The Evolutionary Basis of Disease

Usain Bolt shutterstock_151187450_1024wMost of modern medicine focuses on how disease occurs – what affects our risk of getting sick and which diseases we are particularly vulnerable to. Evolutionary medicine, a relatively new field, looks further to consider why we get sick at all and why particular diseases exist and affect us in the way they do?

Simply put evolution is the process by which species change to become better suited to their environment1. A common misconception is to see evolution as a way of overcoming disease. While the idea of a society free of sickness is appealing, it will not be evolution that makes it possible. The picture is quite the opposite – there are a number of ways in which evolution can be held responsible for the presence of certain diseases in our population. A few are explained below.

Trade-Offs

Importantly evolution does not select for longevity or health, but rather for increased survival1. Traits like the ability to outrun predators would have been desirable in human ancestors, leaving us with the sprinting capabilities of Usain Bolt. In order for a trait to be selected it must be passed down to the next generation. So for evolution to act on a trait, it must be valuable before and/or during the reproductive years. Traits that are only advantageous after childbearing has stopped, cannot be selectively passed down through the generations2. And some characteristics that were advantageous during younger years could increase susceptibility to disease in later life. Trade-offs such as this prioritise early survival at the cost of later morbidity.

Cancer is an example of this. While we are seeing an increasing incidence in modern society, cancer is by no means a new disease. It arises when a DNA mutation in a single cell allows it to divide uncontrollably and invade its surrounding tissues. The propensity for mutation comes from a cell’s capacity to repair and regenerate DNA, which is needed to survive the younger years3. As modern populations live longer than ever, we can see the damaging side of this trade-off. In our ancestors, individuals would rarely have lived long enough to develop these cancers and hence their effects were negligible3.

metabolic_syndrome diagramMismatch

Modern diseases could also be due to the mismatch between our current lifestyles and those our ancestors evolved with1. Modernisation has drastically changed our diets, daily routine, social structures and reproductive lives. Metabolic syndrome fits this picture. It is linked to unhealthy and large diets that overload the body’s metabolism and the decrease in activity seen in modern lifestyles4. We eat far more food and burn far fewer calories that at any other point in our evolutionary history, and the effects of our sedentary lifestyles are seen in the increasing incidence of diabetes, heart disease and obesity1.

A further example is short-sightedness in children – caused by abnormal growth of the eyeball during childhood. Development of the eye can be affected by the light coming into it1. Children who spend a lot of time indoors may never develop normal eyesight. Myopia can be seen as a mismatch between our ancestral outdoor life and the indoor lifestyles of many children1.

Defence Mechanisms

Some features of disease are the product of our own defences that are taken to excess. One example would be fever as a response to infection. The increase in body temperature makes for an environment unfavourable to invading microbes but is normally tolerable to humans. However with too large an increase, fever can be dangerous or even fatal5. Fear works on similar principle. In appropriate situations it is vital in the recognition and avoidance of danger. However if inappropriately triggered or conditioned onto innocuous simulants it becomes a crippling mental condition – better known as a phobia1.

Considering where disease comes from is a vital part of understanding how to prevent and treat it. Looking back gives us a much better understanding of why it exists and why we cannot be rid of it – at least not naturally – and why it takes the form it does. Disease is a part of our evolutionary history and will no doubt be a part of our future too.

  1. Gluckman, P. D., Low, F. M., Buklijas, T., Hanson, M. a. & Beedle, A. S. How evolutionary principles improve the understanding of human health and disease. Evol. Appl. 4, 249–263 (2011).
  2. Jones, J. H. The Force of Selection on the Human Life Cycle. Evol. Hum. Behav. 30, 305–314 (2009).
  3. Merlo, L. M. F., Pepper, J. W., Reid, B. J. & Maley, C. C. Cancer as an evolutionary and ecological process. Nat. Rev. 6, 924–935 (2006).
  4. Edwardson, C. L. et al. Association of sedentary behaviour with metabolic syndrome: a meta-analysis. PLoS One 7, e34916 (2012).
  5. Nesse, R. M. & Williams, G. C. Why We Get Sick: The New Science of Darwinian Medicine. (Vintage, 1996)

Nicole Samuel is a 5th year student studying Medicine; a shortened version of this is printed in Issue 30 (Spring 2015)

Images:  Usain Bolt runs at the World Athletics Championships on August 17, 2013 in Moscow, Shutterstock; Metabolic Syndrome (Wikimedia Commons)

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