- Vestigial structures
- eyes of the blind marsupial mole
- Vestigial anatomical configurations
- paths of the left and right recurrent laryngeal nerves
- path of the phrenic nerve
- embryological placement of mammalian gonads
- Vestigial reflexes
- emotive and thermoregulatory contraction of human Arrectores pilorum
- hiccuping (homologous to amphibian water gulping)
Vestiges are structures, anatomical configurations, and reflexes that are most parsimoniously explained as remnants of common ancestors in the evolutionary history of a given organism. As such, they are one of many sources of evidence for the model common ancestry, on which the evolutionary model is based.
The most common example of a vestige is the presents of small, internal hind leg bones in modern whales, passed on to them by their land-dwelling mammalian ancestors. The response by many creationists—most notably of which; Kent Hovind, in his series, Lies in the Textbooks—is that if a proposed vestige has any function at all, it isn’t really a vestige, and that this applies to whales since their leg bones play a roll in reproduction. What these creationists fail to understand is that the definition of a vestige has never been an evolutionary remnant with no function—it’s always been an evolutionary remnant that has lost or nearly lost its original primary function. If it has any current function, it’s either a persisting secondary function, or a function gained sometime after the lost of its primary one—a process called exaptation, or co-opting.
This concept was known even as far back as Charles Darwin. Now, in Darwin’s day, the term ‘vestigial organ’ didn’t exist; but the idea the term describes was something Darwin wrote on in both his publication, The Descent of Man, and in this very publication. He used the terms ‘rudiments’ and ‘rudimentary,’ rather than ‘vestiges’ and ‘vestigial.’ On pages 451-452 of the original publication of On the Origin of Species, Darwin wrote:
An organ serving for two purposes, may become rudimentary or utterly aborted for one, even the more important purpose; and remain perfectly efficient for the other … Again, an organ may become rudimentary for its proper purpose, and be used for a distinct object: in certain fish the swim-bladder seems to be rudimentary for its proper function of giving buoyancy, but has become converted into a nascent breathing organ or lung. Other similar instances could be given (Darwin, 1859, p.451-452).
Understanding exaptation and how it occurs is central to understanding why the creationist objection fails to address the issue of vestigiality. But before going over some of the paths to exaptation, let’s take a look at the analogy given in the second figure (on page 362) in the publication entitled “The Evolution of Complex Organs” by Dr. T. Ryan Gregory:
Fig. 2 A simple example of exaptation and secondary adaptation. A The original and still primary adaptive function of coins is as currency. B A coin co-opted into a new exaptive role as an instant lottery ticket scraper. Coins would always have been capable of scraping tickets, but this function did not become apparent until an environment arose in which instant lottery tickets were abundant. Though functional as scrapers, coins are somewhat difficult to hold and may not reliably be on hand when needed. C A secondary adaptation that enhances the novel function of a coin as a ticket scraper by incorporating it into a keychain that is easier to grip (US Patent #6009590, “Lottery ticket scraper incorporating coin” by K.M. Stanford 2000). In this case, a second preexisting structure (key ring) was co-opted into a function as a carrier for a lottery ticket scraper (Gregory, 2008, p.362).
Now that the basic idea has been explained, here are six ways exaptation can occur—with examples of each—as explained by Dr. Gregory on pages 361 through 363 of his publication on complex organ evolution:
Now that it’s been established what vestiges are, how they form, how they provide evidence for common ancestry, and how the creationist objection from functionality is invalid, let's take a look at some specific examples of structural, anatomical, and reflexive vestiges.
There are several possible routes by which an organ, components of an organ, or genes can become exaptations (Gould and Vrba 1982; Arnold 1994; Gould 2002; McLennan 2008):
1. One organ (or gene) has an existing function but takes on or switches to a new function as a result of selective pressures experienced after the organism moves into a new environment or adopts a new ecological lifestyle...
Example: the middle ear bones of mammals are derived from former jaw bones (Shubin 2007).
2. One organ (or gene) has an existing function but at some stage modification of the feature for the initial function makes it amenable to modification in a new role and this allows the organism to move into a new environment or adopt a new ecological lifestyle.
Example: early tetrapod limbs were modified from lobe-fins and probably functioned in pushing through aquatic vegetation; at some point, they became sufficiently modified to allow movement on to land (Shubin et al. 2006).
