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.