had larger jaws. The human jaw has become much smaller than its ancestor’s jaw. There are four wisdom teeth in total and a person can get zero to four teeth. The pain caused by the wisdom tooth is because our jaw cannot fit the tooth inside. Therefore, it is recommended to remove the tooth by surgery at a younger age as they get more problematic with time. 

• External Ear: The outer rim of our ears is made of underdeveloped muscles which make us incapable of moving our ears. It is a vestigial organ. 

• Plica Semilunaris: Also known as the third eyelid is observed in the inside corner of human eyes.  It has no role in protecting our eyes from any contaminant. (In animals, it’s known as the nictitating membrane and protects it from foreign particles and also keeps it moist whilst maintaining visibility)

• Tonsils: Tonsils play a role in protecting the body from harmful microorganisms. However, when they become inflamed and infected, they are removed.

• The Palmaris Longus Muscle: About 16% of the human population does not have this muscle. Scientists think that this muscle helped early humans with their grip. Since we’ve begun to walk straight, over generations it has lost its purpose because we did not need to hand one to something for a very long while.

• Body Hair: When we are surprised or tense or under stress, the hair on the body raises a little. This is when our body forms goosebumps. They are also considered vestigial reflexes. In our ancestors, the main purpose of goosebumps was to scare their predators. By the rise of the hair on their body, their body appeared to be larger. In some cases, it also gave warmth to the ancestors. Animals such as cats and dogs also use this to cover their body in winters. 

Vestigial Organs in Animals

• Female Cockroach Wings:  Female cockroaches have rudimentary wings which they cannot use to fly. The wings are small and underdeveloped. 

• Boa Limbs: Boa constrictors have residual pelvic bones and back legs which are not immediately noticeable but can be observed with an X-ray. As snakes are descended from lizards, these are the skeletal remnants of their ancestors. It is believed that the snakes lost their legs when their ancestors modified their way of living and started hunting in burrows. Burrows is now the shelter for many snakes. 

• Dog’s Dew Claw: The toe is found higher up on a dog’s leg and it does serve a function. When a dog runs, its front feet bend in a way that the dewclaws come in contact with the ground. When they turn at high speed or run on slippery surfaces, the dewclaws provide them extra support and help stabilize the joint. Some dogs also use their dewclaws to help them climb trees and hold objects. Since they are useful they should not be removed, how in some cases dew claws also may be infected and might cause a disease. These are removed by veterinarians via surgery. The dewclaws can also be maintained easily as they are just like the dog’s nails. 

• Palmar Grasp Reflex: The palmar grasp reflex is a vestigial behavior that appears as early as 16 weeks in utero and remains until a child reaches six months old. This kind of reflex also lasts in your toes also. 

Vestigial Organs in Plants

• Gynoecium: In Shatavari (Asparagus Racemosus) the gynoecium and the staminode are vestigial organs

• Androecium: In Septic Weed (Cassia occidentalis) the androecium is vestigial and is called a palinode.

Conclusion

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