   #copyright

Vestigial structure

2007 Schools Wikipedia Selection. Related subjects: General Biology

   The human vermiform appendix is a vestigial structure; it no longer
   retains its original function.
   Enlarge
   The human vermiform appendix is a vestigial structure; it no longer
   retains its original function.

   Vestigial structures are anatomical structures of organisms in a
   species which are considered to have lost much or all of their original
   function through evolution. These structures are typically in a
   degenerate, atrophied, or rudimentary condition or form. Vestigial
   structures are often referred to as vestigial organs, though not all of
   them are actually organs.

   Although the structures most commonly referred to as "vestigial" tend
   to be largely or entirely functionless, a vestigial structure need not
   necessarily be without use or function for the organism. Vestigial
   structures have lost their original main purpose, but they may retain
   lesser functionalities, or develop entirely new ones. Thus, a
   "vestigial wing" need only be useless for flight to be vestigial; it
   may still serve some other purpose than that of a wing.

History

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

   Vestigial structures have been noticed since ancient times, and the
   reason for their existence has long been speculated upon. In the 4th
   century BC, Aristotle was one of the earliest writers to comment, in
   his History of Animals, on the vestigial eyes of moles, calling them
   "stunted in development". However, only in recent centuries have
   anatomical vestiges become a subject of serious study. In 1798, Étienne
   Geoffroy Saint-Hilaire noted on vestigial structures:

   “  Whereas useless in this circumstance, these rudiments... have not
       been eliminated, because Nature never works by rapid jumps, and She
        always leaves vestiges of an organ, even though it is completely
     superfluous, if that organ plays an important role in the other species
                               of the same family.                          „

   His colleague, Jean-Baptiste Lamarck, named a number of vestigial
   structures in his 1809 book Philosophie Zoologique. Lamarck noted "
   Olivier's Spalax, which lives underground like the mole, and is
   apparently exposed to daylight even less than the mole, has altogether
   lost the use of sight: so that it shows nothing more than vestiges of
   this organ."

   Charles Darwin was very familiar with the concept of vestigial
   structures, though the term for them did not yet exist. He listed a
   number of them in The Descent of Man, including 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 noted, in The Origin of
   Species, that a vestigial structure could be useless for its primary
   function, but still retain secondary anatomical roles: "An organ
   serving for two purposes, may become rudimentary or utterly aborted for
   one, even the more important purpose, and remain perfectly efficient
   for the other.... [A]n organ may become rudimentary for its proper
   purpose, and be used for a distinct object."

   In 1893, Robert Wiedersheim published a list of 86 human organs that
   had, in his words, "lost their original physiological significance".
   Theorizing that they were vestiges of evolution, he called them
   "vestigial". Since his time, the function of some of these structures
   has been discovered, while other anatomical vestiges have been
   unearthed, making the list primarily of interest 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". This is
   why the zoologist Newman stated in the Scopes Monkey Trial that "There
   are, according to Wiedersheim, no less than 180 vestigial structures in
   the human body, sufficient to make of a man a veritable walking museum
   of antiquities."

Evidence of evolution

   Vestigial structures are often homologous to structures that are
   functioning normally in other species. Therefore, vestigial structures
   can be considered evidence for evolution, the process by which
   beneficial heritable traits arise in population over an extended period
   of time. The existence of vestigial organs can be attributed to changes
   in the environment and behaviour patterns of the organism 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.

   The vestigial versions of the structure can be compared to the original
   version of the structure in other species in order to determine the
   homology of a vestigial structure. Homologous structures indicate
   common ancestry with those organisms that have a functional version of
   the structure.

Humans

   The muscles connected to the ears of a human do not develop enough to
   have the same mobility allowed to monkeys.
   Enlarge
   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 is still more or less
   the same, the relative usefulness of certain structures on the list is
   a 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 was
   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 most
   scientists and physicians believe that it has little or no function.

   The coccyx, or tailbone, is the remnant of a lost tail. 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, has lost its original function in assisting
   balance and mobility, though it still serves some secondary functions,
   such as being an attachment point for muscles, which explains why it
   has not degraded further. In rare cases it can persist after birth and
   must be surgically removed.

   The plica semilunaris is small fold of tissue on the inside corner of
   the eye. It is the vestigial remnant of the nictitating membrane (the
   "third eyelid") which is present in other animals.

