In this so-called RNA World—where RNA reigned supreme—the molecule known for its role as a messenger and intermediary would have needed to accomplish a much broader array of activities. Breaker has zeroed in on RNA's need to monitor the available nutrients in its environment. Survival for all organisms requires them to conserve nutrients and shut down cellular activities that are not needed—otherwise they starve, says Breaker, an HHMI investigator at Yale University. "To do that," he adds, "RNA would have had to sense its own surroundings."

Illustration: Christian Northeast