Some cancers became lineages that outlive their hosts
In rare cases, the transmissible agent is the cancer cell itself. Canine transmissible venereal tumour and several bivalve leukaemias consist of living clonal cells that pass between animals, evade rejection and keep evolving. The cancer lineage can survive the death of each individual host.
Most cancers arise from a host’s own cells and cannot establish themselves in another animal. Genetic markers showed that tumours sampled from dogs on different continents were more closely related to one another than to the dogs carrying them: they descend from one ancient cancer clone. In soft-shell clams, neoplastic cells likewise carry genotypes distinct from their current hosts and nearly identical across locations. Transmission routes differ—direct contact in dogs, seawater-associated spread in bivalves—but both turn a somatic lineage into an evolving parasite.
An organism’s evolutionary future usually travels through reproductive cells. These cancers found another route: the body cell became the transmissible lineage. They expose immune recognition as an ecological barrier and show that individuality is maintained by mechanisms, not guaranteed merely because cells share a species.
Within one host, cancer cells compete for resources and escape controls. A transmissible cancer adds selection between hosts: variants that survive transfer, avoid allogeneic immunity and reach new animals can persist. Over time, the lineage accumulates mutations independent of any host germ line. It is still derived from its original species, but its evolutionary interests are no longer identical to those of the individual carrying it.
Foreign cells usually fail because they are fragile outside a body, lack a route into new tissue and display immune markers that reveal them as non-self. Low genetic diversity, unusual mating behaviour, injury, seawater dispersal or immune modulation can weaken those barriers. The rarity is informative: transmission needs several ecological and immunological doors to align, not merely an aggressive tumour.
Are these really contagious cancers rather than viruses that trigger similar tumours?
The transmitted cells carry the nuclear and mitochondrial genotype of the original tumour lineage, not that of the new host. Tumours from separate animals cluster genetically with one another, and their integration sites or chromosomal signatures recur. A virus can cause cancer, but it would leave each tumour composed mainly of the newly infected host’s transformed cells. The strongest version of the objection is therefore a testable alternative, not a semantic re-description.
Showing that each tumour’s cells match its current host while only a pathogen is shared would replace the clonal-cell transmission account.