3. One organ (or gene) has two functions and is modified as it becomes increasingly specialized for one of them. Sometimes, the organ is specialized for one of the initial functions in one lineage and for the other initial function in a different lineage.
Example: an early gas bladder that served functions in both respiration and buoyancy in an early fish became specialized as the buoyancy-regulating swim bladder in ray-finned fishes but evolved into an exclusively respiratory organ in lobe-finned fishes (and eventually lungs in tetrapods; Darwin 1859; McLennan 2008).
4. Two organs (or genes) perform the same function and then one becomes more specialized for the original function while the other takes on a different role. This is particularly significant when duplication generates multiple copies that subsequently diverge (see below).
Example: some of the repeated limbs in lobsters are specialized for walking, some for swimming, and others for feeding.
5. Afeature that had become vestigial in terms of its original function takes on a new function in its reduced state.
Example: the vestigial hind limbs of boid snakes are now used in mating (Hall 2003).
6. Afeature that formerly had no function and was present for non-adaptive reasons (a "spandrel"; Gould and Lewontin 1979; Gould 1997, 2002) takes on a function and may become specialized for that function. This, too, can occur at both the genetic level and the organ level.
Example 1: the sutures in infant mammal skulls are useful in assisting live birth but were already present in non-mammalian ancestors where they were simply byproducts of skull development (
Example 2: some formerly parasitic transposable elements in the genome, which had no function at the organism level, have been co-opted into a variety of other roles, such as in the vertebrate adaptive immune system (e.g., Zhou et al. 2004).
The important point regarding exaptations, then, is that the current function of a feature may not reflect the reasons for its origin. Rather, the feature may only have come to occupy its current role comparatively recently.
II. Vestigial structures:
1. Unlike most animals with eyes, “the marsupial mole (Notoryctes) is blind and has vestigial eyes that are hidden under the skin. Furthermore, the lens and pupil are absent, and the optic nerve is reduced (Springer et al., 1997, p.13758).” The deterioration of Notoryctes’ eyes are also accompanied by the deterioration of the interphotoreceptor retinoid binding protein gene; a genetic vestige.
III. Vestigial anatomical configurations:
1. The left recurrent laryngeal nerve (RLN) branches from the left vagus nerve near the heart, and the right RLN braches off a bit further up from the right vagus nerve. The left RLN loops under the ductus arteriosus, and right RLN loops under the right subclavian artery. They both then travel up into the neck to innervate the larynx and its surrounding muscles. Both of these recurrent paths are quite inefficient in mammals; adding over a foot of unnecessary length to the human left RLN. This is especially true in giraffes, where the added length is absurd—more than fourteen signal-slowing, energy inefficient feet are added to the left RLN alone. This data doesn’t fit the model of creation, as it would be easier and more beneficial to simply branch the RLNs off higher on the vagus nerve and avoid arterial looping completely. The evolutionary model, however, is a precise fit; especially when one considers fish anatomy. In fish, several branches extend from the vagus nerve, each looping around arterial arches that connect the dorsal and ventral aorta between each gill slit (Ridley, 2004, p.281-282; Berry & Hallam, 1989, p.83). This is powerful evidence that mammals and fish share ancestry, that the RLNs and ductus arteriosus are remnants of the vagus nerve branches and sixth arterial arch in those ancestors, and that the configuration in mammals is a vestige resulting from decent with modification.
2. The gonads of sharks, other fish, and even humans develop in same place—the chest. This works well for sharks, since they stay there, but in human males, the gonads need to travel all the way down into the scrotum to keep cool. This causes an unnecessary looping of the spermatic cord, which causes a weakness in there body wall, leaving them prone to developing a hernia (Shubin, 2009, p.64-66). This is consistent with decent, with modification, from an ancestor we share with modern fish.
3. The path of the human phrenic nerves begins at the base of the skull, and goes through the body cavity to the diaphragm. This is an efficient path to amphibians’ gills, which are in the neck, but is an inefficient path to the diaphragm, in humans. The irritation of these nerves—made likely by their placement—can cause problems with breathing, including hiccups (Shubin, 2009, p.66-67); a reflexive vestige.
