Top 10 Useless Limbs
(and Other Vestigial Organs)
http://www.livescience.com/animals/top10_vestigial_organs.html
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In Charles Darwin's On the Origin of Species (1859) and his next publication, The Descent of Man
(1871), he referred to several "vestiges" in human anatomy that were left over from the course of
evolution. These vestigial organs, Darwin argued, are evidence of evolution and represent a
function that was once necessary for survival, but over time that function became either
diminished or nonexistent.
The presence of an organ in one organism that resembles one found in another has led biologists
to conclude that these two might have shared a common ancestor. Vestigial organs have
demonstrated remarkably how species are related to one another, and has given solid ground for
the idea of common descent to stand on. From common descent, it is predicted that organisms
should retain these vestigial organs as structural remnants of lost functions. It is only because of
macro-evolutionary theory, or evolution that takes place over very long periods of time, that these
vestiges appear.
The term "vestigial organ" is often poorly defined, most commonly because someone has chosen
a poor source to define the term. The Oxford English Dictionary (OED) defines vestigial organs as
organs or structures remaining or surviving in a degenerate, atrophied, or imperfect condition or
form. This is the accepted biological definition used in the theory of evolution.
In the never-ending search for scientific truth, hypotheses are proposed, evidence is found, and
theories are formulated to describe and explain what is being observed in the world around us.
The following are ten observations of vestigial organs whose presence have helped to flesh out
the structure of the family tree that includes every living creature on our planet.
-- Brandon Miller
The Wings on Flightless Birds
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The Cassowary, the sexier, but lesswell-known flightless bird.
In 1798, sixty years before Charles Darwin's
first book was published, a French anatomist,
Étienne Geoffroy St. Hilaire, traveled to Egypt
with Napoleon where he witnessed and wrote
about a flightless bird whose wings appeared
useless for soaring. The bird that Hilaire
described was an ostrich, but he described it
as a "cassowary", a term used back then to
describe various birds of ostrich-like
appearance. Ostriches and cassowaries are
among several birds that have wings that are
vestigial. Besides the cassowary, other
flightless birds with vestigial wings are the
kiwi, and the kakapo (the only known
flightless and nocturnal parrot), among
others. In general, wings of a bird are
considered complex structures that are
specifically adapted for flight and those
belonging to these flightless birds are no
different. They are, anatomically, rudimentary
wings, but they could never give these bulky
birds flight. The wings are not completely
useless, as they are used for balance during
running and in flagging down the honeys
during courtship displays.
Hind Leg Bones in Whales
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Whale skeleton showing pelvis and
thigh bones (see inset).
Biologists believe that for 100 million years
the only vertebrates on Earth were waterdwelling creatures, with no arms or legs. At
some point these "fish" began to develop hips
and legs and eventually were able to walk out
of the water, giving the earth its first land
lovers. Once the land-dwelling creatures
evolved, there were some mammals that
moved back into the water. Biologists
estimate that this happened about 50 million
years ago, and that this mammal was the
ancestor of the modern whale. Despite the
apparent uselessness, evolution left traces of
hind legs behind, and these vestigial limbs
can still be seen in the modern whale. There
are many cases where whales have been
found with rudimentary hind limbs in the wild,
and have been found in baleen whales,
humpback whales, and in many specimens of
sperm whales. Most of these examples are of
whales that had only leg bones, but there
were some that included feet with complete
digits. It was reported recently that whales
and hippos were distantly related.
Erector Pili and Body Hair
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When a rabbit is scared, its hair stands
on end. When a human is scared, he or
she calls the police.
The erector pili are smooth muscle fibers that
give humans "goose bumps." If the erector
pili are activated, the hairs that come out of
the nearby follicles stand up and give an
animal a larger appearance that might scare
off potential enemies and a coat that is
thicker and warmer. Humans, though, don't
have thick furs like their ancestors did, and
our strategy for several thousand years has
been to take the fur off other warm looking
animals to stay warm. It's ironic actually that
an animal, sensing danger is near, would puff
up its coat to look scarier, but the human
hunter would see the puffier coat as a warm
prize, leaving the thinner haired weaker
looking animals alone. Of course, some body
hair is helpful to humans; eye brows can
keep sweat out of the eyes and facial hair
might influence a woman's choice of sexual
partner. All the rest of that hair, though, is
essentially useless.
The Human Tailbone (Coccyx)
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The human tailbone doesn't do much,
but really hurts if you land on it.
These fused vertebrae are the only vestiges
that are left of the tail that other mammals still
use for balance, communication, and in some
primates, as a prehensile limb. As our
ancestors were learning to walk upright, their
tail became useless, and it slowly
disappeared. It has been suggested that the
coccyx helps to anchor minor muscles and
may support pelvic organs. However, there
have been many well documented medical
cases where the tailbone has been surgically
removed with little or no adverse effects.
