  Another type of “byproduct” explanation of cancer—though one that appeals to a much more distant event in our evolutionary history—is that cancer is a product of the emergence of multicellularity. At one point in the very distant past, single celled organisms formed collectives that cooperate; these collectives eventually became multicellular organisms. Any collective of cells, especially collectives whose survival and reproductive success depends on functional organization, are potentially vulnerable to breakdown in cooperative organization. On this view, then, cancer is a product of breakdown in mechanisms that permitted the emergence of multicellularity—the mechanisms that protect us from “revolt from within” are not error-free, and over time will fail. According to Greaves and others (Greaves, et al. 2012; Merlo, et al. 2006) the evolutionary picture of cancer dovetails nicely with the somatic mutation theory; somatic cells divide and acquire mutations during our lifetimes; some of these mutations involve failures in regulatory pathways that ordinarily “enforce” functional organization, and thus cooperation. In this way, an evolutionary perspective—understanding how evolution of multicellularity required the emergence of cooperative organization—is essential to understanding cancer. Moreover, on this view, cancer itself may be viewed as an evolutionary process—the emergence of adaptive features of cancer cells, where these “adaptations” enable short term “fitness,” or relative success at survival and reproduction. Several scientists have developed theoretical models of this process, linking it to empirical data, e.g., on the emergence of chemotherapy resistance (Frank 2007; Wodarz &amp; Komarova 2015).