It's true that cancer is made of human cells but it can still evolve on it's on. Evolution applies to anything that can reproduce while making small changes between generations. Cancer cells do this better than healthy ones, at least within the time scale of a single human life. In some species cancers have even evolved to become contagious and a single tumor has spread to thousands of hosts. https://en.wikipedia.org/wiki/Canine_transmissible_venereal_...