Vestigiality of the Human Appendix

Abstract

This review sought to analyze and discuss the vestigiality of the appendix in humans as a result of evolution. By definition, the term vestigial-organ means those organs which have lost their original functions. Evolutionists consider them to be leftover remnants from an evolutionary ancestor. Several biological structures are considered to be vestiges based on our current knowledge of comparative anatomy and phylogeny. Examples are the coccyx, appendix, auricular muscles of the ear, etc. Evolutionists believe that these organs had major roles and functions in ancestral species but became useless as the species evolved. In the process they decreased in size and were eventually lost. Just like humans have the appendix, herbivores and vegetation eating animals have the caecum. The caecum serves the purpose of hosting a large number of bacteria that aid in enzymatic digestion of plant matter like cellulose.

Appendix is a small blind ended tubular/vermiform organ fixed to the colon close to the point where the ileum also joins the colon (ileocecal junction). Since the appendix  does not play a role in digestion, it is commonly assumed to be a vestigial organ - a remnant of our plant-eating ancestors.  More than it being considered useless, the appendix is viewed as an organ that as no direct role in the absorption and digestion of food.

Appendix plays a crucial role in the digestive system by determining which families of positive bacteria are allowed to live in the colon and which ones are not. Large intestine relies on these harmless fermenting bacteria which are of the Bifidobacteria and Lactobaccillus genera. These bacteria are responsible for digesting cellulose in order to produce short chain fatty-acids, propionate, and butyrate, among others, that are required by the body.  The appendix is located strategically at the start of the colon so that it could replenish these harmless bacteria when their numbers dwindle. Whether the masses of lymphoid tissue play a role in the immune system is also discussed as this usage can have the potential to salvage the appendix from the bucket of useless organs.

By looking at the anatomical structure and size of the appendix, evolutionary theories that regard the  human appendix as a vestigial organ are reviewed while drawing the analogy of the human appendix to the caecum in herbivores and comparing both their functions. From the reviewed material, the contention of whether the appendix is a vestige or not is discussed in detail.

Introduction

Coccyx and human vermiform appendix are the two most notable vestigial organs in modern man but the appendix discussed more by evolutionists since some consider it a vestigial organ while others refute it. Evolutionary vestiges refer to organs with diminished structure, size, associated tissues, and reduced usage, which previously had a greater physiological significance in the ancestor. A vestigial organ can also be defined typologically as a diminished and degenerate organ which when compared to a similar organ in other organisms lacks the original functions that are found in the latter.  Charles Darwin classified the appendix as a vestigial organ in 1871 saying that it was a remnant of our leaf eating ancestor. The now degenerate human appendix held an important role in our primate ancestors by aiding in the digestion of cellulose leaves, barks, and fruits. However, in modern men the appendix plays a different role by providing a safe house for harmless, digestion aiding bacteria and lymphoid tissue  (Radford-Smit, 2008).

[pic 1]

Figure 1: Cross section of an appendix

It is said that the human appendix has lost its major and essential role of cellulose digestion (Rebecca, 2000). Through primate evolution, the appendix has reduced in size to a mere vermiform and is retained as a structure that was earlier developed for housing bacteria and extending the period of digestion. Based on this observation it can be considered that the human vermiform appendix is vestigial, regardless of whether or not it has functions related to the immune system.

From a non-evolutionary, typological point of view, the human appendix can be directly compared to the cellulose fermenting caeca of herbivorous mammals. Though humans still eat cellulose, the contribution of the appendix to cellulose digestion is negligible or almost nonexistent. Hence it can be said that regardless of evolution theory, the human appendix is a smaller, useless form of the caecum (Douglas, 2007) and therefore the vermiform appendix in human beings remains a valid example of a human vestige just like the coccyx.

                            [pic 2]

                      Figure 2: Position of the appendix relative to colon is highlighted

[pic 3]

Figure 3: the size of appendix relative to the colon is highlighted

The term vermiform is derived from Latin and means ‘worm-like’. The name reflects the narrow, elongated shape of the intestinal appendage.  In adults it is an average of four inches long. It is longer during childhood and reduces in length gradually as a person grows older. Structure wise the appendix is made up of all layers that are typical of the intestine (mucosa, submucosa, mascularis, and serosa) but it is more compacted and contains many masses of lymphoid tissue. Similar to any other lymphatic node, the lymphatic tissue in the appendix is prone to inflammations (Samaha, Tawfik, Abbas, & Abdelhamid, 2011). The internal diameter of the appendix is a millimeter or two. This diameter reduces with age, and it may close completely by middle age, but not in all the people  (Laurin, Everett, & Parker, 2011). Vermiform appendix is found in many evolved apes including humans, chimpanzees, gorillas, orangutans, and gibbons. A study examining the caeca of two different groups of cynomolgus and rhesus monkeys, and two orangutans and a chimpanzee showed that the length and size of the caecum relative to that of the colon decreased as the position of the species on the primate scale rose towards the level of man (Scott, 1980). The reason partly being that the more evolved species of the primates adopted omnivorous eating habits.

Caecum in herbivorous mammals and birds

The appendix that human beings ended up retaining was once the caecum that helped our ancestors digest leaves, barks, fruits and roots  (Iqbal, Rabnawaz, Iqbal, Tahir, & Zaman, 2011). The term caecum means blind in Latin and it is named as such because the end of the caecum is a blind conical pouch. In most herbivores the caecum is a large and complex organ, full of mucosal lymphatic tissue, and specialized for digestion of plants  (Yapp, 1955).The caecum varies in size and complexity among species. The size of the caecum in a particular organism is proportional to the percentage of plant intake in the organism's diet. However, correlation between caecal size and it’s positioning is weak in most species except among the closely related ones. Caecum is largest in strict herbivores and it could be as large as the intestines. For Eucalyptus eating animals like the koala bear it is coiled in order to accommodate it’s large length. The important fact is that in herbivorous mammals, caecum's most important work is digestion of cellulose, which is a common molecule in all parts of a plant. The caecum accomplishes its work by housing specialized, symbiotic bacteria and fungi which secrete cellulase, an enzyme that digests cellulose. Without cellulase it would be impossible for mammals to digest cellulose by any other enzyme which is produced via the means of the stomach or ileum.