What do appendix do?

What Is the Appendix? A Vestigial Organ, or One With a Purpose?

The ‘Safe House’ Theory of the Appendix

Some scientists now believe the appendix is not useless after all, and may help our guts recover after a gastrointestinal disease strikes.

The appendix contains a particular type of tissue associated with the lymphatic system, which carries the white blood cells needed to fight infections. In recent years, scientists have found that lymphatic tissue encourages the growth of some beneficial gut bacteria, which play an important role in human digestion and immunity.

Studies have also shown that the lining of the gut contains a biofilm, or a thin layer of microbes, mucus, and immune system molecules — and these biofilms appear to be most pronounced in the appendix. (5)

According to the so-called “safe house” theory, the appendix protects a collection of beneficial gut bacteria when certain diseases wipe them out from elsewhere in the GI tract. Once the immune system has rid the body of the infection, the bacteria emerge from the appendix biofilm and recolonize the gut. (6)

A review of the relevant available research, published in October 2016 in the journal Clinical & Experimental Immunology, concluded that the appendix is not a rudimentary organ but an “important part” of the immune system. (7)

Researchers have recently found that numerous animals, including great apes, other primates, opossums, wombats, rabbits, and certain rodents all have structures similar to the appendix. (8) The appendix, it seems, may have independently evolved in different animals at least 32 times over the course of history, suggesting the organ does have an important function. (4)

Potential Health Issues Involving the Appendix

Sometimes, the appendix can become inflamed and infected, resulting in a condition called appendicitis.

Appendicitis is often the result of an abdominal infection that has spread to the tiny organ, or some kind of obstruction that has blocked the small opening of the appendix. Sources of blockage include, among other things:

  • Hard pieces of stool
  • Parasites or intestinal worms
  • Ingested objects, including air gun pellets and pins
  • Abdominal trauma
  • GI tract ulcers
  • Enlarged appendix lymphatic tissue

The infection or obstruction causes the bacteria in the appendix to grow out of control, and the organ can fill with pus and swell. Appendicitis causes intense abdominal pain and other GI symptoms, including vomiting and diarrhea. Removal of the appendix (an appendectomy) is often the necessary course of action, though increasingly, antibiotics may be recommended and used to treat the infection without the need for surgical intervention — depending on the severity of the case and other health factors in the individual patient. If the problem is left untreated, the pressure in the organ will increase until the appendix ruptures, or bursts.

When the appendix bursts, it spreads its content throughout the abdomen, potentially infecting the peritoneum, which is the silk-like membrane that lines the abdominal cavity. A peritoneum infection, called peritonitis, can then lead to sepsis, a complication that’s potentially deadly if not treated aggressively. (9)

Additional reporting by Deborah Shapiro.

The gastrointestinal tract as an organ of immunity

The body is continuously exposed to damage by viruses, bacteria, and parasites; ingested toxins and chemicals, including drugs and food additives; and foreign protein of plant origin. These insults are received by the skin, the respiratory system, and the digestive system, which constitute the interface between the sterile body interior and the environment.

The defense of the body is vested largely in the lymphatic system and its lymphocytes. A substantial part of the gastrointestinal tract is occupied by lymphoid tissue, which can be divided into three sectors. The first is represented by the pharyngeal tonsils, the appendix, and the large aggregates of nodules known as Peyer patches located at intervals throughout the small intestine. The second sector includes the lymphocytes and plasma cells that populate the basement membrane (lamina propria) of the small intestine, the area of loose connective tissue above the supporting tissue of the mucosal lining extending into the villi. The third sector comprises lymphocytes that lie between the epithelial cells in the mucosa. The interaction between these cells of the lymphatic system and the threatening agent is the basis of defense in the gastrointestinal tract.

Lymphocytes are of two types, B and T, according to whether they originate in the bone marrow (B) or in the thymus gland (T), located in the chest. On leaving their tissue of origin, both types end up in the peripheral lymphoid structures. These include the peripheral lymph glands, the spleen, the lymph nodes in the mesentery of the intestine, the Peyer patches, and the spaces between the epithelial cells of the mucosa.

