Lets take the lymphocyte system we all have. Wich is spread out throughout the body. What if that started as something bad for the body. But as generations past by it got more sophisticated and developed into what we know today.
Its not a theory i have, but its just a new way of thinking about this problem.
As it would take very many generations for it to evolve.
One way of ruling out this arguement of mine would be to prove that people get cancer in different areas of the body, and arent syncronous for every case. Meaning the cancer dont appear in the same places on every human that gets cancer.