There have been documented cases of
infants born with tails, an extended version of
the tailbone that is composed of extra
vertebrae. There are no adverse health
effects of such a tail, unless perhaps the child
was born in the Dark Ages. In that case, the
child and the mother, now considered
witches, would've been killed instantly.
The Blind Fish Astyanax Mexicanus
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Astyanax mexicanus: growing
up in the wrong neighborhood.
In an experiment designed by nature, the
species of fish known as Astyanax
mexicanus, dwelling in caves deep
underground off the coast of Mexico, cannot
see. The pale fish has eyes, but as it is
developing in the egg, the eyes begin to
degenerate, and the fish is born with a
collapsed remnant of an eye covered by flap
of skin. These vestigial eyes probably formed
after hundreds or even thousands of years of
living in total darkness. As for the experiment,
a control is needed; and luckily for us, fish of
the same species live right above, near the
surface, where there is plenty of light, and
these fish have fully functioning eyes. To test
if the eyes of the blind mexicanus could
function if given the right environment,
scientists removed the lens from the eye of
the surface-dwelling fish and implanted it into
the eye of the blind fish. It was observed that
within eight days an eye started to develop
beneath the skin, and after two months the
fish had developed a large functioning eye
with a pupil, cornea, and iris. The fish were
blind, but now they see.
Wisdom Teeth in Humans
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They need regular
brushing.
With all of the pain, time, and money that are
put into dealing with wisdom teeth, humans
have become just a little more than tired of
these remnants from their large jawed
ancestors. But regardless of how much they
are despised, the wisdom teeth remain, and
force their way into mouths regardless of the
pain inflicted. There are two possible reasons
why the wisdom teeth have become vestigial.
The first is that the human jaw has become
smaller than its ancestors' and the wisdom
teeth are trying to grow into a jaw that is
much too small. The second reason may
have to do with dental hygiene. A few
thousand years ago, it might be common for
an 18 year old man to have lost several,
probably most, of his teeth, and the incoming
wisdom teeth would prove useful. Now that
humans brush their teeth twice a day, it's
possible to keep one's teeth for a lifetime.
The drawback is that the wisdom teeth still
want to come in, and when they do, they
usually need to be extracted to prevent any
serious pain.
The Sexual Organs of Dandelions
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Send in the clones ...
Dandelions, like all flowers, have the proper
organs (stamen and pistil) necessary for
sexual reproduction, but do not use them.
Dandelions reproduce without fertilization;
they basically clone themselves, and they are
quite successful at it. Look at any lawn for the
proof. If dandelions were to revert to sexual
reproduction, they might not retain whatever
traits they have that allow them to be pests to
gardeners everywhere. If flowers can begin
reproducing in this manner, does that mean
animals, even humans could too? Asexual
reproduction can be a good strategy in an
environment that is constant if a species is
well suited to those conditions. It doesn't take
a scientist to figure out that humans wouldn't
last long if the condition set forth was no
sexual contact with others. Therefore, the
human sexual organs are probably in no
danger of becoming vestigial.
Fake Sex in Virgin Whiptail Lizards
(Vestigial Behavior)
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Feminist lizards take the male
out of the picture.
Only females exist in several species of the
lizards of the genus Cnemidophorus, which
might seem like a problem when it comes time
to propagate the species. The females don't
need the males though, they reproduce by
parthenogenesis, a form of reproduction in
which an unfertilized egg develops into a new
individual. So basically, the females don't need
the males; they just produce clones of
themselves as a form of reproduction. Despite
the fact that it is unnecessary and futile to
attempt copulation with each other, the lizards
still like to try, and occasionally one of the
females will start to "act like a male" by
attempting to copulate with another female.
The lizards evolved from a sexual species and
the behavior to copulate like a male -- to
engage in fake sex -- is a vestigial behavior;
that is, a behavior present in a species, but is
expressed in an imperfect form, which in this
case, is useless.
Male Breast Tissue and Nipples
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Oh yeah, really handy those ...
The subject of male nipples is a
sensitive, and maybe confusing, topic to
many. Those who wish to invalidate
evolutionary theory might pose the
question, "Was man descended from
woman?" The answer, of course, is no.
Both men and women have nipples
because in early stages of fetal
development, an unborn child is
effectively sexless. Nipples are present
in both males and females; it is only in a
later stage of fetal development that
testosterone causes sex differentiation
in a fetus. All mammals, male and
female, have mammary glands. Male
nipples are vestigial; they may perform a
small role in sexual stimulation and a
small number of men have been able to
lactate. However, they are not fully
functional and, because cancer can
grow in male or female breast tissue, the
tissue can be dangerous.
The Human Appendix
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Leonardo da Vinci's
sketch of the intestines
that included the
appendix.