Lymphocytes are immature until they come into contact with antigens. If foreign material is recognized as such by T cells (T lymphocytes), the lymphocytes undergo a process of maturation in which they proliferate and divide into subclasses. The first subclass comprises the “helper” T cells, which are mediators of immune function. The second class consists of “suppressor” T cells, which modulate and control immune responses. The third class comprises the “killer” T cells, which are cytotoxic (i.e., they are able to destroy other cells). Most of the lymphocytes lying between the epithelial cells of the mucosa are killer T cells.

When B cells (B lymphocytes) recognize antigen, they also mature, changing to the form known as plasma cells. These cells elaborate a highly specialized protein material, immunoglobulin (Ig), which constitutes antibodies. There are five varieties of immunoglobulin: IgA, IgM, IgG, IgD, and IgE. B cells and plasma cells are found mainly in the cells in the spaces of the basement membrane. Another group of specialized cells are known as M cells. These are stretched over and around ordinary epithelial cells of the mucosa. The M cells package antigenic material into vesicles and move it through the cell and into the surrounding spaces.

Lymphocytes of the Peyer patches pass through lymph vessels to the nodes in the mesentery and then to the thoracic duct. This is the collecting channel in the abdomen, which passes up through the thorax to drain into the venous system at the junction of the left internal jugular and left subclavian veins. The various ramifications of the abdominal lymphatics all drain into the thoracic duct. From there the lymphocytes are carried back to the intestine as well as being dispersed to other organs. It is these migrated lymphocytes that come to populate the basement membrane and to occupy the spaces between epithelial cells.

Most cells in the mesenteric nodes and the basement membrane are plasma cells that produce immunoglobulin of class IgA, while IgM and, to a lesser extent, IgE are produced by other cells, and IgG is formed by cells in the spleen and peripheral lymph nodes. The IgA of plasma cells is secreted into the lumen of the intestine, where it is known as “secretory IgA” and has a different molecular structure from that of the IgA circulating in the blood. When secreted, it is accompanied by a glycoprotein that is produced by the epithelial cells of the mucosa. This substance, when attached to the IgA molecule, protects it from digestion by protein-splitting enzymes. This IgA complex can adhere to virus and bacteria, interfering with their growth and diminishing their power to invade tissue. It is also capable of rendering toxic substances harmless.

Formed by B cells, IgE coats the surface of mast cells, which are specially adapted to deal with the allergic challenge posed by parasites and worms.

The newborn infant is protected by already-matured immunoglobulin with which the colostrum, the initial secretion of the lactating breast, is richly endowed. As time passes, the gastrointestinal tract of the infant is increasingly exposed to various insults, and the lymphocytes and other cells of the immune system become adapted to deal with these. In this way, the body also develops a tolerance to potentially offending substances. If invasion of tissue occurs despite these various defenses, then a generalized systemic immune reaction is marshaled. Some of the features of this reaction, such as fever and a massive increase in the white blood cells, are the evidence of illness.

William Sircus

Loren G. Martin, professor of physiology at Oklahoma State University, replies:

“For years, the appendix was credited with very little physiological function. We now know, however, that the appendix serves an important role in the fetus and in young adults. Endocrine cells appear in the appendix of the human fetus at around the 11th week of development. These endocrine cells of the fetal appendix have been shown to produce various biogenic amines and peptide hormones, compounds that assist with various biological control (homeostatic) mechanisms. There had been little prior evidence of this or any other role of the appendix in animal research, because the appendix does not exist in domestic mammals.

“Among adult humans, the appendix is now thought to be involved primarily in immune functions. Lymphoid tissue begins to accumulate in the appendix shortly after birth and reaches a peak between the second and third decades of life, decreasing rapidly thereafter and practically disappearing after the age of 60. During the early years of development, however, the appendix has been shown to function as a lymphoid organ, assisting with the maturation of B lymphocytes (one variety of white blood cell) and in the production of the class of antibodies known as immunoglobulin A (IgA) antibodies. Researchers have also shown that the appendix is involved in the production of molecules that help to direct the movement of lymphocytes to various other locations in the body.

“In this context, the function of the appendix appears to be to expose white blood cells to the wide variety of antigens, or foreign substances, present in the gastrointestinal tract. Thus, the appendix probably helps to suppress potentially destructive humoral (blood- and lymph-borne) antibody responses while promoting local immunity. The appendix–like the tiny structures called Peyer’s patches in other areas of the gastrointestinal tract–takes up antigens from the contents of the intestines and reacts to these contents. This local immune system plays a vital role in the physiological immune response and in the control of food, drug, microbial or viral antigens. The connection between these local immune reactions and inflammatory bowel diseases, as well as autoimmune reactions in which the individual’s own tissues are attacked by the immune system, is currently under investigation.