IV. Vestigial reflexes:
1. Signals are sent from the human brain, through the phrenic nerves, that induce synchronous spasms of the diaphragm. The subsequent sharp inhalation of air closes the epiglottis. This is commonly known as the hiccups. The evidence, as laid out by Dr. Straus (2003), that it is a vestigial reflex is as follows:
- Amphibians have homologues motor pathways that cause similar inhalation (of water into their gills) and closing of the epiglottis (to prevent aspirating water into their lungs).
- Motor pathways necessary for hiccupping complete embryological development before those of lung functionality do.
- Both hiccups and amphibian water gulping are inhibited by the detection of high CO2 blood concentration by chemoreceptors.
- Both hiccups and amphibian water gulping are outright stopped by the binding of (RS)-4-amino-3-(4-chlorophenyl) butanoic acid (Baclofen) to gamma-aminobutyric acid B receptors.
2. In many particularly hairy vertebrates, such as apes, dogs, and rats, the involuntary erection of hairs via the contraction of muscles, called Arrectores pilorum, act as an efficient means of thermoregulation. This is achieved by trapping insulating air against the skin. Humans, however, are not hairy enough for this reflex to have any significant effect in regulating their temperature. The involuntary erection of hair is also an emotive reflex in many vertebrates, increasing their apparent size to intimidate predators when afraid, to intimidate rivals when angry, or to convey various other emotions (Darwin, 1872, p.95-102). The only secondary purpose Arrectores pilorum serve is in aiding the sebaceous gland in secreting sebum (Song et. al., 2007).
Berry, R. J., and A. Hallam, eds. Encyclopedia of Animal Evolution. 1st ed. Facts on File, 1989.
Darwin, Charles. The expression of the emotions in man and animals. 1st ed. London: John Murray, 1872. <http://darwin-online.org.uk/content/frameset?viewtype=side&itemID=F1142&pageseq=106>.
Darwin, Charles. On the Origin of Species by Means of Natural Selection, or the preservation of favoured races in the struggle for life. 1st ed. London: John Murray, 1859. <http://darwin-online.org.uk/content/frameset?viewtype=side&itemID=F373&pageseq=469>.
Gregory, T. R. "The Evolution of Complex Organs." Evolution: Education and Outreach 1.4 (2008): 358-89. <http://www.springerlink.com/content/t125078h5p201442/>.
Springer, M. S., A. Burk, J. R. Kavanagh, V. G. Waddell, and M. J. Stanhope. "The interphotoreceptor retinoid binding protein gene in therian mammals: Implications for higher level relationships and evidence for loss of function in the marsupial mole." Proceedings of the National Academy of Sciences USA 94.25 (1997): 13754-3759. <http://www.pnas.org/content/94/25/13754.full>.
Ridley, M. Evolution. 3rd ed. Malden, Massachusetts: Blackwell, 2004. <http://tinyurl.com/ybw2p3m>.
Shubin, N. H. "This Old Body." Scientific American Jan. 2009: 64-67. <http://www.mukto-mona.com/Special_Event_/Darwin_day/2009/english/SA_old_bodyShubin.pdf>.
Song, W. C., K. S. Hu, H. J. Kim, and K. S. Koh. "A study of the secretion mechanism of the sebaceous gland using three-dimensional reconstruction to examine the morphological relationship between the sebaceous gland and the arrector pili muscle in the follicular unit." The British journal of dermatology 157.2 (2007): 325-30. <http://www.ncbi.nlm.nih.gov/pubmed/17596168>.
Straus, C., K. Vasilakos, R. J. Wilson, T. Oshima, M. Zelter, J. P. Derenne, T. Similowski, and W. A. Whitelaw. "A phylogenetic hypothesis for the origin of hiccough." Bioessays 25.2 (2003): 182-88.<http://www.ncbi.nlm.nih.gov/pubmed/12539245>.
Dr. T. Ryan Gregory's References
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Gould SJ, Vrba ES. Exaptation—a missing term in the science of form. Paleobiology. 1982;8:4–15.
Hall BK. Descent with modification: the unity underlying homology and homoplasy as seen through an analysis of development and evolution. Biol Rev Camb Philos Soc. 2003;78:409–33. doi:10.1017/S1464793102006097.