In plant-eating vertebrates, the appendix is
much larger and its main function is to help
digest a largely herbivorous diet. The human
appendix is a small pouch attached to the large
intestine where it joins the small intestine and
does not directly assist digestion. Biologists
believe it is a vestigial organ left behind from a
plant-eating ancestor. Interestingly, it has been
noted by paleontologist Alfred Sherwood Romer
in his text The Vertebrate Body (1949) that the
major importance of the appendix "would appear
to be financial support of the surgical
profession," referring to, of course, the large
number of appendectomies performed annually.
In 2000, in fact, there were nearly 300,000
appendectomies performed in the United States,
and 371 deaths from appendicitis. Any
secondary function that the appendix might
perform certainly is not missed in those who had
it removed before it might have ruptured.
Duke University Medical Center. "Appendix Isn't Useless At
All: It's A Safe House For Good Bacteria." ScienceDaily 8
October 2007. 28 May 2009 <http://www.sciencedaily.com
/releases/2007/10/071008102334.htm>.
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Appendix Isn't Useless At All: It's A Safe House For Good Bacteria
ScienceDaily (Oct. 8, 2007) — Long denigrated as vestigial or useless, the appendix now appears to have a reason to be – as a "safe house" for the beneficial
bacteria living in the human gut.
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Drawing upon a series of observations and experiments, Duke University Medical Center investigators postulate that the beneficial bacteria in the appendix that
aid digestion can ride out a bout of diarrhea that completely evacuates the intestines and emerge afterwards to repopulate the gut.
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"While there is no smoking gun, the abundance of circumstantial evidence makes a strong case for the role of the appendix as a place where the good bacteria
can live safe and undisturbed until they are needed," said William Parker, Ph.D., assistant professor of experimental surgery, who conducted the analysis in
collaboration with R. Randal Bollinger, M.D., Ph.D., Duke professor emeritus in general surgery.
The appendix is a slender two- to four-inch pouch located near the juncture of the large and small intestines. While its exact function in humans has been
debated by physicians, it is known that there is immune system tissue in the appendix.
The gut is populated with different microbes that help the digestive system break down the foods we eat. In return, the gut provides nourishment and safety to the
bacteria. Parker now believes that the immune system cells found in the appendix are there to protect, rather than harm, the good bacteria.
For the past ten years, Parker has been studying the interplay of these bacteria in the bowels, and in the process has documented the existence in the bowel of
what is known as a biofilm. This thin and delicate layer is an amalgamation of microbes, mucous and immune system molecules living together atop of the lining
the intestines.
"Our studies have indicated that the immune system protects and nourishes the colonies of microbes living in the biofilm," Parkers explained. "By protecting these
good microbes, the harmful microbes have no place to locate. We have also shown that biofilms are most pronounced in the appendix and their prevalence
decreases moving away from it."
This new function of the appendix might be envisioned if conditions in the absence of modern health care and sanitation are considered, Parker said.
"Diseases causing severe diarrhea are endemic in countries without modern health and sanitation practices, which often results in the entire contents of the
bowels, including the biofilms, being flushed from the body," Parker said. He added that the appendix's location and position is such that it is expected to be
relatively difficult for anything to enter it as the contents of the bowels are emptied.
"Once the bowel contents have left the body, the good bacteria hidden away in the appendix can emerge and repopulate the lining of the intestine before more
harmful bacteria can take up residence," Parker continued. "In industrialized societies with modern medical care and sanitation practices, the maintenance of a
reserve of beneficial bacteria may not be necessary. This is consistent with the observation that removing the appendix in modern societies has no discernable
negative effects."
Several decades ago, scientists suggested that people in industrialized societies might have such a high rate of appendicitis because of the so-called "hygiene
hypothesis," Parker said. This hypothesis posits that people in "hygienic" societies have higher rates of allergy and perhaps autoimmune disease because they -and hence their immune systems -- have not been as challenged during everyday life by the host of parasites or other disease-causing organisms commonly
found in the environment. So when these immune systems are challenged, they can over-react.
"This over-reactive immune system may lead to the inflammation associated with appendicitis and could lead to the obstruction of the intestines that causes
acute appendicitis," Parker said. "Thus, our modern health care and sanitation practices may account not only for the lack of a need for an appendix in our
society, but also for much of the problems caused by the appendix in our society."
Parker conducted a deductive study because direct examination the appendix's function would be difficult. Other than humans, the only mammals known to have
appendices are rabbits, opossums and wombats, and their appendices are markedly different than the human appendix.
Parker's overall research into the existence and function of biofilms is supported by the National Institutes of Health. Other Duke members of the team were
Andrew Barbas, Errol Bush, and Shu Lin.
This theory appears online in the Journal of Theoretical Biology.
Adapted from materials provided by Duke University Medical Center.
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