Darwin and the appendix

monkey with skull

Appendicitis was recognized as a disease relatively late, in 1886, largely because of the classification of the organ as vestigial - of lost function. Over centuries, most medical physicians have looked to disease as a result of functional impairment. Therefore, if an organ has no apparent function, it is less likely to be a cause of disease.

Interesting story. The carotid artery was not identified as a cause of strokes until the 1951, when the Pathologist C. Miller Fisher first described the hemorrhagic plaques in the great vessel of the neck that led to emboli and debris breaking off and occluding vessels in the brain. The reason for this very late finding of the cause of disease was that undertakers used the carotid artery in the neck as a convenient access for embalming. Therefore, in order to preserve access for later embalming, the carotid was not examined during autopsies or dissections of people who had died of stroke. (click to read Conrado Estol's lovely history of this discovery) One of the most frequent surgeries in the United States now is on the carotid artery for the prevention or treatment of stroke.

The moral of the story is that discovery requires a mind open to possibilities beyond the strict generally accepted paradigms of the day.

Darwin and the appendix

1871 Descent F937.1 040

No one has ever been entirely sure of the function of the appendix;(Click to view the exhibit on the function of the appendix)it is attached to the gut, but appears to have no role in digestion. In the Origin of Species, Darwin was quite blunt in his classification of the appendix as "rudimentary." (see picture to right).



The Variable appearance of the Appendix

The appendix is only variably present in animals – hominid apes, such as orangutans, have appendices, but monkeys do not. Rabbits and wombats have appendices, but dogs and cats do not.

golden retriever GI

Sir Frederick Treves, in his position as dissector for the London Zoo, produced a handsome series of watercolor drawings of the GI tracts of various animals and the presence or absence of the appendix. These are housed in the Archives of the Royal College of Surgeons in London, and are reproduced in an exhibit in this museum.(The dissection watercolors of Sir Frederick Treves) The lack of obvious function and the variability of presentation led the Natural Philosophers, Darwin included, to classify the appendix as vestigial, a harmless little evolutionary accident that could be safely ignored.

Theresult of the vestigial classification was that the sudden onset of pain in the right lower quadrant associated with fever, chills, and then peritonitis was thought for centuries to be due to an inflammation of the cecum, and diseases of the cecum were noted with the Greek roottyphlos, or blind, since the cecum represents a “blind pouch” of the colon.(Williams) Pain in the right lower quadrant with evidence of inflammation wastyphlitis, except when there was bloody dysentery as well, in which case the diagnosis wastyphoid.

Arguments againstvestigiality

Several arguments may be made against classifying the appendix as vestigial, however.

The first problem with classifying the human vermiform appendix as vestigial, however, is its inconsistent appearance in other animals, particularly primates. A basic principal of biology is that "ontogeny recapitulates phylogeny." Under this theory, a developing fetus progresses through the development phases of "lower" orders of animals including primates, before progressing on to human characteristics; that the development of animals follows the same general paths, with an embryo developing as a disc of cells, then forming a neural tube, a gi tract, a heart, pharyngeal pouches and gills, then lungs.

Haeckelembryodrawings

In this regard Ernst Haeckel, produced a series of famous embryo drawings in 1892, pictured at right, which superficially demonstrate recapitulation. If one accepts recapitulation, however, it does not follow logically that the appendix would be variably present in primates and in other animals, a vestigial organ, under recapitulation, should be disappearing as one rises up evolutionarily.

Embryology of the Appendix

gut embryology

Embryologically, the appendix has developed as a bud off of the cecum, or cecal diverticulum by the 10th week of gestation. By the 11th or 12th week of gestation the bowel has resettled into the abdomen and the appendix has assumed its position in the right lower quadrant of the abdomen. By the 17th week of gestation the appendix has developed copious numbers of the white blood cell lymphocytic aggregates that will characterize its anatomy ex utero. This finding suggests a developmental role for the appendix in the fetus, perhaps involvement with the fetal immune system. At any rate, it is incredibly unusual to be born without an appendix, and estimates are that fewer than 1 in 100,000 persons are born without an appendix. By contrast, about 1 in 1000 children are born missing some other section of intestine, and 1 in 750 children are born missing one or both kidneys. It would appear that, to put the matter in Darwinian terms, that there is a survival advantage for a fetus to have an appendix.

Survival of the Fittest?

Second, again using Darwinian reasoning, if inflammation of the vermiform appendix leads to rupture, peritonitis and death in a percentage of people, and the most frequent age of developing appendicitis is in youth, before reproduction, by the application of "survival of the fittest" principals, the appendix should have beenexapted from the specias long ago. By this reasoning, therefore, the presence of the appendix must have a greater survival advantage to overcome this pressure.

Therefore the appendix is not vestigial. Darwin was wrong in his classification. We just need to look harder for the function of the appendix!