Despite an obvious focus of cancer as a medical phenomenon affecting human lifespan, cancer occurs across multicellular life. We argue that cancer research could benefit from moving from considering other species’ cancers as mere models of those of humans to embracing the differences across species, as these dictate the logic of natural selection and its ability to “see” cancer as a relevant problem in an organism’s ecology and life history. Simultaneously, cancer should be incorporated more strongly in evolutionary thinking itself. At the origin of multicellularity, there is a gray zone between offspring production and cancer, and the association between sexual reproduction and a unicellular stage in a metazoan life history could prove interesting in this context. The idea that links sex with possibilities to discard “faulty” products of cell divisions is particularly clear in a basal metazoan, the hydra. In larger species with clearly differentiated tissues, there is much to gain from investigating the coevolution of senescence and cancer robustness: prolonging the lifespan of cell lineages (e.g., via telomerase) can be counterproductive for the lifespan of the entire organism, and organisms that live long and are now miniaturized compared with their ancestors (such as birds) should show great promise as study species.