Discovering Religion: Episode 9 – Vestigial Organs
Not only do we find multitudes of transitional species all throughout the fossil record, we also find within currently living animals a great variety of evidence displaying ongoing transitional processes. For example: homologous structures, vestigial organs, and even tucked away within the genetic code. A common theme within evolutionary biology is “descent with modification,” which is evidenced by observing anatomical similarities between species grouped within related taxonomic categories.
For example, many mammals, such as humans, cats, whales, and bats, have closely related skeletal elements that make up their forelimbs, although these appendages have very different functions. In their defense theists often claim God created life according to a plan, therefore it is not surprising we observe similarities in anatomical structures between different species. Although this might explain the similarities between the forearms of a primates and humans, surely the best way to design the infrastructure of a bat’s wing is not also the best way to build a whale’s flipper. Such anatomical peculiarities make no sense if the structures are uniquely engineered and unrelated.
The forelegs, wings, flippers, and arms of different mammals are variations on a common structural theme. Similarity in characteristics resulting from common ancestry is known as homology, and such anatomical signs of evolution are called homologous structures. Comparative anatomy is consistent with all other evidence in testifying evolution is a remodeling process in which ancestral structures that function in one capacity become modified as they take on new functions. Biologists are very concerned with structure and function. For instance, the structure of the stomach serves the function of digestion. However, all throughout the animal kingdom we find structures within animals that have no apparent function.
Non-functional structures are known as vestigial organs — the historical remnants of structures that once did have important functions in ancestral organisms. Anti-evolutionists often claim a loss of anatomy is not evidence for evolution, where one might expect to see the emergence of new and beneficial structures. However, vestigial organs do in deed provide concrete evidence of natural selection, because it would be wasteful for the body to continue to provide blood, nutrients, and space to an organ that, as a result of evolution, an animal no longer requires as part of its physiology. Natural selection tends to favor individuals with reduced versions of such organs, thereby phasing out obsolete structures.
If you’ve ever seen the skeleton of a Blue Whale, either in person or in a picture, you may have noticed two relatively small bones precariously hanging off the back third of the skeleton. These bones are actually remnants of the pubis and femur; however, many ancestral whales such as Durodon and Basilosaurs, still displayed their hind legs that over time became progressively reduced in size. As discussed in episode 7, the benefits these appendages offered were so negligible that eventually their descendants lost them entirely in the effort to become more streamlined in the water.
In fact, the only reason modern whales even have a pelvis at all is because these bones are still utilized in the birthing process. Keep in mind, even though the whale’s pelvis is reduced, it would not have been beneficial to lose it all together, as in the case of their hind legs, because only useless structures are discarded by natural selection. However, bones are not the only evidence that whales once walked on the land.
In the mammalian embryo limbs first appear as little buds on the sides of the developing animal. In fact, many species of snakes and legless lizards, such as the “slow worm”, initially display limb buds in their embryonic development, only to reabsorb them before hatching. In the same regard, modern adult whales, dolphins, and orcas do not display hind legs, even though hind legs, complete with various developing leg bones, nerves, and blood vessels, temporarily appear in the cetacean fetus and subsequently degenerate before birth.
As can be seen in the example of the dolphin embyro, where the circled hind limb buds develop, grown, and then once again become reduced. Sometimes these rudimentary hindlimb buds are observed to persist longer in the embryos of baleen humpback whales and dolphins, which may explain the rare occurrence of external hindlimbs found in the fully matured animal.
We also find vestigial structures within cave dwelling animals. The Mexican tetra is a fish that lives in complete darkness. Although it is blind, the tetra still contains a rudimentary eye with a lens, sclera, degenerate retina and optic nerve. Evolution makes the prediction that the ancestors of the Mexican tetra made the lightless caverns their home and over time natural selection favored tetras that diverted blood, nutrients, and space away from eyes that were longer used. In fact, this is exactly what happened, as there are many other species of tetra that live in open water and have fully functioning eyes.
