Vestigial - Doesn't mean what you think

"Supposedly, summer vacation happens because that's when the kids are home from school, although having the kids home from school is no vacation. And supposedly the kids are home from school because of some vestigial throwback

to our agricultural past" ~ P.J. O'Rourke

Introduction

That the term vestigial has been and is currently being misused is supremely unfortunate. The definition of “ without function” is only half correct. The correct definition is without function, diminished function, or the loss of the original function.  Compounding this many scientists are also unaware of the exact definition. As will be demonstrated, this has been the correct concept for many decades and applies at the anatomical and genetic levels. It is especially unfortunate that even biology textbooks have not used a proper definition. This issue is similar to the misuse and confusion over the terms atheism and theory in many cultures.  Dictionaries are often of no help in clarifying because their primary function is usage and not delineating best or correct utilization. 

 “The writer of a dictionary is a historian, not a lawgiver. . . To regard the dictionary as an 'authority,' therefore, is to credit the dictionary writer with gifts of prophecy which neither he nor anyone else possesses.” 

https://www.thoughtco.com/what-is-a-dictionary-1690450

First, if one looks at better definitions of the term we can see that without function or without the original function not only is the more accurate definition but it goes back at least to the 1960’s. This is not a dodge by evolutionary biologists, who have always looked at the concept of vestigial this way.

For example:

1. Vestigial structures are those that appear in a rudimentary form in some organisms but in a fully developed, functional form in other, closely related animals; or they are poorly developed structures thought to have been fully functional in ancestors of the modern form (pg. 777) ~ Biology by Leland Johnson 1983

2. [Vestigial] - refers to an organ or part which is greatly reduced from the original ancestral form and is no longer functional or is of reduced or altered function. ~ www.biologyonline.com

3. Vestigial - a degenerate or imperfectly developed organ or structure that has little or no utility, but in an earlier stage of the individual or in preceding evolutionary forms of the organism performed a useful function ~ www.dictionary.com

4. Vestigial - (of certain organs or parts of organs) having attained a simple structure and reduced size and function during the evolution of the species: the vestigial pelvic girdle of a snake ~ Collins English Dictionary, 2014.

5. Vestige - a small, imperfectly developed, or degenerate structure which represents an organ or structure which was fully developed and functional in an ancestor or in an earlier state of development. ~ Dictionary of Biology, Edwin Steen. 1971

Secondly, it clarifies much regarding evolution and what we see in nature. A few examples will be discussed that show the loss of original function applies to many of the structures and facts we observe in nature.

1. The pelvis of modern cetaceans (whales and dolphins) are vestigial. We know this because the entire fossil record shows it changing from associated with a small 4 legged terrestrial animal to the vestigial bone today. Anatomists can identify within the modern whale vestigial pelvis what represents the ischium, ilium, and other pelvic structures. All the atrophied muscles and other structures are still attached in modern cetaceans but at least in males in terms of copulation the vestigial pelvis still has some functions. But they sure can’t walk with them.The current tiny vestigial pelvis sits outside the vertebra and is not attached to any bones. See Figure 1.

Figure 1a: Modern whale tiny vestigial pelvises showing anatomical structures that were previously only useful for walking. Several have vestigal femurs and all are not attached to the spinal cord. For educational purposes only. Fair use attribution From: www.http://um.uib.no/fagsider/osteologi/hvaler/b_bekken.htm

Figure 1b shows the evolutionary shrunken hind limbs of the fossil Basiolsaurid that lived 37 - 40 million years ago (left) compared to a modern Right Whale's vestigial hind limbs (right).

Figure 1b. Left = Basiolsaurid fossil whale species showing actual size of vestigial pelvis. NOTE ARROW. Magnified view shows bones used for walking. Right = photograph of actual living Right whale vestigial leg bones.

From: https://faculty.msj.edu.kritskg/evolab/Site/Vestigial_structures.html

Fair use, for educational use only.

B. The appendix has been shown to have functions in terms of repopulating the intestinal flora after severe diarrhea and it also contains lymphatic tissue. But the shape and location of the structure indicates it probably served other primary purposes in the past. It is officially called the “vermiform appendix” because in some people it’s very long like a worm and can loop back towards the liver behind the cecum. Not an efficient design for repopulating the gut after a wash out of severe diarrhea. Having a function today does not invalidate it being vestigial. See Figure 2.

Figure 2. Diagram of human vermiform appendix. From: Henry Vandyke Carter, Public domain, via Wikimedia Commons

C. Pseudogenes are genes we share with other species that have been disabled and knocked out by various mutations. By noting which pseudogenes are shared between species and which are not we can even construct evolutionary phylogenetic trees that rise to the level of proof of macroevolution (see pseudogenes, this site). We have about 20,000 pseudogenes and some have functions. Some are only partially transcribed whereas others have even taken on new functions. They still are vestigial even if they are partially transcribed or have acquired new functions because they have lost their original functions. Finlay notes, Pseudogenes may develop new functions “...but they are defined by their loss of the original parent gene function and not whether they have functions or not currently" ... and "The progressive changes in base sequence provide the history of a pseudogene, and this history defines evolutionary relationships of those species that share the pseudogene. Current functionality is irrelevant to the value of pseudogenes as evolutionary markers." (1)  A few pseudogenes have been noted to exhibit gain of function as noncoding RNAs. (2) Moran notes: “The idea that most duplicated genes will become pseudogenes is consistent with a ton of data and fits well with our understanding of mutation rates and genome evolution. This is an important point. We don't arbitrarily assign the word "pseudogene" to any old DNA sequence. The designation is based on the fact that the duplicated region is no longer transcribed, or it is no longer correctly spliced, or that it carries mutations rendering the product nonfunctional... There are some examples of DNA sequences that appear to be pseudogenes but they also have functional regions. The best examples are duplicates that contain small RNA genes within their introns or genes that contain other functional regions like SARs and origins of replication. In those cases, the inactivated gene is still a pseudogene but the other functional regions are best characterized as something else.

