The certainty of death was absent at the origin of life. Unlike humans and other mammals, many organisms do not age and die. The process of programmed, inevitable death evolved only after our symbiotic microbial ancestors, some two billion years ago, became sexual individuals.

I propose that it did so as an accidental consequence of a desperate strategy for survival. Sex began when unfavorable seasonal changes in the environment caused our protoctist predecessors to engage in attempts at cannibalism that were only partially successful. The result was a monster bearing the cells and genes of at least two individuals (as does the fertilized egg today).

The return of more favorable environmental conditions selected for survival those monsters able to regain their simpler, normal identity. To do so, each had to slough off half or more of the "extra" cell remains. Death and the genes that caused death evolved.