This new year, researchers concluded that 2/3 of the difference in cancer risk between different parts of the body can be attributed to the number of stem cell divisions those parts undergo. More cell divisions reflect a higher risk as errors that occur naturally during the DNA replication process can contribute to the development of cancer. In other words, the same genetic mutability that enables evolution also ensures that many people will be afflicted by a terrible disease. On Pharyngula, PZ Myers suggests this is one reason our cells naturally get old and stop dividing: because if they continued forever, too many mutations would accumulate in the individual.
Of course, mutations are rare and unpredictably distributed, and not all of them are dangerous, making who gets cancer largely a matter of chance. The new study shows which cancers are most influenced by lifestyle factors such as using tobacco. PZ writes,"colorectal and lung cancers do have a significant risk beyond what can be accounted for by stochastic errors, so pursuing a reduction in exposure to risk factors, like diet and smoking, can have a useful role in reducing the incidence of these cancers." On the flip side, the incidence of pancreatic cancer (for example) can be totally accounted for by random mutation.