Evolutionary Anatomy – Vestigial Structures

20 Sep

Welcome to a new series on Evolutionary Anatomy by Dr. Tony Friscia.  Dr. Friscia’s research program has focused on the evolution of early Tertiary mammals and their diversification into modern families, using extensive anatomical and functional knowledge as a basis for comparison.

A message from UCLA professor, Dr. Tony Friscia.

A message from UCLA professor, Dr. Tony Friscia.

With the constant drive to prepare for allied health careers, one thing both students and even instructors of anatomy often forget is that anatomy is a sub-field of biology. Theodosius Dobzhansky, a founding member of the field of population genetics and one of the architects of the Modern Synthesis of evolutionary biology, famously said, “Nothing in biology makes sense except in light of evolution.” So by corollary, nothing in anatomy makes sense except in light of evolution. As recognition of this fact, this is the first in a multi-part blog arc about the evidence for evolution in human anatomy. For me, this connection offers a richer picture of anatomy, and often helps to illustrate points that even our allied health students can appreciate.

There are some well-known, obvious examples of this connection. The appendix is one of these. It’s what’s called a vestigial structure. This means that it although it has no obvious use in humans, it is a holdover from our evolutionary history. Looking at other mammals we can see that the appendix is used by many of them, especially herbivores (plant-eaters) as a key part of their digestive processes. The fact that we retain the appendix speaks to our shared ancestry with these animals. (Having said this, there is some evidence that the appendix in humans acts as a store of the various ‘good’ bacteria that aid in our digestion, but even this function hints at the connection to similar, but more extensive, use in other mammals.)

A comparison of appendices among various mammals

A comparison of appendices among various mammals (from Talk Origins)

One question I often get asked about these sorts of vestigial structures, whether it be appendix or wisdom teeth or something else, is “Will we ever lose this structure?” Student wonder if it will be ‘evolved’ away, and the answer is a bit more complex than it might appear, and we need to go back to the fundamentals of evolutionary theory.

Darwin’s theory of Natural Selection added the necessary mechanism to evolutionary theory. Evolutionary thought was around before Darwin, by some accounts going back to the Greeks, but there was no good mechanism for it. One of the pre-Darwinian contenders for an evolutionary mechanism was Lamarckian evolution via acquired characteristics. Lamarck believed that the usage of morphological adaptations provided the selective mechanism for evolution. We now know this to not be the case. (Just think about it for a minute – if you work out, will your children be born with bigger muscles?) Nevertheless that Lamarckian idea hangs on a bit, because it makes some amount of logical sense. It is from this that the students’ questions about the loss of vestigial characters arises; if we aren’t using the appendix it should go away.

A key part of Darwin’s Natural Selection is the concept of fitness – a combination of the ability to survive to adulthood and to reproduce. When students ask me if the appendix is ‘going away’ I retort with, “Does anyone die of appendicitis anymore?”. In places with access to modern medicine the answer is no. Even if someone has the bad luck to have an infected appendix burst, spewing its biological weapons into the peritoneal cavity, we have a slew of antibiotics that can help most victims. So if the appendix doesn’t affect survival or the ability to reproduce, will it ‘go away’? Not via the mechanism of Natural Selection.

Modern genetics (a piece Darwin was lacking, and was added in the Modern Synthesis of the 20th century) adds a caveat to this answer. We now know about the concept of mutation. Mutations can alter the development of body structures. Obviously a mutation in a critical gene, like one controlling head or heart development, will never last long in a population, because the individuals will probably not survive (or even be born; many of the most horrible malformations will be spontaneously aborted early in pregnancies). So there is strong selection to maintain working forms of these developmental genes.

If a mutation occurred in the gene controlling appendix development, it may not even be noticed. Eventually in a population, these mutations may build up, to the point where the appendix never even forms. With no Natural Selection maintaining the appendix, it may simply be mutated out of existence, although this process may take a long time.

Other evolutionary factors affect our morphology, and in the next installment, we’ll tackle the trade-offs that are often made through the course of evolution, and examples of these trade-offs in humans.

(Here’s a link to a more detailed discussion of the vestigiality of the appendix)

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