“In the past, the appendix was often routinely removed and discarded during other abdominal surgeries to prevent any possibility of a later attack of appendicitis; the appendix is now spared in case it is needed later for reconstructive surgery if the urinary bladder is removed. In such surgery, a section of the intestine is formed into a replacement bladder, and the appendix is used to re-create a ‘sphincter muscle’ so that the patient remains continent (able to retain urine). In addition, the appendix has been successfully fashioned into a makeshift replacement for a diseased ureter, allowing urine to flow from the kidneys to the bladder. As a result, the appendix, once regarded as a nonfunctional tissue, is now regarded as an important ‘back-up’ that can be used in a variety of reconstructive surgical techniques. It is no longer routinely removed and discarded if it is healthy.

Evolution Of The Human Appendix: A Biological ‘Remnant’ No More

Now, some of those same researchers are back, reporting on the first-ever study of the appendix through the ages. Writing in the Journal of Evolutionary Biology, Duke scientists and collaborators from the University of Arizona and Arizona State University conclude that Charles Darwin was wrong: The appendix is a whole lot more than an evolutionary remnant. Not only does it appear in nature much more frequently than previously acknowledged, but it has been around much longer than anyone had suspected.

“Maybe it’s time to correct the textbooks,” says William Parker, Ph.D., assistant professor of surgical sciences at Duke and the senior author of the study. “Many biology texts today still refer to the appendix as a ‘vestigial organ.'”

Using a modern approach to evolutionary biology called cladistics, which utilizes genetic information in combination with a variety of other data to evaluate biological relationships that emerge over the ages, Parker and colleagues found that the appendix has evolved at least twice, once among Australian marsupials and another time among rats, lemmings and other rodents, selected primates and humans. “We also figure that the appendix has been around for at least 80 million years, much longer than we would estimate if Darwin’s ideas about the appendix were correct.”

Darwin theorized that the appendix in humans and other primates was the evolutionary remains of a larger structure, called a cecum, which was used by now- extinct ancestors for digesting food. The latest study demonstrates two major problems with that idea. First, several living species, including certain lemurs, several rodents and a type of flying squirrel, still have an appendix attached to a large cecum which is used in digestion. Second, Parker says the appendix is actually quite widespread in nature. “For example, when species are divided into groups called ‘families’, we find that more than 70 percent of all primate and rodent groups contain species with an appendix.” Darwin had thought that appendices appeared in only a small handful of animals.

“Darwin simply didn’t have access to the information we have,” explains Parker. “If Darwin had been aware of the species that have an appendix attached to a large cecum, and if he had known about the widespread nature of the appendix, he probably would not have thought of the appendix as a vestige of evolution.”

He also was not aware that appendicitis, or inflammation of the appendix, is not due to a faulty appendix, but rather due to cultural changes associated with industrialized society and improved sanitation. “Those changes left our immune systems with too little work and too much time their hands – a recipe for trouble,” says Parker.

That notion wasn’t proposed until the early 1900’s, and “we didn’t really have a good understanding of that principle until the mid 1980’s,” Parker said. “Even more importantly, Darwin had no way of knowing that the function of the appendix could be rendered obsolete by cultural changes that included widespread use of sewer systems and clean drinking water.”

Parker says now that we understand the normal function of the appendix, a critical question to ask is whether we can do anything to prevent appendicitis. He thinks the answer may lie in devising ways to challenge our immune systems today in much the same manner that they were challenged back in the Stone Age. “If modern medicine could figure out a way to do that, we would see far fewer cases of allergies, autoimmune disease, and appendicitis.”

Colleagues who contributed to the study include lead author Heather Smith, of the Arizona College of Osteopathic Medicine; Rebecca Fisher, of Arizona State University; and Mary Lou Everett, Anitra Thomas and R. Randal Bollinger from the Department of Surgery at Duke.

Your Appendix Might Serve an Important Biological Function After All

One of the first things you learn about evolution in school is that the human body has a number of ‘vestigial’ parts – appendix, wisdom teeth, tailbone – that gradually fell out of use as we adapted to more advanced lifestyles than our primitive ancestors.