The same process is responsible for the blind texas salamander, who has also lost the ability to see due to the absence of light in its natural habitat. However, there are other species of salamander that still live on the surface and hence have retained their sight.
We also find vestigial structures within plants. The dandelion, like all flowers, contains the proper sexual organs necessary to fertilize the other members of its species. However, dandelions exclusively reproduce through asexual reproduction. Dandelion seeds are produced without pollination through an asexual process known as apomixis, resulting in offspring that are genetically identical to the parent plant, that is, they are clones.
The dandelion contains the same organs used in sexual fertilization found in all other flowers, such as the stamen, pistil, and even its pollen, all of which are viewed as vestigial structures within the dandelion because they are not utilized in any biological application, neither for the originally intended function of sexual reproduction nor for any other conceivable purpose.
Certain species of the whiptail lizard, of the genus Cnemidophorus, only exist in the female sex. Although this might seem like a problem when it comes to propagating the species, females do not require a male for reproduction as they engage in parthenogenesis, a form of asexual reproduction where an unfertilized egg develops into a new individual. Despite the fact these lizard reproduce asexually, some female lizards try to act like a male by mounting another female in a futile attempt at copulation. Because these lizards evolved from a species that reproduced through sexual fertilization, the unnecessary practice of engaging in fake sex is viewed as a vestigial behavior, a remnant of their ancestral nature.
Both reptiles and birds lay eggs. The emerging young use either an “egg-tooth” to cut through leathery eggshells made of keratin, as found in lizards and snakes, or they use a specialized structure called a caruncle to crack their way out of hard eggshells made of calcium, as found in turtles and birds. Mammals evolved from a reptilian-like ancestor.
Placental mammals, like humans and dogs, have lost the egg-tooth and caruncle, and, yes we have even lost our eggshells well. However, monotremes, such as the platypus and echidna, are primitive mammals that have both an egg-tooth and a caruncle, even though the monotreme eggshell is thin and leathery.
Even more striking is what happens during the embryonic development of marsupials, which express transient eggshells that are later reabsorbed before live birth. Although they have no need to hack through a hard egg-shell, several marsupial newborns, such as baby Brushtail possums, koalas, and bandicoots, retain a vestigial caruncle as a clear indicator of their reptilian, egg-laying ancestry.
We also find vestigial structures within insects. There are many examples of flightless beetles, such as the weevils of the genus Lucanidae, which retain perfectly formed wings housed underneath fused wing covers.
There are also many varieties of flightless bird, such as the ostrich, kiwi, and penguin, to name a few, whose ancestors were once able to fly. Take for example the penguin. In regions of the world accessible to land based predators, such as the Arctic, birds like the Little Auk, which very closely resemble the penguin, require the ability to fly not only to find food but to escape predation.
However, on the remote cotenant of Antarctica, which is inaccessible to land based predators, the ancestors of the penguin were free to move along the ground as they pleased, encouraged to spend more time hunting in the water than in the air and thereby modifying their wings once used for flight into flippers that now allow them to swim.
All of the examples discussed in this video can be explained in terms of structures which have been, leftover, reduced, or modified from the original organs found within the animals predicted ancestors. And humans are of no exception. Please continue to part 2 two of this episode where we will examine in detail the many vestigial structures found within our own bodies.
The appendix is one of the most commonly perceived vestigial organs found within the human body. In plant-eating animals the appendix often makes up the majority of the intestine, where large amounts of cellulose are broken down by bacteria. Humans are unable to digest cellulose and our appendix is greatly reduced and seemingly nonfunctional.
However, anti-evolutionists argue the appendix serves an important function in aiding immunity due to the presence of lymphoid tissue.
Antibodies are produced in all lymphoid tissue; however, it is misleading to suggest the appendix is vital to acquired immunity. The production of antibodies is a direct result of T and B-lymphocytes being entrapped in lymphatic tissue after an antigen is detected in the body. The lymphocytes themselves are produced initially prior to birth, from foetal stem-cells and continue to reproduce throughout one’s lifetime. Because the lymphocytes are simply trapped by lymphoid tissue, the percentage trapped in appendiceal lymphoid tissue is miniscule in comparison with the rest of the lymphatic system.
