Recently, I read one of the funniest articles I’ve ever read in my life about vestigial organs and why they don’t exist according to a creationist. It was funny, not because of the author’s intention, but because of the blatant ignorance that kept repeating itself over and over again in its paragraphs.
But, what are vestigeal organs, and why do they show evidence of evolution? Does the vestigeality of an organ mean that it cannot have been “roped in” to serve another function?
For centuries we have had questions about strange things on our bodies:
- Why do we have appendices?
- Why do we have wisdom teeth?
- What is that tiny pink spot in the inside corner of our eye?
- Why are there muscles in our ears that don’t move?
- Why do we get goosebumps when we’re cold?
- Why do babies grab anything that their tiny hands touch?
- Why do men have nipples?
Let’s tackle each of these questions one by one and see if we can come up with answers, remembering that vestigeality in evolutionary biology doesn’t always mean that that particular organ has no function, but that its function may have changed from an original use in an animal with a homologous organ (an organ which has the same embryonic or historical orgin).
1. The Vermiform Appendix
The appendix is a small tube which is connected to the caecum (a part of the colon / large intestine), from which it develops embryologically. It is located near the junction of the small the large intestines. It is widely present in the Euarchontoglires (rodents, lagomorphs, treeshrews, colugos and primates) and is analogous to a variety of organs in other families. It is highly diverse in size and shape.
The human appendix has been proposed to be a vestigial structure, a structure that has lost all or most of its original function through the process of evolution. The vermiform appendix is proposed to be the shrunken remainder of the caecum that was found in a remote ancestor of humans. Caeca, which are found in the digestive tracts of many extant herbivores, house “good” bacteria which help animals digest the cellulose molecules that are found in plants. As the human appendix does not house a significant number of these bacteria, humans are not capable of digesting more than a minimal amount of cellulose per day.
So as we can see here, the appendix is a vestigial feature which may have a useful but not essentiel function. We do know that 12.5% of people get appendicitis and that 17 in 1000 people die when their appendix bursts before surgery. Of course, before the advent of surgery, every single one of those 1000 people would have died. That’s 12.5% of the population. So, what was the evolutionary advantage of keeping the appendix?
A possible function of the human vermiform appendix as a “safe house” for beneficial bacteria in the recovery from diarrhoea. Every human in the world has had diarrhoea at some point in their lives and this may well have been the evolutionary advantage that we are looking for.
Loren G. Martin, a professor of physiology at Oklahoma State University, asserts that the vermiform appendix really does have a function in foetuses and adults. Endocrine cells have been found in the appendix of 11-week-old fetuses that contribute to “biological control mechanisms”, also known as “homeostatic systems”. In adults, Martin states that the vermiform appendix acts as a lymphatic organ. The appendix has been confirmed to contain a rich reservoir of infection-fighting lymphoid cells, suggesting that it might play a role in the immune system. Zahid suggests that it plays a role in both manufacturing hormones in foetal development as well as functioning to “train” the immune system, exposing the body to antigens so that it can produce antibodies. He notes that doctors in the last decade have stopped removing the appendix during other surgical procedures as a routine precaution, because it can be successfully transplanted into the urinary tract to rebuild a sphincter muscle and reconstruct a functional bladder.
Although it was long accepted that the immune tissue, called gut associated lymphoid tissue, surrounding the appendix and elsewhere in the gut carries out a number of important functions, explanations were lacking for the distinctive shape of the appendix and its apparent lack of importance as judged by an absence of side effects following appendectomy. William Parker, Randy Bollinger, and colleagues at Duke University proposed that the appendix serves as a haven for useful bacteria when illness flushes those bacteria from the rest of the intestines. This proposal is based on a new understanding of how the immune system supports the growth of beneficial intestinal bacteria, in combination with many well-known features of the appendix, including its architecture, its location just below the normal one-way flow of food and germs in the large intestine, and its association with copious amounts of immune tissue. Research performed at Winthrop University-Hospital showed that individuals without an appendix were four times more likely to have a recurrence of Clostridium difficile. However, other research showed that there is a significantly greater rate of C. difficile infection among people with an appendix, with more than 80% of the infections occurring among patients with an intact appendix.
The vermiform appendix may be the only vestigeal feature that may still have a function in our list of 7 features. What wbout the others?
