Philip Ball has an interesting article in last week’s issue of Nature entitled Why a person does not evolve in one lifetime.
Of course he notes that evolution is really happening in our organism and at least in one case (the immune system) evolution happens to our advantage. The point is nonetheless, that our organism has been evolved to avoid evolving itself (with the mentioned exception), or at least, to make this somatic evolution as unlikely as possible. The way to do this is by means of stem cells and transient amplifying cells.
This hypothesis was tested computationally by John Pepper and colleagues when working at the Santa Fe Institute. If every cell in the organism is in charge of making sure that there are enough cells of their own type then you don’t need many alterations to have cancer. The route is a bit more complicated. Stem cells are in charge of making sure that there are enough cells of a given type. They divide very slowly and have been evolved to be especially immune to mutations. They divide creating a copy of themselves and a transient amplifying cell. These TACs divide in a way that the offspring is different from the mother so even if they mutate, given that the offspring is different, it is unlikely that their faulty behaviour will be inherited and spread in a tissue.
Based on these assumptions the computer models show that even though energetically wasteful, this system minimises significantly the possibility of cancer.