But while our wisdom teeth are definitely causing us more pain than good right now, the human appendix could be more than just a ticking time bomb sitting in your abdomen. A new study says it could actually serve an important biological function – and one that humans aren’t ready to give up.

Researchers from Midwestern University traced the appearance, disappearance, and reemergence of the appendix in several mammal lineages over the past 11 million years, to figure out how many times it was cut and brought back due to evolutionary pressures.

They found that the organ has evolved at least 29 times – possibly as many as 41 times – throughout mammalian evolution, and has only been lost a maximum of 12 times.

“This statistically strong evidence that the appearance of the appendix is significantly more probable than its loss suggests a selective value for this structure,” the team reports.

“Thus, we can confidently reject the hypothesis that the appendix is a vestigial structure with little adaptive value or function among mammals.”

If the appendix has been making multiple comebacks in humans and other mammals across millions of years, what exactly is it good for?

Conventional wisdom states that the human appendix is the shrunken remnant of an organ that once played an important role in a remote ancestor of humans millions of years ago.

The reason it still exists – and occasionally has to be removed due to potentially fatal inflammation and rupturing – is that it’s too ‘evolutionarily expensive’ to get rid of altogether. There’s little evolutionary pressure to lose such a significant part of the body.

In other words, the amount of effort it would take for the human species to gradually lose the appendix though thousands of years of evolution is just not worth it, because in the majority of people, it just sits there not hurting anyone.

But what if it’s doing more than just sitting there?

For years now, researchers have been searching for a possible function of the human appendix, and the leading hypothesis is that it’s a haven for ‘good’ intestinal bacteria that help us keep certain infections at bay.

One of the best pieces of evidence we’ve had for this suggestion is a 2012 study, which found that individuals without an appendix were four times more likely to have a recurrence of Clostridium difficile colitis – a bacterial infection that causes diarrhoea, fever, nausea, and abdominal pain.

As Scientific American explains, recurrence in individuals with their appendix intact occurred in 11 percent of cases reported at the Winthrop-University Hospital in New York, while recurrence in individuals without their appendix occurred in 48 percent of cases.

Now the Midwestern University team has taken a different approach to arrive at the same conclusion.

First they gathered data on the presence or absence of the appendix and other gastrointestinal and environmental traits across 533 mammal species over the past 11.244 million years.

Onto each genetic tree for these various lineages, they traced how the appendix evolved through years of evolution, and found that once the organ appeared, it was almost never lost.

“he appendix has evolved independently in several mammal lineages, over 30 separate times, and almost never disappears from a lineage once it has appeared,” the team explains in a press statement.

“This suggests that the appendix likely serves an adaptive purpose.”

Next, the researchers considered various ecological factors – the species’ social behaviours, diet, habitat, and local climate – to figure out what that “adaptive purpose” could be.

They found that species that had retained or regained an appendix had higher average concentrations of lymphoid (immune) tissue in the cecum – a small pouch connected to the junction of the small and large intestines.

This suggests that the appendix could play an important role in a species’ immune system, particularly as lymphatic tissue is known to stimulate the growth of certain types of beneficial gut bacteria.

“While these links between the appendix and cecal factors have been suggested before, this is the first time they have been statistically validated,” the team concludes in their paper.

“The association between appendix presence and lymphoid tissue provides support for the immune hypothesis of appendix evolution.”

The study is far from conclusive, but offers a different perspective on the hypothesis that humans have been keeping the appendix around for its immune support this whole time.

The challenge now is to prove it, which is easier said than done, seeing as most people who have had their appendix removed don’t suffer from any adverse long-term effects.

But it could be that when people get their appendix removed, immune cell-producing tissues in the cecum and elsewhere in the body step up to compensate for the loss.

One thing’s for sure in all of this – while we’re probably not going to regain our tails, it’s too soon to write off the appendix just yet.

The research has been published in Comptes Rendus Palevol.

