Vestigial organ

From New World Encyclopedia
The human vermiform appendix is a vestigial structure; it no longer retains its original function.

In evolutionary biology and comparative anatomy, vestigiality is a term that describes homologous biological structures of organisms (such as anatomical structures (vestigial structures), behaviors, and biochemical pathways) that have seemingly lost all or most of their original function in a species through descent with modification. These structures are typically in a degenerate, atrophied, or rudimentary condition, and are often called vestigial organs, despite some of them not being actual organs. Examples include the eyes of some cave dwelling fish, the leg and hip bones found in whales, the teeth that quickly disappear in duck billed platypuses, pollen in dandelions, and various structures of the human body, including the appendix and wisdom teeth.

Although structures deemed "vestigial" are often identified as largely or entirely functionless, in modern conceptions of the term in evolutionary biology, a vestigial structure may still retain lesser functions. This broader definition (versus no function), as well as the use of vestigial organs as one of the main arguments toward evolutionary theory, has led to significant debate between evolutionists and creationists.

History

The blind mole rat (Spalax typhlus) has tiny eyes completely covered by a layer of skin.

Prior to the theory of Darwinian evolution, the reason for the existence of vestigial structures was long speculated upon. In the 4th century B.C.E., Aristotle commented in his History of Animals on the vestigial eyes of moles, which lack pattern recognition due to a layer of skin that completely covers them. Despite this long-held curiosity, it is only in recent centuries that anatomical vestiges have become a subject of serious study.

In 1798, Étienne Geoffroy Saint-Hilaire hypothesized that vestigial structures exist because nature leaves vestiges of an organ as long as it plays an important role in species of the same family. His colleague, Jean-Baptiste Lamarck, also researched vestigial structures, naming a number of them in his 1809 book Philosophie Zoologique. Lamarck speculated that Guillaume-Antoine Olivier's blind mole rat Spalax had lost its eyesight because it rarely came in contact with daylight, thus leaving behind vestiges of the unnecessary organs and passing on these traits to its offspring. Thus, he advocated the once popular concept of inheritance of acquired characters, or that the individual efforts of an organism during its lifetime were the main mechanism driving species to adaptation.

Several decades later, the inheritance of acquired characters concept was largely diminished by Charles Darwin's theory of natural selection (though Darwin was loathe to abandon the concept), which used such vestigial structures as the muscles of the ear, wisdom teeth, the appendix, the tail bone, body hair, and the semilunar fold in the corner of the human eye as evidence for his theory. He also made the important distinction in The Origin of Species (1859), that if a structure had lost its primary function, but still retained secondary anatomical roles, it could still be described as vestigial.

In the late 19th century, Robert Wiedersheim published a list of 86 human organs that, he claimed, had lost their original function. He then labeled them vestigial, theorizing that they were vestiges of evolution. Since the publication of his list, the true function of some of these structures has been discovered, while other anatomical vestiges have been unearthed, resulting in the use of the list as a record of the knowledge of human anatomy at the time. Later, versions of Wiedersheim's list were expanded to as many as 180 human "vestigial organs," which was later referenced in the landmark Scopes Monkey Trial by the zoologist Newman, as evidence of evolution.

Currently, the term vestigial organ is used in the media to refer to a wholly useless organ, while it retains Darwin's functional distinction in evolutionary biology.

Evidence of evolution

Further information: Evidence of evolution

The fact that vestigial structures reveal a similarity in structure and position with organs in presumed ancestors, but lack the function found in the ancestors, can be considered evidence for evolution—specifically, the "theory of descent with modification", or "theory of common descent." That is, vestigial organs support the view that all organisms have descended from common ancestors by a continuous process of branching; in other words, all life evolved from one kind of organism or from a few simple kinds, and each species arose in a single geographic location from another species that preceded it in time. The presence of a vestigial organ suggests a shared ancestry with another organism that has similar, but functional organs.

The "theory of natural selection" offers a proposed mechanism by which vestigial organs can come about. That is, the existence of vestigial organs may be hypothesized as resulting from changes in the environment and behavior patterns of the species in question. As the function of the structure is no longer beneficial for survival, the likelihood that future offspring will inherit the "normal" form of the structure decreases. In some cases, the structure becomes detrimental to the organism (for example the eyes of a mole can become infected). In many cases, while the structure may not be directly harmful, all structures require extra energy to build and maintain, providing some selective pressure for the removal of parts that do not contribute to an organism's fitness. A structure that is not harmful is theorized to take longer to be phased out by natural selection than one that is.

Darwin's theory of descent with modification remains the most widely accepted scientific theory regarding why such organs exist, with Darwin's theory of natural selection the most widely accepted scientific theory regarding how such organs came about. However, technically, vestigial organs only strongly support the theory of descent with modification (the non-causal "pattern" of evolution between ancestral and descendant taxa), not the theory of natural selection (the "process" by which evolution took place to arrive at the pattern).