   Wisdom teeth are vestigial third molars that humans' ancestors used to
   help in grinding down plant tissue. It has been proven that the skulls
   of human ancestors had larger jaws with more teeth, which were probably
   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
   humankind's diet changed, a smaller jaw was selected for by evolution,
   but the third molars, or "wisdom teeth", still commonly grow in.
   Goose bumps are an example of a vestigial human reaction to stress.
   Enlarge
   Goose bumps are an example of a vestigial human reaction to stress.

   Humans also bear some vestigial behaviors and reflexes. For example,
   the formation of goose bumps in humans under stress is a vestigial
   reflex; its purpose in humans' evolutionary ancestors was to raise hair
   to make the animal appear bigger and scare off predators. Raising the
   hair is also used to trap an extra layer of air, to keep the animal
   warm. This reflex formation of goosebumps when cold is not vestigial in
   humans, but the reflex to form them under stress is.

   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. This
   inability is compensated mainly by the ability of humans to turn their
   heads on a horizontal plane, an ability which is not common to most
   apes. Therefore, a function once provided by one structure is now
   replaced by another.

   There are also 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.

In other organisms

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

   In whales and other cetaceans, one can find small vestigial leg bones
   deeply buried within the back of the body. These are remnants of their
   land-living ancestors' legs. Many whales also have undeveloped, unused,
   pelvis bones in the anterior part of their torsos.

   Penguins and Dodo birds (now extinct) have hollow bones, a feature
   usually reserved for flying birds. (The weight reduction is crucial to
   staying in the air. Since both birds don't fly, having hollow bones is
   unneeded, and therefore vestigial.) The wings of ostriches, emus, and
   other flightless birds are vestigial; they are remnants of their flying
   ancestors' wings.

   The eyes of certain cavefish and salamanders are vestigial, as they no
   longer allow the organism to see, and are remnants of their ancestors'
   functional eyes.

   Crabs have small tails tucked between their rear legs that are probably
   vestigial, as they are no longer in use. The working version of these
   tails can be found in their close crustacean relative, the lobster.

   Vestigial structures are not only found in animals; plants also are
   known to have vestigial parts. Dandelions and other asexually
   reproducing plants produce unneeded flower petals. These petals were
   once used to attract pollinating insects, but are now no longer needed.

   Grass and other non-flowering angiosperms often have small, undeveloped
   structures which strongly resemble those of flowering plants.

   The eyes of a flounder start off on either side of the flounder's head.
   Shortly after birth, one of the eyes moves around to the other side of
   the fish's head.

   Fruit flies have been known to occasionally grow legs where their
   antennae should be. This suggests that the antennae and legs have a
   similar origin, or that during development appendages are signaled to
   develop into their respective forms. In this case, it is possible the
   signal did not occur correctly.

   Certains species of moths (for example the Gypsy moth) have females
   that although flightless, still carry small wings. These wings have no
   use, and are vestigial to the versions in species whose females can
   fly.

Controversy

   Because vestigial organs are used as supporting evidence for evolution,
   some creationists oppose the validity of the idea. They question
   whether these organs are actually useless, since they believe that God
   gave each organism its organs for a specific reason and use.

   Those who question the existence of vestigial organs usually claim a
   different definition for vestigial, giving a strict interpretation that
   an organ must be utterly useless to qualify. This is a definition often
   used in dictionaries and children's encyclopedias. Biology textbooks
   and scientific encyclopedias usually describe an organ as vestigial if
   it does not serve the same function in the modern animal as the cognate
   organ served in an ancestor, even if the modern organ serves a
   completely different use ( preadaptation).

   Those who consider the true meaning of vestigial to be "completely
   without use" tend to claim that the meaning has been changed over time
   as structures thought to be vestigial were found to have other uses.
   However, documentation indicates that from the theory's beginnings in
   the 19th century, vestigial structures have invariably been understood
   to "sometimes retain their potentiality", becoming either "wholly or in
   part functionless". It was thought that "not infrequently the
   degenerating organ can be turned to account in some other way".

   An example of the dispute is the gas bladder of many fish, which is
   thought to be a vestigial lung, "left over" from the
   occasionally-air-gasping common ancestor of ray-finned fish and land
   vertebrates.

   Retrieved from " http://en.wikipedia.org/wiki/Vestigial_structure"
   This reference article is mainly selected from the English Wikipedia
   with only minor checks and changes (see www.wikipedia.org for details
   of authors and sources) and is available under the GNU Free
   Documentation License. See also our Disclaimer.