Appendix

Appendix, formally vermiform appendix, in anatomy, a vestigial hollow tube that is closed at one end and is attached at the other end to the cecum, a pouchlike beginning of the large intestine into which the small intestine empties its contents. It is not clear whether the appendix serves any useful purpose in humans. Suspected functions include housing and cultivating beneficial gut flora that can repopulate the digestive system following an illness that wipes out normal populations of these flora; providing a site for the production of endocrine cells in the fetus that produce molecules important in regulating homeostasis; and serving a possible role in immune function during the first three decades of life by exposing leukocytes (white blood cells) to antigens in the gastrointestinal tract, thereby stimulating antibody production that may help modulate immune reactions in the gut. While the specific functions of the human appendix remain unclear, there is general agreement among scientists that the appendix is gradually disappearing from the human species over evolutionary time. Blockage of the appendix can lead to appendicitis, a painful and potentially dangerous inflammation.

  • appendicitisAn appendix showing inflamed and damaged tissue due to acute appendicitis.Ed Uthman, M.D.
  • The appendix is a hollow tube that is closed at one end and is attached at the other end to the cecum at the beginning of the large intestine.Encyclopædia Britannica, Inc.

The appendix is usually 8 to 10 cm (3 to 4 inches) long and less than 1.3 cm (0.5 inch) wide. The cavity of the appendix is much narrower where it joins the cecum than it is at its closed end. The appendix has muscular walls that are ordinarily capable of expelling into the cecum the mucous secretions of the appendiceal walls or any of the intestinal contents that have worked their way into the structure. If anything blocks the opening of the appendix or prevents it from expelling its contents into the cecum, appendicitis may occur. The most common obstruction in the opening is a fecalith, a hardened piece of fecal matter. Swelling of the lining of the appendiceal walls themselves can also block the opening. When the appendix is prevented from emptying itself, a series of events occurs. Fluids and its own mucous secretions collect in the appendix, leading to edema, swelling, and the distention of the organ. As the distention increases, the blood vessels of the appendix become closed off, which causes the necrosis (death) of appendiceal tissue. Meanwhile, the bacteria normally found in this part of the intestine begin to propagate in the closed-off pocket, worsening the inflammation. The appendix, weakened by necrosis and subject to increasing pressure from within by the distention, may burst, spilling its contents into the abdominal cavity and infecting the membranes that line the cavity and cover the abdominal organs (see peritonitis). Fortunately, peritonitis is usually prevented by the protective mechanisms of the body. The omentum, a sheet of fatty tissue, often wraps itself around the inflamed appendix, and an exudate that normally develops in the areas of inflammation behaves like glue and seals off the appendix from the surrounding peritoneal cavity.

A person experiencing an attack of appendicitis may feel pain all over the abdomen, only in the upper abdomen, or about the navel. This pain is usually not very severe. After one to six hours or more the pain may become localized to the right lower abdomen. Nausea and vomiting may develop sometime after the onset of the pain. Fever is usually present but is seldom high in the early phases of the attack. The patient’s leukocytes (white blood cells) are usually increased from a normal count of 5,000–10,000 in an adult to an abnormal count of 12,000–20,000; this phenomenon can be caused by many other acute inflammatory conditions that occur in the abdomen.

In a person with a normally sited appendix, the pain of appendicitis is situated at a point between the navel and the front edge of the right hipbone. But many people have the appendix lying in an abnormal position and may feel the pain of an appendicitis attack in a different or misleading location, which makes their symptoms difficult to distinguish from the abdominal pain caused by a variety of other diseases. Careful diagnostic examination by a physician can usually determine if acute appendicitis is indeed causing a patient’s abdominal pain. Ultrasound or computed tomography (CT) scanning may also be useful in the diagnosis of appendicitis.

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The basic treatment of appendicitis is the surgical removal of the appendix in a minor operation called an appendectomy. The operation itself requires little more than a half hour under anesthesia and produces relatively little postoperative discomfort. If a diagnosis of acute appendicitis cannot immediately be made with reasonable certainty, it is common to wait and observe the patient’s symptoms for a period from 10 to 24 hours so that a definitive diagnosis can be made. This wait does slightly increase the risk that the appendix will rupture and peritonitis set in, so the patient is kept under careful medical surveillance at this time.

Appendix may have important function, new research suggests

Heather F. Smith, Ph.D., Associate Professor, Midwestern University Arizona College of Osteopathic Medicine, is currently studying the evolution of the appendix across mammals. Dr. Smith’s international research team gathered data on the presence or absence of the appendix and other gastrointestinal and environmental traits for 533 mammal species. They mapped the data onto a phylogeny (genetic tree) to track how the appendix has evolved through mammalian evolution, and to try to determine why some species have an appendix while others don’t.