It is often stated that vestigial versions of a structure can be compared to the original version of the structure in other species in order to determine the homology of a vestigial structure, and that homologous structures indicate common ancestry with those organisms that have a functional version of the structure. However, technically, the presence of homology, as currently defined, does not offer evidence of common descent. That is because homology is commonly defined as any similarity between structures that is attributed to their shared ancestry. Anatomical structures that are similar in different biological taxa (species, genera, etc.) would be termed homologous if they evolved from the same structure in some ancestor. It would be incorrect to state that homology, as presently defined, provides evidence of evolution because it would be circular reasoning, with homology defined as similarity due to shared ancestry.

Note that this definition on homology is a redefinition from the classical understanding of the term, which predates Darwin's theory of evolution, being coined by Richard Owen in the 1840s. Historically, homology was defined as similarity in structure and position, such as the pattern of bones in a bat's wing and those in a porpoise's flipper (Wells, 2000). Similarity in structures between diverse organisms—such as the similar skeletal structures (utilizing same bones) of the forelimbs of humans, bats, whales, birds, dogs, and alligators—does provide evidence of evolution by common descent (theory of descent with modification). There is evidence that new forms develop on the foundation of earlier stages.

Examples

Animals

Letter c in the picture indicates the undeveloped hind legs of a baleen whale.

There are numerous examples of vestigial organs in the animal kingdom. In whales and other cetaceans, one can find small vestigial leg bones deeply buried within the back of the body. The evolutionary explanation is that these bones are the remnants of their land-dwelling ancestor's legs. Many whales also have undeveloped, unused, pelvis bones in the anterior part of their torsos, added remains from their land-dwelling ancestors.

Several flightless birds, including Dodo birds (now extinct) and penguins, have hollow bones, a feature usually reserved for flying birds (since the weight reduction is crucial to staying in the air). Thus, these hollow bones are explained by evolution to be the remnants of a flying ancestor. The wings of emus, and some other flightless birds, like the dodo, are often identified as vestigial as well, much like the hollow bones. However, there is a difference with penguins. While penguin's wings are not used for flight, they are essential for the penguin to be able to navigate underwater (they essentially act as flippers). Thus, penguin wings are often not identified as vestigial, since they have gained an essential function that has not been found in any ancestral state.

The eyes of certain cavefish, like the blind [[fish] Astyanax mexicanus, are also vestigial, presumably as a result of living in total darkness. While the fish are developing in their eggs, their newly formed eyes start to degenerate, until the fish is left with the collapsed remnant of an eye, covered by a flap of skin. These are thought to be the remnant of their ancestors' functional eyes.

The modern-day duck-billed platypus is said to have vestigial teeth, which it quickly loses in early life, resulting in gums that are more suited for scooping crustaceans and worms out of riverbeds. While fossil records of prehistoric platypus ancestors are not extensive, the fossils that have been found show platypus-like creatures with rows of teeth, thus adding to evolutionists' arguments.

Humans
The muscles connected to the ears of a human do not develop enough to have the same mobility allowed to monkeys.

Although the list of human vestigial structures has remained essentially the same over time, the relative usefulness of certain structures on the list is a continuing subject of debate. The following are some of the structures often included in this list.

The vermiform appendix is a vestige of the cecum, an organ that, according to evolutionists, may have been used to digest cellulose by humans' herbivorous ancestors. Analogous organs in other animals similar to humans continue to perform that function, whereas other meat-eating animals may have similarly diminished appendices. The modern functionality of the appendix is still controversial in the field of human physiology, although evidence has been uncovered for certain useful functions of the appendix in both fetal and adult humans, such as producing important compounds in fetuses (biogenic amines and peptide hormones), working as a lymphatic organ in adult with immune function, and a safe haven for useful bacteria (Rowland 2007; Zahid 2004; AP 2007).

The coccyx, or tailbone, is thought to be the remnant of a lost ancestral tail, selected out as humans began to walk increasingly upright. All mammals have a tail at one point in their development; in humans, it is present for a short time during embryonic development. The tailbone, located at the end of the spine, lacks what is considered its original function in assisting balance and mobility, though it still serves some secondary functions, such as being an attachment point for muscles. In rare cases it can persist after birth and must be surgically removed.

The plica semilunaris is the small, pink fold of tissue on the inside corner of the eye. It is claimed to be the vestigial remnant of the nictitating membrane (or the clear "third eyelid"), which is fully present in other animals, and which can be controlled to protect the eye from debris and moisture loss, while still allowing the animal to see. It forms while a human is an embryo, but then declines afterwards until it is only present in the corner of the eye. Whether or not this structure is vestigial is a matter of popular controversy, partially due to the interpretation of the word vestigial to mean useless. This is because the plica semilunaris in humans currently serves the important purpose of cleaning out dust and other particles from the eye, resulting in a common crusted mucus. However, evolutionists claim that it evolved from the nictitating membrane found in other animals, and is thus vestigial.

Wisdom teeth are considered to be vestigial third molars that human ancestors used to help in grinding down plant tissue. It has been postulated 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, it is posited that a smaller jaw was selected by evolution, yet the third molars, or "wisdom teeth," still commonly develop in human mouths, often leading to extractions.

