Charles Darwin famously puzzled over how to explain altruistic behaviors in animals and humans: Entire castes of social insects dedicate themselves to the good of the colony but are themselves sterile, while humans who save the lives of others might not live to pass on to their children such noble inclinations. How, he wondered, did such selflessness evolve in a world that seemed to overwhelmingly favor fitness above all else? Historians and philosophers have done much work to trace how scientists have sought to come to grips with altruism and its ugly twins, selfishness and cheating, over the course of the nineteenth and twentieth centuries. This paper looks at more recent studies that rely on the social amoeba Dictyostelium discoideum (better known as slime mold) and their bacterial symbionts to investigate altruism. The paper is especially concerned with how slime mold scientists study the phenomenon across different time scales—through genetics, experimental evolution, and game theory. It seeks to place this work in the social and cultural context of the history of altruism by asking what work complex moral concepts (such as selfishness, cooperation, and cheating) must do when applied to the lowly slime mold.
