Why Do We Get Sick? - The New Science of Evolutionary Medicine

Evolutionary medicine identifies six reasons why our bodies are vulnerable to disease. They are: 

1) defences,

2) environmental mismatch, 

3) trade-offs, 

4) conflict between survival and reproduction, 

5) co-evolution with infectious parasites 

6) constraints on natural selection. 

This explanatory framework yields insights about everything from cancer to why we age and die.

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Constraints on natural selection

Natural selection is powerful, but it’s not omnipotent. There are limits. For example, natural selection can only work with the raw material it has; if there is no genetic variation for a given trait, there’s simply nothing that can be done. Natural selection is also constrained by history, or what’s sometimes called “path dependence.” Because of the precursor from which our visual system evolved, we are stuck with the suboptimal design of having a blind spot in both eyes. Evolution is a tinkerer, not an engineer, so it can only proceed in a gradual step-by-step fashion, and each step must be a definite improvement over the last. There’s no such thing as going back to the drawing board and starting from scratch the way an engineer might. Because of this historical inertia, we’re stuck with the inelegant setup of the trachea and the oesophagus, which poses a serious choking hazard. Evolutionarily, where we can go next is limited by where we’ve already been and the existing body plan that we’re saddled with. These and other constraints limit the power of natural selection.

Since its inception, medicine has focused almost exclusively on the “how” questions of disease, largely setting aside the “why.” By tackling the missing why question—why our bodies are vulnerable to disease in the first place—evolutionary medicine supplies an exciting new layer of understanding.

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Evolutionarily informed cancer research is still in its infancy, but even so, it has already led to many new findings, and it promises more insights and practical applications over the next few years. As researchers Athena Aktipis and Randy Nesse note,

An evolutionary approach can help us understand why cancer exists and how it progresses (somatic evolution), how cancer cells interact with environments (ecological approaches), why it is not more common (natural selection for cancer suppression mechanisms), and why cancer suppression mechanisms can never be perfect (constraints, trade-offs, and other evolutionary reasons for vulnerability to disease). Evolution is essential for understanding cancer.

And It’s Not Just Cancer

Evolutionary medicine extends far beyond cancer and illuminates a wide range of health conditions. The science of why we get sick offers new insights into cardiovascular problems, tooth disease, blood iron deficiency, breastfeeding, pregnancy and miscarriage, pain, Alzheimer’s disease, aging and senescence, sepsis and even psychological disorders. We are only in the beginning stages of a revolution that is already transforming how we think about medicine, health and disease.

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