Cancer, or at least the dysregulated proliferation of cells in some manner, is the natural fate of all multicellular organisms if they live long enough. Even plants get a form of cancer.
The remarkable thing is that multicellular life exists at all. For billions of years, all of life was single-celled organisms, and most of it still is. This is a more natural evolutionary state, since every cell may have descendants. As cells compete for survival, every cell has a potential evolutionary future. The most successful single-celled organisms are usually those that follow a straightforward strategy something like scarf up as many nutrients as possible from the environment, metabolize those nutrients as efficiently as possible, then make as many copies of myself as possible as quickly as possible.
By contrast, from an evolutionary perspective, multicellular life seems an odd strategy. In complex multicellular organisms, the billions or trillions of somatic cells have no descendants at all, they have no direct evolutionary future. The vast majority of cells in the organism sacrifice their individual procreative potential for the sake of a tiny number of germline cells (eggs or sperm in humans). This only makes sense because all cells in an organisms share the same DNA. The self-sacrificing somatic cells do have an indirect evolutionary future, because the successful passage of germline cells into the next generation means that identical copies of their own DNA proliferate.
In order for multicellular life to be feasible, the DNA incorporates complex regulatory mechanisms to control the growth of somatic cells to specific limited quantities in specific places for the benefit of the organism as a whole. Make a liver so big, then stop proliferating. Make a certain thickness of skin, then stop proliferating. Make are many leukocytes as necessary to protect the organism, then stop. And, from an evolutionary perspective, all of this is dedicated to increase the chance of the organism’s sperm or eggs making it into the next generation.
The problem is that the body of a multicellular organism is itself an evolutionary environment where cells are constantly replicating and acquiring new DNA mutations, either by routine copying errors or exacerbated by carcinogens and the bombardment of radiation. There are many redundant checks and balances to regulate the growth of cells for the good of the organism as a whole, but sooner or later some cells will acquire enough DNA mutations in key places (usually at least 6 mutations) to override all of the checks and balances. When this happens, the organism reverts to the “wild west” of evolutionary competition among selfish single cells. And the most “successful” cells in that environment are those that follow the ancestral evolutionary strategy: scarf up as many nutrients as possible from the environment, metabolize the nutrients as efficiently as possible, then make as many copies of myself as possible as quickly as possible. This is cancer.