Cancers are dramatic examples of multi-level selection. Because cancers can damage and kill their hosts, there has been selection at the organismal level to suppress cancer. However, natural selection at the cellular level generates cancer. I will illustrate these two levels of selection with two stories from our research. I will first tell the story of our investigations into why elephants and whales don’t get more cancer than humans, even though they have 100X to 1000X more cells than us. I will then discuss why it has been so hard to develop biomarkers that predict whether a benign tumor will become cancerous and whether a malignant tumor will kill a patient. I will show that measures of evolution and ecology can provide better biomarkers that should be universally applicable across all types of cancers.