Can Darwin’s theory explain why we get sick?

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While physicians care for their individual patients and are interested in the factors contributing to the individual’s condition, evolutionary biologists investigate the cause of the biological phenomenon, trying to understand for example why a certain disease has developed in a particular species such as our own Homo sapiens. By combining the view of physicians with the view of evolutionary biologists, the field of evolutionary medicine has the potential to improve our understanding of certain diseases and how to prevent them.

This article was authored by Christina Takke from V-Bio Ventures.

Lessons from history

What can history teach us about identifying new treatments for current and future diseases? Since Charles Darwin published his ideas on evolution in On the Origin of Species (1859), our understanding of biological processes has increased dramatically. The basic principles of evolution by natural selection, as described by Darwin, are still actively used by biologists today in trying to understand the world in which we live.

As a refresher, evolution by natural selection occurs when there is an disequilibrium between the reproductive potential of a population (i.e. the number of offspring a certain organism can generate) and the environmental resources available (e.g. food, space, mating partners, etc.). In these circumstances, individual organisms in a population must compete for survival and reproduction.  Evolution occurs when differing individuals leave behind a different number of offspring that are able to go on to reproduce. Traits which predispose individuals to survive to reproductive age are transmitted more frequently to the next generation through their offspring and are therefore positively selected for.

People often confound the term “survival of the fittest” with “survival of the healthiest”, where this is not necessarily the case. The only parameter on which evolution by selection takes place is the number of fertile offspring an individual organism can produce. In humans, for example, the heritable disorder Huntington’s disease is not usually selected against, despite being a deadly disease. This is because the onset of symptoms typically doesn’t occur until a person is in their 30s or 40s, often after they have already had children. Therefore, the condition persists in the population despite the lethality of it.

Although one might think that historical selective pressures should have caused us to evolve defences to most weaknesses and conditions, evolution’s prioritization of reproduction over health and wellbeing has actually left humanity vulnerable to a range of conditions and diseases. By incorporating aspects of evolution into medical research, this perspective can provide us with new insights into the development of specific diseases and therefore potential methods of tackling them.

Evolutionary medicine can answer the “why” questions

Why do diseases develop and persist in humans? Let us use chronic obstructive pulmonary disease (COPD) as an example. COPD is a common disease and causes millions of annual deaths worldwide. Recent studies have revealed that the heritability of COPD is 37%, meaning that an individual with a genetic predisposition for COPD has a 37% chance to pass that predisposition on to their offspring. Why did humans acquire this genetic trait in the first place, and why has such a negative genetic predisposition persisted?  The evolutionary biologist perspective can provide some insight to both these questions.

In a 2015 publication, Aoshiba et al. discuss some intriguing reasons why COPD developed in humans, based on current evolutionary medical theories.

i) Evolutionary constraints and the inability to correct a basic design flaw

Since humans have a long generation time of about 20 years or more, we are unable to rapidly adapt to “fast” changes like general uptake of tobacco smoking or increased air pollutants, the leading causes of the historically recent disease COPD. Although inhaled particles are detrimental to our health, our low number of offspring and long generational cycles has not led to any real shift in our physiology in the past few hundred years to better cope with this selection pressure. As a point of comparison: it took the human population about 10,000 years to acquire the widespread ability to digest cow’s milk, an ability that was absent in most of our ancestors and is still absent in many cultures today where milk products were not traditionally part of the general diet.

Read this previous BioVox article for more on the interplay between evolution and behaviour, regarding the impact of changing diets on human diseases.

ii) Co-evolution of the immune system and microorganism

Given our long lifespan and low number of offspring, we experience a higher frequency of repeated invasion by the same microorganisms as compared to shorter living animals.  This increases the need for a better immune system, especially in the airways which are frequently exposed to microorganisms. This active immune system in our lungs, which is key to ward off invasion by microorganisms, is also activated by cigarette smoke and other airborne substances. Scientists have also identified that airway inflammation by viral pathogens causes a similar biological reaction as COPD. The system which has protected us for millions of years is now also causing us harm.

iii) Life history trade offs

According to Darwinian theory, traits that lead to more surviving offspring are naturally selected for and passed on more frequently to the next generation. This means that traits that are beneficial to the health of young organisms are positively selected for, even when they are detrimental to the health of the elderly. This seems to be the case for COPD, where strong, reactive immune systems increase the survival rate of the young and are therefore kept in the population despite causing issues for older individuals. Another gene variant associated with a genetic predisposition to COPD is one which increases protease activity. While high proteolytic activity is a genetic risk factor for COPD, it is advantageous for the removal of microorganisms and also promotes reproduction chances by helping sperm penetration of the zona pellucida of the ovum at fertilisation. Therefore, a trait which increases the chances of COPD nevertheless persists and even flourishes in the population. Other examples of reproductive success at the expense of health can be found in oncology, where people with higher testosterone or oestrogen levels are generally more fertile, but also more susceptible to prostate and breast cancer.

Looking at the bigger picture

It goes without saying that evolutionary medicine does not give us the answers how to treat or diagnose a disease in a specific patient. It is also ethically impossible to experimentally prove the effect of natural selection and the trade-offs made during evolution in humans. However, by looking outside the box and considering the evolutionary perspective, we can generate new understandings and insights on a range of human afflictions.

When diagnosed, many patient’s ask their physicians the difficult question: why me? Evolutionary explanations can help shed some light on this otherwise unanswerable question, hopefully helping patients to better cope with their personal situation. A deeper understanding of the development of diseases can also help us find new solutions to these healthcare challenges.