As part of a continuing series of my wild speculations on virtually anything:

Doesn’t it seem possible, given its extreme prevalence in the human population, particularly among people past their reproductive years, that cancer is not some weird mutation of the cells, but in some sense a natural state, perhaps even one that preceded the more familiar restrained, cooperative behavior of cells in our bodies? Think about it: Richard Dawkins has already suggested that organisms may die for the most part not too long after they become incapable of reproduction because, after they cease to be fertile and their offspring are raised, there is not much adaptive value in their continued existence (although they may still be of help in raising the young in a group, especially their grandchildren), so there is not a strong selection pressure weeding out the fatal ailments that afflict them from the gene pool. Cancer is one of the major forms of these post-reproductive afflictions (though it of course hits the young sometimes as well). What I want to add is that, from what I know of cancer, it seems to make cells behave more like they would as single-cell or very simple organisms: they reproduce as fast as possible and try to wipe out all the other cells in the vicinity, as if they were competitors. Now for basically independent cells this strategy seems sensible from an evolutionary standpoint, at least until they start to overpopulate an area. But in a complex multicellular being, where individual cells cannot survive on their own and rely on other cells around them, reproducing onself at the expense of others can be fatal. So my hypothesis is that a form of cooperative behavior between the component cells of large organisms during the period of life in which they reproduce and/or raise offspring proved successful enough to replace the old competitive behavior, but that, among old organisms, the old behavior has been able to hold on and reassert itself simply because there is not much selective pressure against it. Of course environmental factors can induce, or at least favor cancer’s onset, but the very frequency of its appearance in the population seems to indicate its centrality to our genome. Perhaps there is even a positive evolutionary value in its killing off older organisms, insofar as it frees up resources for the younger and more vital generations. Just think how sustainable our biosphere would be if everyone lived to be 500 or older (not that I think such a project to be remotely feasible, but medical scientists who take as their goal the indefinite prolongation of human life might want to keep this issue in mind. While the world birthrate (except in a few places like Africa) has fallen enough to disappoint Malthusian fears, an ever-growing army of the near-undead could still prove them right).