<p>Ageing and cancer are ubiquitous in animals. They are fundamental and generally intrinsic to multicellular life. Nonetheless, ageing and cancer rates vary widely across species and understanding their evolution and interaction is of great biological interest. Although cancer arises from uncontrolled cell proliferation, ageing involves cell loss and degeneration, making them seemingly opposite yet interconnected processes. Because cancer can affect young individuals, natural selection will favour the evolution of cancer resistance over processes that maintain health in later life. As such, I propose that species evolve longer lifespans under the constraints imposed by the need to reduce cancer risk. Mechanisms that suppress cancer, such as telomere shortening and cellular senescence, may inadvertently promote ageing by limiting cell proliferation and tissue regeneration. Selection for tumour suppression may also impact stem cell ageing and contribute to the limited ability of adult tissues to regenerate. Overall, although cancer resistance is essential for the evolution of longevity, tumour suppression mechanisms may also contribute to ageing-related tissue degeneration and functional decline. Studying the trade-offs between the evolution of tumour suppression processes and their impact later in life may provide important insights into ageing processes.</p>