We've found dinosaur remains with cancer. It's also not one thing,it's many, many things and varies from easily treatable to untreatable depending on the type and location and person it's in.
How old is cancer? Is this worse?
These are marks left in us by diseases our ancestors were exposed to. Could range from a cold up to a potential for why humans bottle necked at various points. (Last isn't terribly likely, but an example.)