Cancer happens for a lot of reasons. It happens because cells in the body evolve to proliferate out of control. It happens because our evolved cancer suppression systems aren’t perfect. And it happens because many of the exposures we have in our modern world - like sun, smoking and even too many calories - overwhelm our body’s ability to suppress cancer.

Our cancer suppression mechanisms evolved for a world that is not the world we live in today. This mismatch between our ancestral environment (in which our cancer suppression systems evolved) and the current environment that we live in is part of why we get cancer. It is not the whole story - there are other important factors like tradeoffs,1 constraints,2 and the inevitability of evolution among cells in the body3 - but modern exposures are an important part of the story as well. They don’t explain why we are susceptible to cancer in the first place (it’s because we are multicellular,4 in case you’re wondering), but modern exposures are part of the reason why rates of some cancers - like skin, lung, breast and gastrointestinal cancers - are particularly high.

If you have very light skin, it’s likely that your ancestors came from northern latitudes where there was little sun exposure. We need a certain amount of sun exposure for our bodies to function normally - to synthesize vitamin D and other important nutrients - and so populations that lived in northern latitudes evolved light skin that allowed sun in. Light-skinned individuals who live in or visit areas that are close to the equator are at higher risk for skin cancer5 because their skin lets in more radiation from the sun that can damage DNA and spur on cellular evolution in the body that can eventually result in cancer.

Smoking is another modern exposure that increases cancer risk. Smoking increases cancer risk because the smoke contains chemical carcinogens that can damage DNA, leading to more mutations. There are other chemical carcinogens in our modern world that contribute to cancer: asbestos, arsenic, some flame-retardants, waste from some manufacturing processes.6 But smoking is the big contributor of the chemical carcinogens - heavy smoking increases cancer risk by about 4 - 40 times.7

We eat much more and exercise much less than our ancestors ever did. This means that we have a huge amount of excess calories that our bodies have to store and manage. When we have higher body fat, this alters our physiology. It raises our hormone levels, alters our metabolism and insulin regulation and can increase inflammation. All of these processes can contribute to increased cancer risk. Eating lots of meat8 and simple carbohydrates9 is associated with increased risk of cancer and regular exercise is associated with lower cancer risk.10

Modern reproductive patterns also contribute to breast cancer risk. In hunter-gatherer populations women typically start having children around age 18, have 5 children and wean them around age 3.11 This is very different from modern populations where women typically start having children at age 26, have an average of 1.86 children and typically wean them before 6 months of age.12 Our ancestors probably had reproductive patterns similar to modern-day hunter gatherers and therefore had far fewer menstrual cycles than we modern humans have. Modern reproductive patterns like these are associated with higher risk of hormone positive breast cancers.13

Another modern exposure that may contribute to cancer risk is - somewhat ironically - lower exposure to infectious diseases during childhood. If the immune system doesn’t get the stimulation from diverse microbes in the first years of life, it can end up overreacting when it does encounter an infectious agent; sometimes with devastating consequences including childhood leukemia. Studies from around the world have found that day care attendance in infancy (which is considered a good proxy for infectious disease exposure in early life) is associated with lower risk of Acute Lymphoid Leukemia.14

So, is cancer a disease of civilization? Yes, in that cancer risk is shaped by many distinctly modern exposures like smoking. And no, because our susceptibility to cancer is simply part of being a multicellular organism made of of cells that can evolve inside the body.4 Cancer happens because cells in our bodies evolve and because our cancer suppression systems aren’t perfect. So, cancer is partly a disease of modernity, but it is also an ancient disease: one with roots that go back deep into our evolutionary history as multicellular organisms. Nevertheless, there is a lot we can do to reduce our risk of certain cancers by change our lifestyle and exposures. And a lot of it comes down to simple advice that your mom has probably been giving you for decades: Eat well and not too much, exercise, don’t smoke, and maybe go outside and play in the dirt every once in a while - just don’t forget the sunscreen.


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