Here is a nice Science News article on contagious cancers. Apparently, a second version of the Tasmanian Devil cancer has evolved. Contagious cancers are know in 3 animals (Tasmanian Devils (there're kind of cute!), dogs, and some clams and mussels). A contagious cancer is a cancerous cell that acts as a normal cancer in one animal but can cause cancers in other individual animals (same species, except the clams and mussels) if some how transferred to them. This can involve fighting (Tasmanian Devils), sex (dogs), or through a shared watery medium (the ocean for clams and mussels). In vertebrates (including dogs, tasmanian devils, and humans), the immune system would normally cause rejection (killing) of any cells introduced from a different individual of the same species due to mismatches among the tissue typing genes (HLA genes) the cells express. These same genes are what makes transplantation difficult in human medicine. Drugs are needed to suppress the immune system. It has been hypothesized that these cancers can more easily arise in inbred populations where there would be less molecular diversity in the HLA genes. Inbred populations in mouse genetic lines was one of the reasons the first inbred mouse lines were made. Inbred lines allow transplantation to be made among the mice of one line (HLA genes alike) but not to individuals of a different line (HLA genes different). After overcoming this initial hurdle, the cancerous cells may acquire other protections against the immune system (mutations protecting from immune detection) which could allow them to infect more immunologically diverse individuals. Cancers can also be spread among humans by transplantation. In this case, however these cancers cells are usually dependent on anti-rejection drugs are given to the patient to suppress the immune system rejection of the transplant. The drugs allow cancers cells from a different individual to thrive. Drug removal can usually take care of the cancer but could also threaten the transplanted organ. Thus screening transplantation donors for cancers is important. Clams and mussels are molluscs (invertebrates) and do not have the vertebrate's adaptive immune system, they only have an innate immune system. An innate immune system uses molecules and cells that have been evolved over long periods of time against well established bodily threats. An adaptive immune system combines a random generation of diverse antibody molecules with a developmental system where the immune system learns that what is present early in its development is "supposed" to be in the body and should not be attacked and anything else should be killed. I find it interesting that the contagious cancer in dogs is hypothesized to be 11,000 years old (being passed down through the generations from dog to dog) because it brings into question its place in life and evolution. In many ways it is like a parasite (recently derived from vertebrates) that has evolved in parallel with (and parasitic upon) dogs for 11,000 years. Is it a different species? Its genome has presumably changed quite a bit, although it would maintain many similarities with its dogian (canine) ancestors.