Goose bumps are an example of a vestigial human reaction to stress.

Evolutionists also state that humans bear some vestigial behaviors and reflexes. For example, the formation of goose bumps in humans under stress is considered a vestigial reflex; its purpose in human evolutionary ancestors was believed to raise the body's hair, making the ancestor appear larger and scaring off predators. Raising the hair also could be used to trap an extra layer of air, keeping an animal warm. This reflex formation of goosebumps when cold is not vestigial in humans, but the reflex to form them under stress is considered to be so.

In the case of the ears of a human, it is believed that a function once provided by one structure has been replaced by another. The ears of a Macaque monkey and most other monkeys, have muscles that are more developed than those of humans and therefore have the capability to move their ears to better hear potential threats. In humans, 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.

Delving deeper into vestigiality arguments, there are also thought to be 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-gulono-gamma-lactone oxidase, a gene, found functional in most other mammals, which produces a Vitamin C-catalyzing enzyme. In humans, an earlier mutation may have caused it to become disabled (unable to produce the enzyme), and it now remains in the human genome only as a vestigial genetic sequence.

Plants

Vestigial structures are not only found in animals; plants are also said to have vestigial parts. Dandelions and other asexually reproducing plants retain flowers, and produce pollen, both of which are only necessary for the process of sexual reproduction.

Grass often has small, undeveloped structures that strongly resemble those of flowering plants.

Controversy

Further information: Creationism

The concept of vestigial organs as supporting evidence for evolution is opposed by creationists in that narrower sense of creationism that does not allow descent with modification or evolution by natural selection. A common point of contention between creationists in this sense and evolutionists is the actual definition of vestigial. Creationists often define vestigial organs as having no purpose, whereas evolutionists view vestigial organs as those that have lost their primary function, but are not necessarily functionless. (This contradiction is shown above, in the examples of the appendix and plica semilunaris). Thus, based on these opposing definitions, some creationists believe that there are no true vestigial organs, because some function can be discerned and because God is considered to have had a specific plan and purpose for all the structures of living beings. It is also argued that over one hundred years ago, scientists made claims that certain structures, such as the tonsils, were vestigial, simply because medical science had not advanced to the point where the function of the tonsils could be well understood. Today, the function of the tonsils in disease prevention has been identified, and creationists use this as further evidence in the case against vestigial organs.

Another common argument is that the observed similarity of structures between species essentially proves nothing. Creationists maintain that such structures, as observed, can indicate a repeated, effective model put in place by God in different species. They also point out that some organs, such as the appendix, are present in apes and humans, but not in the lower apes, and then appear again in even lower animals, such as opossums. Such gaps in the presumed chain of evolution (the gap being the lower apes) indicate evidence in their view of logical flaws in the theory of evolution, further showing that similar organs do not provide evidence of evolution, but instead creationism.

At the same time, some branches of creationistism will allow for microevolution (variations in individual species) but not macroevolution (speciation or higher changes, such as the origin of new designs). Thus, the existence of vestigial traits as support of microevolution is not opposed, but their use as an argument in support of macroevolution is opposed.

Religious adherents who accept the reality of descent with modification, but not natural selection as the main causal agent of macroevolutionary change, do not find conflict with the concept of vestigial structures as evidence for evolution (in the sense of descent with modification). Rather, they view newer creations coming on the foundation of earlier creations.

References
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  • Bergman, J., and G. Howe. 1990. Vestigial Organs Are Fully Functional. Kansas City: Creation Research Society Books. ISBN 0940384094.
  • Darwin, C. 1859. On the Origin of Species by Means of Natural Selection. London: John Murray.
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  • Muller, G. B. 2002. "Vestigial organs and structures." Pages 1131-1133 in M. Pagel, eds., Encyclopedia of Evolution. New York: Oxford University Press. ISBN 0195122003.
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  • St. Hilaire, G. 1798. Observations sur l'aile de l'Autruche, par le citoyen Geoffroy. La Decade Egyptienne, Journal Litteraire et D'Economie Politique 1: 46-51.
  • Wells, J. 2000. Icons of Evolution. Washington, D.C.: Regnery Publishing. ISBN 0895262762.
  • Wiedersheim, Robert. 1895. The Structure of Man: An Index to His Past History. London: Macmillan and Co.
  • Zahid, A. 2004. The vermiform appendix: Not a useless organ. J Coll Physicians Surg Pak 14: 256-258.
Basic topics in evolutionary biology (edit)
Processes of evolution: evidence - macroevolution - microevolution - speciation
Mechanisms: natural selection - genetic drift - gene flow - mutation - phenotypic plasticity
Modes: anagenesis - catagenesis - cladogenesis
History: History of evolutionary thought - Charles Darwin - The Origin of Species - modern evolutionary synthesis
Subfields: population genetics - ecological genetics - human evolution - molecular evolution - phylogenetics - systematics

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