Cancer comprises a group of diseases characterised by the uncontrolled proliferation of abnormal somatic cells in the body. It is considered a major health challenge in humans, and its global burden is projected to increase in the coming decades. However, cancer is not unique to humans, and it is hypothesised to have appeared alongside multicellularity. Therefore, it should be discussed and studied as an inherent pathology for all multicellular organisms. Guided by this evolutionary perspective, we highlight key research avenues through which marine ecosystems could improve our understanding of cancer biology and support the development of improved prevention and therapeutic strategies. In particular, marine ecosystems (1) harbour animals with some of the most extreme life history traits, thereby highlighting how natural cancer suppression might have evolved; (2) harbour animals exposed to some of the strongest oncogenic environmental contaminations, thereby providing natural systems to investigate how pre-existing anticancer mechanisms shape cancer susceptibility, and (3) harbour the most diverse set of transmissible cancers in marine bivalves, allowing us to explore mechanisms underlying inter-individual differences in resistance and disease spread within and between populations. These systems provide unique opportunities to uncover how cancer resistance evolves across deep evolutionary timescales. They also present opportunities to investigate naturally evolved anticancer mechanisms and bioactive compounds that could inspire biomimetic approaches to cancer prevention and therapy. By integrating evolutionary biology, marine ecology, and oncology, marine systems could become a powerful frontier for discovering fundamental principles of cancer resistance and exploring their potential relevance for biomedical innovations.
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