What's more, without modern medicine (and sanitation and nutrition) people with cancer would tend to die earlier, or perhaps of secondary factors. If you're some occasionally malnourished farm hand (read: the vast majority of all individuals who lived in pre-industrial times) and you get, say, leukemia you are going to die of whatever the fuck disease happens to be going around, and it's going to happen quick. The result is that there may be less evidence of cancer in, say, ancient bones but not due to decreased incidence of cancer per se.
There is a line of reasoning (and evidence in mice) that the body seems to be designed to operate in starvation mode (e.g. calorie restriction). One line of thought behind this is that food abundance results in signalling along certain metabolic pathways that have not experienced heavy evolutionary pressure.
