Nothing in Medicine Makes Sense Except in the Light of Evolution

The title of this post is a direct allusion to Theodosius Dobzhansky's famous 1973 article Nothing in Biology Makes Sense Except in the Light of Evolution [1] in which he outlined facts from areas such as ecology and molecular biology, and stated that "all these remarkable findings make sense in the light of evolution: they are nonsense otherwise." In a similar manner, aspects of human anatomy, developmental biology, physiology, and genetics which look bizarre, poorly designed, and otherwise defy rational description (not to mention difficult to memorise and store for quick recall) become perfectly understandable when understood in the light of an evolutionary origin for the human species. Examples include extra nipples along the milk line [2], the inverted retina [3], the multiple  pseudogenes, retrotransposons, and endogenous retroviral elements we share with primates [4], not to mention the presence of non-coding intronic DNA, which leads to mutations at intron-exon borders that "often disrupt premRNA splicing in ways that alter gene products and lead to countless genetic disabilities, including various cancers and other metabolic defects." [5]

I am not the first to allude to Dobzhansky in reflecting on the increasing importance evolution has for both understanding the basic clinical sciences on which medicine is based, and for the practice of medicine itself. Ajit Varki, Distinguished Professor of Medicine and Cellular & Molecular Medicine at the University of California,  San Diego wrote in 2012 a paper with the same title as this post where he states that the 'scientific aspects of medicine are rooted in understanding the biology of our species and those of other organisms that interact with us in health and disease. Thus, it is reasonable to paraphrase Dobzhansky, stating that, 'nothing in the biological aspects of medicine makes sense except in the light of evolution'" and proceeds to outline his experience in teaching evolution to medical students. [6] Given that there are more than 4.7 million hits in Google Scholar when searching for evolution and medicine, Varki's desire (and that of many other physician and scientists) to integrate evolutionary biology in the teaching of medicine is eminently sensible.

Endogenous Retroviruses and the Evidence for Common Descent

Overview

Common descent has not been an issue in the mainstream scientific community for over one hundred years. The case for common descent, which was considered solid in the early 20th century, has now become overwhelming based in no small part on the evidence from molecular genetics. The evidence is overwhelming when it comes to endogenous retroviral inclusions. ERVs are remnants of prior retroviral infections that have become integrated into the DNA of organisms. If they pass into the germline of an organism, its descendants can inherit them. Their presence is clear evidence of past infection in an ancestral organism.

Common descent would predict that the descendants of a species infected by a retrovirus, which subsequently became integrated into the germline, would inherit that retroviral inclusion at the same point in the DNA of the descendant organisms. We find evidence of this in organisms ranging from primates to sheep to crocodiles. It is difficult to imagine a more comprehensive demonstration of common descent, particularly when closely related species such as humans and chimpanzees share significant numbers of ERVs at the same point in their genome.

One analogy that should drive home this point is that of a mathematics teacher who receives ten exam papers that not only get the same questions wrong, but show the same errors in the working of the problem, and even share the same spelling errors at the same questions. Copying is the only reasonable conclusion, as it stretches credibility to assume that the ten students independently got the same questions wrong, made exactly the same mistakes in derivation and made the same spelling error. This is similar to what we see with closely related species with shared ERVs at exactly the same point in their genomes.

The following article should serve not only as an introduction to retrovirology for the layperson, but point out in detail the evidence for common descent from ERVs, examples where evolution has co-opted ERV components to perform new functions, and refutations of common evolution denialist arguments against the evidence for common descent from ERVs.

This review is large, but no apology is made for this. Shared ERV elements in related species is overwhelming evidence for common descent, which is why creationists, who are unable to refute this evidence raise objections which while superficially appealing to the layperson do not pass the critical scrutiny of the scientists whose professional life involves working with them. Dismissing the molecular evidence as “circumstantial evidence” is a merely an attempt to evade the burden of proof expected of anyone who opposes a consensus view. If one challenges a long-established position, one needs to understand what one us challenging intimately, and produce hard evidence for critical review by the scientific community. Otherwise, if that is not done, no one will take such attempted rebuttals seriously - and rightly so. No creationist refutation of this evidence from shared ERV elements exists - the consensus view that they support common descent remains unchallenged.