All cancer evolves—cells multiply and the ones that are better able to survive and grow in the body will increase in number—but in humans this evolution usually has a short timescale. The unique transmissible cancers in clams and other species live far beyond a single host body—for hundreds or even thousands of years. This long-term evolutionary interaction allows us the opportunity to understand the selective pressures which drive evolution of cancer cells, which may enable us to identify previously unrecognized cancer-host interactions or pinpoint vulnerabilities in cancer cells.