2. Wisdom Teeth
Wisdom teeth are vestigial third molars that usually appear between the ages of 16 and 25. They used to help human ancestors in grinding down plant tissue, however, they serve no known purpose in modern humans, except to cost the NHS £30 million a year. Most adults have four wisdom teeth, but it is possible to have fewer (hypodontia), or more, in which case they are called supernumerary teeth. Wisdom teeth commonly affect other teeth as they develop, becoming impacted or “coming in sideways.” They are often extracted when this occurs.
The common postulation is that the skulls of human ancestors had larger jaws with more teeth, which were possibly 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 (ironically, wisdom teeth are in some ways an evolutionary response to the reduction in size of the appendix). As human diets changed, smaller jaws gradually evolved, yet the third molars, or “wisdom teeth”, still commonly develop in human mouths.
Wisdom teeth therefore are or first vestigeal feature which actually serve no useful purpose as with or without them, it makes no difference. To attest to this further, the agenesis (non-appearance) of wisdom teeth in human populations ranges from practically zero in Tasmanian Aborigines to nearly 100% in indigenous Mexicans. If the Tasmanians can do without them, why can’t the Mexicans?
3. The plica semilunaris
Your plica semilunaris (what many believe to be a vestigial remnant of your third frakking eyelid) is a small fold of tissue located on the inside corner of your eye (not the little bump in the very innermost corner, but the small flap right next to it). Your plica semilunari are the vestigial remnants of what is referred to as “nictitating membrane” which is most commonly found in amphibians and reptiles (including birds, which are strictly reptiles).
Shown above is the nictitating membrane of a masked lapwing bird as it blinks its inner eyelid. This eyelid is typically translucent, and serves to moisten the eye, clear debris, and help stare down Weeping Angels! :
Humans are unfortunately not equipped with these third eyelids, but don’t let stop you from staring into a mirror and willing yourself to make them blink. It certainly didn’t stop me.
4. The Coccyx
Your tail-bone, or coccyx, is the very last section of your vertebrae, and is the remnant of a lost tail. Almost every mammal on earth had a tail at some point in their lives, even if it was while they were developing in the womb. Humans all have tails in the womb (as well as a full coat of thick hair and even gills).
Between stages 14 and 22 of human embryogenesis, one you can actually observe a tail-like structure that is later absorbed (but, sometimes it isn’t which results in certain birth abnormalities such as tailed humans).
This “tail” is not so much a vestigial feature as an “atavism”, a feature which existed in an ancestor that is still written in the DNA, but not transcribed (for example, a chicken was recently produced with teeth, which all birds have lacked for at least 70 million years since they diverged from the theropod dinosaurs.
Ernst Haeckel’s theory holds that the process of developing from embryo into adult parallels various stages of the organism’s evolution from an ancestral state to its current one. This may well be true as many examples attest to it. Embryos develop tails which then shrink (99.999% of the time), lanugo, a thick coat of fur which then disappears, and even gills, which are co-opted into various structures of the jaw.
5. Arrector Pili
When you’re cold or stressed out, your arrector pili , are the muscle fibres that contract involuntarily to give you “goose pimples.” If you’re a furry woodland creature, this can provide insulation (thick, standing fur traps air between the erect hair follicles, helping you retain heat), or make you look bigger, which can mean the difference between being eaten and being passed over for less troublesome prey, a particularly good example being a porcupine (and cats which also exhibit this hair expanding behaviour). Since most humans aren’t hairy enough to fit the “furry woodland creature” description, our arrector pili provide neither of these benefits.
6. The Palmar Grasp Reflex
The palmar grasp reflex isn’t so much a vestigial feature as it is a vestigial behaviour. When a finger or similar object is placed in the palm of an baby, as many as 37% of infants are able to grasp with enough power to support their own body weight were they to be suspended. A similar grasping motion can be observed in the feet of babies. These behaviours typically last until four or five months of age, and might have been useful to our hairier ancestors, who could have been clung to by their offspring while they were travelling. Interestingly, the palmar grasp reflex along with the arrector pili are vestigial features associated with our hairy past.
7. Male Nipples
The biological function of male nipples is still a mystery, even though there have been cases of males who can lactate. Male nipples are vestigial in a different way from many of the other vestigial features in this article, in that they aren’t left over from an evolutionary past, but rather an embryological developmental one. All foetuses for all practical purposes begin life in the womb as females. In the absence of a Y-chromosome, the embryo will develop into a female. but when a Y-chromosome is present, the foetus will produce testosterone and other “male” hormones and develop into a male, but the nipples pretty much stay where they are. This essentially makes them decorative… and play things… yes, play things
For more in the “Evidence of Common Descent”series of blog posts, click on the tag “CommonDescent“.