They discovered that the appendix has evolved independently in several mammal lineages, over 30 separate times, and almost never disappears from a lineage once it has appeared. This suggests that the appendix likely serves an adaptive purpose. Looking at ecological factors, such as diet, climate, how social a species is, and where it lives, they were able to reject several previously proposed hypotheses that have attempted to link the appendix to dietary or environmental factors. Instead, they found that species with an appendix have higher average concentrations of lymphoid (immune) tissue in the cecum. This finding suggests that the appendix may play an important role as a secondary immune organ. Lymphatic tissue can also stimulate growth of some types of beneficial gut bacteria, providing further evidence that the appendix may serve as a “safe house” for helpful gut bacteria.

They also found that animals with certain shaped ceca (tapering or spiral-shaped) were more likely to have an appendix than animals with a round or cylindrical cecum. Therefore, they concluded that the appendix isn’t evolving independently, but as part of a larger “cecoappendicular complex” including both the appendix and cecum.

Researchers collaborating with Dr. Smith on this study are William Parker, Ph.D., Department of Surgery, Duke Medical Center, Durham, North Carolina; Sanet H. Kotzé, Ph.D., Department of Biomedical Sciences, Faculty of Medicine and Health Sciences, University of Stellenbosch, Tygerberg, South Africa; and Michel Laurin, Ph.D., from the Muséum National d’Histoire Naturelle in France. Midwestern University Senior Research Associate Brent Adrian also contributed illustrations for the study.

The appendix may not be useless after all. The worm-shaped structure found near the junction of the small and large intestines evolved 32 times among mammals, according to a new study. The finding adds weight to the idea that the appendix helps protect our beneficial gut bacteria when a serious infection strikes.

Charles Darwin was one of the first scientists to theorize on the function of the appendix, which in his day had been identified only in humans and other great apes. He hypothesized that the distant ancestors of these animals survived on a diet of leaves, and so they required a large cecum, a portion of the gut that houses bacteria that can break down stubborn plant tissue. Later, he speculated, these ancestors shifted to a largely fruit-based diet that was easier to digest. A large cecum was no longer necessary, and it began to shrink; today our cecum is tiny. Darwin thought the appendix, which juts off of the cecum, is one of its former folds that shriveled up as the cecum shrank. Consequently, he thought it carried no function.

But some scientists have challenged the idea that the appendix serves no purpose. It’s been clear for about a century that the structure contains a particular type of tissue belonging to the lymphatic system. This system carries the white blood cells that help fight infections. Within the last decade, research has shown that this lymphatic tissue encourages the growth of some kinds of beneficial gut bacteria. What’s more, careful anatomical study of other mammals has revealed that species as diverse as beavers, koalas, and porcupines also have a structure jutting off of their guts in exactly the same place as our appendix—in other words, the feature is much more common among mammals than once thought.

Now, an international team of researchers that includes Heather F. Smith, an evolutionary biologist at Midwestern University in Glendale, Arizona, and William Parker, a surgeon who studies the immune system at Duke University Medical Center in Durham, North Carolina, says it has the strongest evidence yet that the appendix serves a purpose. In a new study, published online this month in Comptes Rendus Palevol, the researchers compiled information on the diets of 361 living mammals, including 50 species now considered to have an appendix, and plotted the data on a mammalian evolutionary tree. They found that the 50 species are scattered so widely across the tree that the structure must have evolved independently at least 32 times, and perhaps as many as 38 times.

By plotting the dietary information onto the evolutionary tree, the researchers could work out whether the appendix appears when a particular group of mammals changes its diet. In most cases, there was no sign of a dietary shift, suggesting appendix evolution doesn’t necessarily proceed as Darwin thought. He may have correctly identified the origin of the ape appendix, though, which the analysis confirms did appear when our ancestors switched diets.

Randolph Nesse, an evolutionary biologist at the University of Michigan, Ann Arbor, is impressed by the new study. “I salute the authors for creating an extraordinary database,” he says. “The conclusion that the appendix has appeared 32 times is amazing. I do find their argument for the positive correlation of appendix and cecum sizes to be a convincing refutation of Darwin’s hypothesis.”