There are also quite a few examples of pseudogenes that have secondarily acquired a distinct new function such as producing a small RNA that might have a regulatory function. The review by Cheetham et al. contains several examples of such pseudogenes. They are still pseudogenes but the region may now specify a new lncRNA gene or some other gene such as an siRNA gene.” (3)

If so disabled that they have lost all functions they are basically molecular fossils. If they are shared with the same random knock-out mutations between sepecies we are justified in claiming common ancestry as the only rational explanation. For examples and discussion see pseudogenes this site.

D. The coccyx is vestigial. It obviously looks like a shrunken tail because it has 5 small bones. It still supports some attachments but it’s small enough that if a Designer didn’t want to make it look like a shrunken tail from our past ape descendants why not just use one or at the most two bones? Since it still has muscles and other tissues attached it still plays a small role in human anatomy, but not it's original function.

Figure 3. Diagram of the human coccyx showing how this small bone is really a shrunken remnant of a tail. Functionality is not critical to the vestigial designation. The bone is small enough that one or two bones could easily suffice to be functional instead of having the appearance of 5 fusions.

E. Yolk sac . As an embryo we make an empty yolk sac that is vestigial. It still has important functions today such as providing lymphatic and blood cell precursors for the developing embryo but no yolk is produced. Yet it is indeed called a yolk sac because that's what it resembles in other animals that lay eggs. More conclusively, scientists have found 3 human yolk pseudogenes that match the same genes and locations in chickens. We do indeed hark back in deep evolutionary time to a point where our ancestors laid eggs and we have the structure and genes to demonstrate it. Our temporary yolk sac no longer has the function it had in the past and is vestigial. See Figure 4.

Figure 4. Human yolk sac which begins development in the embryonic stage and then disappears about the 3rd month. It has important functions but has lost its original evolutionary function. For attribution and additional information, see Why Not Intelligent Design?.

Figure 5. Diagram showing three human egg yolk pseudogenes (VIT 1, 2, 3) found also in homologous locations in chickens. Conclusive evidence that the human yolk sac is vestigial, having lost it's original function. For attribution and additional information, see Why Not Intelligent Design?.

Third, sometimes a modern species will form an atavistic structure. The DNA is still present to form most of an ancient structure and becomes accidentally turned on. This exposes the ancient connection to it’s past ancestors. These are technically not a vestigial structure because they are uncommon or rare; obviously every individual of the species does not have them, unlike vestigial structures.

Notice the decapitated snake in Figure 5. It accidentally grew a leg. It was beaten to death by a homeowner in China, who thought it was trying to crawl up a wall. Notice the leg grew at about the anatomical location we would expect. It did not grow a feather and it did not grow elsewhere like a HOX gene mistake. It grew where an evolutionary ancestor would have had a leg.

Figure 5. Atavistic snake leg. From: http://pleiotropy.fieldofscience.com

Also: Evolutionary throwback. Snake with foot. Gizmodo.com. 2009, Sept. 15

About 1:5,000 sperm whales grow hind limbs. The DNA instructions for doing that are still present but damaged and normally not expressed. At least six cases are known of whales being caught with hind legs growing. A dolphin was caught in 2006 off the coast of Japan with hind flippers. See Figure 6.

Figure 6. Left - a dissected sperm whale atavistic hind limb exposing a femur, tibia and metatarsal caught in 1919 off the coast of British Columbia. Right - a photo of hind flippers growing from a dolphin caught near Japan in 2006.

For more discussion of cetacean leg atavisms and attribution see Overwhelming Evidence for Whale Evolution, this site Part 1

Conclusion

The proper definition of vestigial is loss of function, diminished function or loss of original function. Repeatedly anti-evolutionists will claim "checkmate" when a function is found for a structure or gene. The finding of a function does not discount that a form can be vestigial. That this complete definition for vestigial is nothing new was demonstrated. It is unfortunate that even within the sciences researchers and teachers may be communicating a wrong concept of vestigial. The actual definition as defened here of vestigial applies not only to structures but also down to the molecular and genetic level and has resulted in unnecessary controversy more recently over pseudogenes in the field of genetics, where some claim there is no junk DNA and few or no pseudogenes (see Junk DNA, this site for example). Vestigial structures, which are present in each individual in a population, were contrasted with atavisms which are rare occurrence of structures that hark back to appropriate evolutionary ancestry. Whales can grow hind limbs but never feathers. Boas and pythons have a vestigial pelvis with rudimentary femurs that are used in courtship which in no way negates them as being vestigial since the original function of the structures was for walking.

Citations

1. Finlay, Graeme. 2021. Human Evolution: Genes, Genealogies and Phylogenies. Cambridge University Press. 283 pp. not including References and Index. Paperback edition 2021 - ISBN 978-1-009-00525-8. Hardback edition 2013.

2. https://onlinelibrary.wiley.com/doi/abs/10.1002/9780470015902.a0020836.pub2

3. https://sandwalk.blogspot.com/2020/01/are-pseudogenes-really-pseudogenes.html#more