According to the American Journal of Epidemiology there are an estimated 250,000 appendectomies annually performed in the United States. Surgery is the only way to resolve a bout of appendicitis, which would otherwise result in painful death due to peritonitis, intra abdominal abscess, or severe infection following a rupture. In fact, so great is the risk and so severe are the consequences of developing appendicitis, it has become routine for surgeons to remove a healthy appendix if they happen to be in the abdominal cavity performing an unrelated procedure.
The appendix is another sound piece of evidence that reinforces the understanding our ancestors at one time had a diet solely consisting of plant matter. However, now that our diets have so dramatically changed the appendix has become more harmful than beneficial. And this is exactly how natural selection works, for if we did not intervene in our own natural evolution by providing medical assistance to the sick, those more likely to suffer from appendicitis would die, unable to propagate their genes. And those of the population with reduced version of the appendix would survive and hence their offspring would have reduced versions as well, until the appendix disappears all together.
A herbivore ancestor is also supported by the presence of wisdom teeth, another set of structures that have become more harmful than beneficial. These molars were required for chewing and grinding plant material. Although over 90% of adults develop third molars, usually these teeth never erupt from the gums, and in one third of all individuals they are malformed and impacted. These useless structures can cause significant pain, increased risk for injury, and may result in illness and even death if the impacted teeth become infected and go untreated.
The plantaris muscle is used by animals for gripping and manipulating objects with their feet — something you see with apes who seem to be able to use their feet just as well as their hands. Humans have this muscle as well, but it is now so underdeveloped that it is often taken out by doctors when they need tissue for reconstruction in other parts of the body. The muscle is such an unimportant part of our anatomy that 9% of all humans are now born without this structure.
Homo sapiens are taxonomically classified as apes; and one of the defining characters of apes is the lack of an external tail. However, human embryos display the initial signs of a tail during our early development. Between four and five weeks of age, the normal human embryo has 10-12 developing tail vertebrae that extend beyond the anus and legs, accounting for more than 10% of the embryo’s total length.
The embryonic tail is composed of several complex tissues besides the developing vertebrae, including a secondary neural tube, a notochord, tail gut, and mesenchyme. By the eighth week of gestation, the sixth to twelfth vertebrae have disappeared via cell death, and the fifth and fourth tail vertebrae are still being reduced. Likewise, the associated tail tissues also undergoing cell death.
However, sometimes there maybe a problem in the programmed regression of vestigial embryonic structures, just as in the previously examined case of a dolphin’s vestigial limb buds failing to regress, causing the adult animal to be born with hind limbs. So too has a failure in the regression of the human tail been observed, where children have been born with a vestigial appendage.
Although the expression of a human tail is a very rare occurrence, the coccyx is a more permanent reminder of our tree-swinging days. Over time we lost the need for a tail, but we did not lose the need for the coccyx, which, much like the rudimentary femur found in whales, now functions as a support structure for various muscles, providing support for a person when sitting down and leaning back, as well as supporting the position of the anus.
Humans also have remnants of ear-wiggling muscles. There are three small muscles around each of the human ears that apparently have no function whatsoever. In other mammals these muscles are often large and serve an important function, such as deer, that use them to turn their ears toward a source of sound. However, few humans can wiggle their ears, and none can turn them toward sound.
Another seemingly wasteful structure is the recurrent laryngeal nerve, which regulates the ability to swallow in mammals, and in humans has the added function of controlling speech. In our mammalian ancestors the nerve took a direct route from the brain to the throat, passing directly in front of the aorta. However, during the course of evolution the aorta shifted inferiorly. Being such an important structure the nerve could not be broken, so instead it has become longer in order to loop back up and reach the throat. In giraffes the recurrent laryngeal nerve is 15 feet long, a full 14 feet longer than necessary.
