Thursday, 9 April 2015

Evolutionary medicine (or why nothing in medicine makes sense except in the light of evolution)

There are many reasons why I accept the fact of evolution. One of the most powerful reasons for me as a doctor is that the evidence for evolution from human anatomy, genetics, and developmental biology is beyond rational dispute. Add to that the considerable utility evolution has for rational medical practice, and the case for the importance of evolution for medical studies is rock-solid. Evolutionary geneticist Theodosius Dobzhansky famously said that nothing in biology makes sense except in the light of evolution. The same can readily be said about medicine.

Emily Thompson, writing at the Panda's Thumb blog notes how the importance of evolutionary biology for medicine is increasingly being recognised:

In his keynote address at the inaugural meeting of the International Society for Evolution, Medicine, and Public Health (held March 19-21 in Tempe, AZ), Dr. Harvey Fineberg stated that an understanding of evolution is central to health. Fineberg, the former Dean of the Harvard School of Public Health, argued that an evolutionary viewpoint is necessary to explain structures and functions of the human body (like the fact that wisdom teeth were helpful in some way to our ancestors but serve no purpose now) and evolution can provide insight into diseases that develop and spread under evolutionary mechanisms, like infectious disease and cancer.

Antimicrobial resistance occurs when bacteria, viruses, and other infection-causing microorganisms evolve and develop mutations that enable them to resist drug therapies. Drug-resistant bacteria alone affect over two million Americans each year, according to the CDC. The process of microbial evolution follows the guiding principles of natural selection, so scientists can use their knowledge of evolution to understand how microbes attain resistance and perhaps even prevent it. For example, the current methods of treating bacterial infections target a mechanism of mutation called de novo mutation, but scientists have learned that antibiotic resistance mostly develops from a different method called horizontal gene transfer (Sterns, 2012), which suggests that we may need new therapies for bacterial infection. 

Evolutionary medicine has also started to play a role in cancer research. Some scientists are using an evolutionary background to understand how cancers develop, spread, and metastasize as well as to find effective treatments. For instance, a group of scientists is trying to understand how large animals with long lifespans, like the blue whale, have evolved and developed cancer suppression techniques that are reportedly 1000 times better than those of humans. Many hypotheses attempting to explain this phenomenon exist: the lower metabolic rate of large animals might lead to a lower mutation rate, or perhaps tumors are so much bigger in large animals that they are actually less likely to become malignant than smaller tumors (Nagy et. al., 2007). Whatever the explanation, understanding why and how large animals evolved to gain such effective tumor suppressor mechanisms could provide new therapies for cancer in humans. (Caulin and Maley, 2012.)
This does not mean that your average surgeon needs to understand or accept evolution, any more than an electronics engineer needs to understand quantum physics or quantum electrodynamics in order to design monolithic microwave integrated circuits. However, when you ignore evolution in your medical practice in areas where an awareness of evolution is critical, such as microbiology or transplant medicine, the consequences can be lethal, as the case of 'Baby Fae' in which heart surgeon Leonard Bailey transplanted a baboon heart into an infant with a significant cardiac abnormality:

Baby Fae was not the first human to receive a primate xenograft. In a review of xenografts, the Council of Scientific Affairs of the American Medical Association noted a rapid rejection of all baboon transplants to humans. Nevertheless, Bailey claimed that the problems of rejection could be overcome by the "immature" state of an infant's immune system. After the operation, immunologists from around the world pointed out that the part of the immune system that rejects unmatched transplants is fully mature at birth, Furthermore, there is no way to match baboon hearts to human recipients, because baboons have no antigens in common with human tissue. Bailey has always maintained that Baby Fae's death was unrelated to the species of the organ "donor." An editorial in the Journal of the American Medical Association called Bailey's claim "wishful thinking."
Bailey's use of baboons was somewhat surprising, given their relatively distant evolutionary relationship to humans compared to other primates. The reason came to light when the Times of London published an interview between Bailey and an Australian radio crew. The reporters had been forbidden to ask direct questions about the operation, so they queried Bailey on the issue of why he had chosen a baboon in view of the baboon's evolutionary distance from humans. Bailey replied, "Er, I find that difficult to answer. You see, I don't believe in evolution."6 It is shocking that Bailey ignored basic biological concepts in formulating a life-threatening human experiment. (Emphasis mine)
The full Panda's Thumb article is here.