The story of cancer begins approximately 1 billion years ago, with the rise of multicellular organisms. Before that point, there was no cancer. Afterwards, cancer was the central problem threatening the integrity of the body. Cancer has become a particular problem in developed countries. In the U.S., men have a 45% and women a 38% lifetime risk of developing cancer. Despite the enormous progress we have made in technology and medicine in the last 50 years, we have made almost no progress reducing deaths from cancer, even when we take into account changes in lifespan.
Why is cancer such a difficult problem to solve? Because cells in tumors evolve. Tumor cells mutate a high rates and compete for space and resources like oxygen. Mutant cells that can reproduce or survive better than their competitors tend to spread in a tumor. Thus, tumors are microcosms of natural selection. By the time we detect a tumor in the clinic, it contains billions of cells carrying tens of thousands of mutations. By chance, some of those mutant cells are often resistant to the anti-cancer drugs we use. The result is temporary remission followed by relapse with a resistant tumor. Therapeutic resistance is so common that attention has recently switched to detecting cancer early in its development when it is still easy to cut out, or preventing cancer altogether.