The human nose gives evidence for past ancestral forms as well. The olfactory receptor (OR) genes once coded for proteins involved in now lost olfactory functions. Our predicted ancestors, like other mammals, had a more acute sense of smell than we do now. Humans have more than 100 Olfactory receptor genes, approximately 70% of which are non-functional, or pseudogenes. Other mammals, such as the mouse, marmoset, and the fox, have many of the same olfactory receptor genes as humans, but all of theirs are functional. An extreme case is the dolphin, which is the descendant of land mammals. Dolphins no longer have any need to smell, yet they contain many olfactory receptor genes, of which none are functional — they are all pseudogenes.
For the most part, humans have relatively little to no body hair. Yet, some people, especially men, have a considerable amount of body hair from which they gain negligible warmth. No advantage can be gained from the amount of body hair found on modern-day humans. However, in light of evolution, we know our predicted ancestors had a coat of hair that covered their bodies and what little hair we currently find on ourselves today is just a vestige of our primate ancestry.
Goose bumps are caused by the contraction of the arrector pillis muscle located around the shaft of the hair follicle. This phenomenon is a vestigial defense mechanism that has since been rendered obsolete after generations of evolution. However, this method of defense has been retained in some modern-day mammals, such as dogs and cats, that intimidate their foe by causing their fur to stand on end, thereby increasing the size of their appearance. This is also the way porcupines project their quills toward an on coming predator. However, humans no longer have control over the arrector pillis muscle, and due to our lack of hair, any benefit our ancestors might have gained by employing these structures has now been rendered useless within homo sapiens.
Another structure with no apparent function is found in the male body, it is the nipple.
Of course, nipples serve a vital function within women, however, men have absolutely no use for theirs. In fact, men even have small amounts of mammary tissue. Under natural conditions men do not produce the appropriate hormonal stimulus to induce lactation; however, if men are exposed increased levels of prolactin, the same hormone that becomes elevated when women are pregnant and give birth, men can indeed produce milk. Male lactation was of some interest to Charles Darwin, who speculated that both sexes of our early mammalian ancestors may have nursed their young. In fact, males of a rare species of fruit bat, known as the Dayak, have been observed breastfeeding their offspring.
Despite all these vestigial structures, the human body also has several flaws in its construction. For example, the retina contains millions of photoreceptor cells responsible for absorbing light, which is sent to the brain for interpretation via the optic nerve. However, at the location where the second cranial nerve innervates the retina, known as the optic disc, there is an absence of any photosensitive cells. As a result, there is a break in the visual field of each eye, known as the “physiological blind spot”. The brain compensates for this flaw by filling in the surrounding details with information from the other eye. However, with one eye closed the blind spot becomes noticeable with a simple test.
“And you have to keep looking right at the bridge of my nose and keep your eye fixed. And now we are going to move it just out a little bit, about 15 degrees, and right about there.
Yeah, its gone.
You cant see it?
Now can you see it?
Now can you see it?
Now can you see it?
It’s a blind spot, that’s really lousy.
All vertebrates display this blind spot in their eyes. However, cephalopods, such as the octopus, have eyes very similar to vertebrates although they do not share the same constructional flaw. The reason cephalopods do not have a blind spot is the result of different origins of eye evolution. The eyes of cephalopods began as invaginations of photo receptors with the head, whereas in vertebrates the eyes began as extensions of the brain. Therefore, the optic nerve of the cephalopod eye was able to attach from the rear of the retina instead of passing through the retina, alleviating blind spot found in the vertebrate eye.
Another flaw in our anatomical structure is the human knee, which is not well made for kneeling and prolonged kneeling can lead to an expansion of the bursa in front of the patella, a condition known as “housemaid’s knee”. Likewise, there is a flaw in the human elbow. At the knob on the lower end of the humerus, the ulnar nerve is exposed just under the skin. A sharp blow by a hard object causes that numbing, painful sensation known as “hitting the funny bone”.
The adult human skull is also too thin to provide adequate protection to such a large brain, where the absence of brow ridges leaves the eyes poorly protected. However, we do see a prominent brow in the skulls of primates as well as other human species, such as the Neanderthal.