“I agree with the general assertion that the appendix evolved numerous times in mammals, but I think the exact count is still up for debate,” adds Olaf Bininda-Emonds, an evolutionary biologist at the University of Oldenburg in Germany. There is some uncertainty over whether all 50 species considered to have an appendix really do possess one. When just the clear-cut cases are included, the appendix evolved 18 times, he says.

Even that figure suggests the appendix performs a useful function, and the hunt is now on to identify what that function is. The research team may already have the answer. In 2007, Parker and his colleagues suggested that the appendix has an immunological role, acting as a “safe house” for beneficial gut bacteria. These bacteria help train the immune system and can prevent diseases by outcompeting dangerous pathogenic bacteria—but there are times when the dangerous microbes gain the upper hand and overrun the gut. The researchers reasoned that when this happens, the beneficial bacteria could retreat to the safety of the appendix, which remains unaffected. Once the immune system has beaten the infection, the beneficial bacteria emerge from the appendix to quickly recolonize the gut.

The “safe house” idea makes sense, says Indi Trehan, a pediatrician at the Institute for Public Health at Washington University in St. Louis who recently studied the importance of maintaining gut bacteria when treating people with malnutrition. “The appendix has a unique anatomical location that is out of the way,” he says. “Bacteria can be kept safe there for repopulation as needed.”

The safe house hypothesis is reasonable, Nesse agrees, but he points out that just 50 of the 361 mammalian species included in the analysis have an appendix. “One wonders why such a trait with such a function would not be universal,” he says. That suggests it is possible we still haven’t completely cracked the mystery of the appendix, he says.

The body’s appendix has long been thought of as nothing more than a worthless evolutionary artifact, good for nothing save a potentially lethal case of inflammation.

Now researchers suggest the appendix is a lot more than a useless remnant. Not only was it recently proposed to actually possess a critical function, but scientists now find it appears in nature a lot more often than before thought. And it’s possible some of this organ’s ancient uses could be recruited by physicians to help the human body fight disease more effectively.

In a way, the idea that the appendix is an organ whose time has passed has itself become a concept whose time is over.

“Maybe it’s time to correct the textbooks,” said researcher William Parker, an immunologist at Duke University Medical Center in Durham, N.C. “Many biology texts today still refer to the appendix as a ‘vestigial organ.'”

Slimy sac

The vermiform appendix is a slimy dead-end sac that hangs between the small and large intestines. No less than Charles Darwin first suggested that the appendix was a vestigial organ from an ancestor that ate leaves, theorizing that it was the evolutionary remains of a larger structure, called a cecum, which once was used by now-extinct predecessors for digesting food.

“Everybody likely knows at least one person who had to get their appendix taken out — slightly more than 1 in 20 people do — and they see there are no ill effects, and this suggests that you don’t need it,” Parker said.

However, Parker and his colleagues recently suggested that the appendix still served as a vital safehouse where good bacteria could lie in wait until they were needed to repopulate the gut after a nasty case of diarrhea. Past studies had also found the appendix can help make, direct and train white blood cells.

Now, in the first investigation of the appendix over the ages, Parker explained they discovered that it has been around much longer than anyone had suspected, hinting that it plays a critical function.

“The appendix has been around for at least 80 million years, much longer than we would estimate if Darwin’s ideas about the appendix were correct,” Parker said.

Moreover, the appendix appears in nature much more often than previously acknowledged. It has evolved at least twice, once among Australian marsupials such as the wombat and another time among rats, lemmings, meadow voles, Cape dune mole-rats and other rodents, as well as humans and certain primates.

“When species are divided into groups called ‘families,’ we find that more than 70 percent of all primate and rodent groups contain species with an appendix,” Parker said.

Several living species, including several lemurs, certain rodents and the scaly-tailed flying squirrel, still have an appendix attached to a large cecum, which is used in digestion. Darwin had thought appendices appeared in only a small handful of animals.

“We’re not saying that Darwin’s idea of evolution is wrong — that would be absurd, as we’re using his ideas on evolution to do this work,” Parker told LiveScience. “It’s just that Darwin simply didn’t have the information we have now.”

He added, “If Darwin had been aware of the species that have an appendix attached to a large cecum, and if he had known about the widespread nature of the appendix, he probably would not have thought of the appendix as a vestige of evolution.”

What causes appendicitis?

