VESTIGIAL ORGANS: CHARACTERISTICS AND EXAMPLES - ANATOMY AND PHYSIOLOGY - 2024

The vestigial organs are remains of structures that once had some function for the ancestor of the species studied but that, today, the organ no longer fulfills any apparent role. Therefore, the importance of these organs for the organism that carries them is marginal or practically nil.

In nature, there are multiple examples of vestigial organs. Among the most notable we have the skeleton of certain species of snakes that still have remains of the pelvis. Interestingly, the same pattern has been observed in whales.

The coccyx. Source: BodyParts3D is made by DBCLS

Vestigial organs are also found in our body. Humans have a series of structures that are no longer useful to us, such as the wisdom teeth, the appendix, the vertebrae of the coccyx, among others.

What are vestigial organs?

The year 1859 was crucial in the development of biological sciences: Charles Darwin publishes his masterpiece The Origin of Species. In his book, Darwin puts forward two main ideas. First, it proposes the mechanism of natural selection as the causal agent of evolution and proposes that species are descendants with modifications of other ancestral species.

There is strong and abundant evidence that supports the Darwinian principles mentioned. We find the evidence in the fossil record, in biogeography, in molecular biology, among others. One of the arguments that supports the idea of ​​"descendants with modifications" is the existence of vestigial organs.

Therefore, the presence of vestigial organs in organisms is important evidence of the evolutionary process. If we ever doubt the veracity of evolution, it will suffice to observe our own vestigial organs (see examples in humans below).

However, vestigial organs had been noted since pre-Darwinian times. Aristotle noticed the paradoxical existence of eyes in animals of underground life, considering them as a delay in development.

Other naturalists made reference to vestigial organs in their manuscripts, such as Étienne Geoffroy Saint-Hilaire.

characteristics

The one common feature of all vestigial structures is their apparent lack of functionality.

We assume that in the past these structures performed an important function, and in the course of evolution the function was lost. Vestigial structures or organs are a kind of "leftover" from the evolutionary process.

Why do vestigial structures exist?

Before the publication of Darwin's theory, naturalists had their own ideas regarding evolutionary changes. One of the most prominent was Jean-Baptiste Lamarck and the inheritance of acquired characters.

For this French zoologist "the frequent and sustained use of any organ strengthens it little by little, giving it a power proportional to the duration of that use, while the constant disuse of such an organ weakens it." However, today we know that it is not the lack of use that promotes the weakening of the structure in question.

Evolutionary processes explain why vestigial structures exist. Due to some environmental, biotic or abiotic change, there is no longer a selective pressure under the organ, and it can disappear or remain.

In the event that the very presence of the organ translates into a disadvantage, the selection will tend to eliminate it: if a mutation arises that eliminates the organ and achieves greater reproductive success than peers who still have the organ. This is how selection works.

If the presence of the organ does not pose any disadvantage to its bearer, it may persist in the course of evolution, becoming a vestigial organ.

Examples

Vestigial structures in humans

There are several examples of vestigial organs from humans, many of them highlighted by Darwin. The human embryo has a tail, which as development proceeds shortens and is lost before birth. The last vertebrae fuse and form the coccyx, a vestigial organ.

The appendix is ​​another iconic example. This structure is previously thought to be related to cellulose digestion - thanks to evidence of the homologous organ in other mammalian species.

Today it is debated whether the appendix is ​​a vestigial organ or not, and some authors argue that it contributes to functions in the immune system.

Molars in vampires

Members of the order Chiroptera are incredible animals from any point of view. These flying mammals have radiated in multiple trophic habits, including insects, fruits, pollen, nectar, other animals, and their blood.

Bats that feed on blood (there are only 3 species, of which one consumes mammalian blood and the remaining two species bird blood) have molars.

From a functional perspective, a blood-sucking mammal (a term used for blood-consuming animals) does not need a food-grinding molar.

The wings in flightless birds

Throughout evolution, birds have modified their upper limbs into highly specialized structures for flight. However, not all the birds we see today move through the air, there are some species with terrestrial habits that move on foot.

Specific examples are the ostrich, the emu, the cassowary, the kiwi and the penguins - and all of these retain their wings, being a clear example of a vestigial structure.

However, the anatomy of flightless birds is not identical to those of flying birds. There is a bone called the keel located in the chest that participates in flight, and in non-flying species it is absent or greatly reduced. Also, the plumage tends to differ and is a bit more abundant.

Pelvis vestiges in whales and snakes

Both whales and snakes are descendants of tetrapod animals that used their four limbs in locomotion. The presence of pelvic vestiges is a "memory" of the evolutionary trajectory of both lineages.

In the course of whale evolution, the absence of hind limbs represented a selective advantage for the group - the body was more aerodynamic and allowed optimal movement in the water.

However, it is not accepted by all authors that these structures are vestigial. For example, for West-Eberhard (2003), the pelvic bones in whales acquired new functions related to the urogenital system of some modern species.

References

1. Audesirk, T., Audesirk, G., & Byers, BE (2003). Biology: Life on Earth. Pearson education.
2. Campbell, NA, & ​​Reece, JB (2007). Biology. Panamerican Medical Ed.
3. Conrad, EC (1983). True vestigial structures in whales and dolphins. Creation / Evolution, 10, 9-11.
4. Dao, AH, & Netsky, MG (1984). Human tails and pseudotails. Human pathology, 15 (5), 449-453.
5. West-Eberhard, MJ (2003). Developmental plasticity and evolution. Oxford University Press.