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Thomas Stossel was lucky he didn't know how difficult phagocytosis would be to figure out. “If I'd known how complicated it was, I might have gone another way,” he says. In the 18th century, some of the first observations with

optical microscopes had shown that cells engulf food and slither along by turning part of their cytoplasm into a semi-solid gel, and then liquefying it again. Stossel and his colleague John Hartwig (both at Harvard University) wanted to know what controlled this gel–sol transformation.

At the time, the discovery that nonmuscle cells contained actin and myosin was fresh. But what the pushy proteins accomplished was uncertain—researchers had just discerned that actin helps form the contractile ring that pinches cells in half during division (Schroeder, 1972). Stossel and Hartwig started by nabbing a...