Darwin was also not aware that appendicitis, or a potentially deadly inflammation of the appendix, is not due to a faulty appendix, but rather to cultural changes associated with industrialized society and improved sanitation, Parker said.

“Those changes left our immune systems with too little work and too much time their hands — a recipe for trouble,” he said. “Darwin had no way of knowing that the function of the appendix could be rendered obsolete by cultural changes that included widespread use of sewer systems and clean drinking water.”

Now that scientists are uncovering the normal function of the appendix, Parker notes a critical question to ask is whether anything can be done to prevent appendicitis. He suggests it might be possible to devise ways to incite our immune systems today in much the same manner that they were challenged back in the Stone Age.

“If modern medicine could figure out a way to do that, we would see far fewer cases of allergies, autoimmune disease, and appendicitis,” Parker said.

The scientists detailed their findings online August 12 in the Journal of Evolutionary Biology.

The appendix is an organ thought to have gone the way of our wisdom teeth and body hair: At one point we all needed them, now people can get by just fine without them.

Scientists, though, have never been certain what the appendix used to do—and if it is still, in fact, useless. On Jan. 9, a team of researchers led by scientists at Midwestern University Arizona College of Osteopathic Medicine published (paywall) a review study proposing an answer: the appendix is a secondary immune function that both catalyzes immune cell responses and floods your gut with beneficial bacteria when they’ve been depleted. And it still plays that role, in a limited fashion, in human body function.

Appendices are not unique to humans. Over the course of eight years or so, Heather Smith, an anatomist and lead author of the paper (who had her own appendix removed at the age of 12), led a team that examined all the available information on mammals with the organ. The total number of species came out to be 533, and included chimps, dogs, and even pikas. These animals are as different as could be: They hail from all parts of the world, eat different diets, and even travel in different types of social groups. Smith was looking for similarities among their various appendices that could explain what kind of purpose the organ could serve in humans.

They thought that maybe, variation among appendices would be related to animals’ lifestyles. But no such luck: “We failed to find a link between appendix presence (or size) and the dietary, ecological, and social characters,” they wrote.

But they did find a common immunity trait. “In animals that have an appendix, there is a higher concentration of lymphoid tissue in the cecum,” says Smith. The cecum is a part of the digestive tract that begins in the large intestine, from which the appendix stems. This lymphoid tissue, she thinks, is full of cells that trigger an immune reaction when the body is under duress.

Additionally, their work supports evidence that the appendix is a reserve of emergency bacteria for when the rest of the gut is wiped clean. Our guts are populated with trillions of microbes (mostly bacteria), that live symbiotically with us. They get a cozy space to live with a constant food supply (both the food we eat and the nutritious mucosal lining coating our intestines), and in return they give us extra nutrients and crowd out infection-causing bacteria. When we take antibiotics or get food poisoning (or anything else that causes the intestines to rapidly evacuate their contents), our guts are completely flushed of all bacteria. “The appendix has a concentration of good gut bacteria that can repopulate the gut,” says Smith. Although they weren’t able to study the specific types of bacteria in each animal, this is a theory that has been described (paywall) in humans before.

Although this work doesn’t catch the appendix in action, the fact that the appendix evolved separately in mammals 30 different times means its existence isn’t random, says Smith. ”We can function okay without it, but the appendix does provide some degree of immunity and beneficial bacteria,” she says.

System: Unknown
Location: Attached to the first part of your large intestine
Physical description: A narrow, muscular, worm-like pouch, usually around nine centimetres long
Function: Unknown

No known function in humans

The appendix has no known function in humans. Evidence suggests that our evolutionary ancestors used their appendixes to digest tough food like tree bark, but we don’t use ours in digestion now. Some scientists believe that the appendix will disappear from the human body.

Rich in infection-fighting lymphoid cells

The appendix is rich in infection-fighting lymphoid cells, suggesting that it might play a role in the immune system. Whether the appendix has a function or not, it can be removed without any ill effects.

Appendicitis

Indigestible food delivered from the small intestine to the large intestine flows into the appendix and is forced out by contraction of the muscular walls of the appendix. A blockage in the opening where the appendix attaches to the large intestine can lead to inflammation of the appendix, known as appendicitis. This can cause acute pain, fever, nausea, vomiting and loss of appetite, but can be cured easily by removing the appendix.

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