Cover Story: This week’s cover is on the promise of human enhancement. Today’s boosters of superhumanity crave immortality and godlike intelligence. But among the quackery and snake oil is a more down-to-earth promise of chemical, biological and technological improvements that could add years and powers to ordinary human lives. Just now, progress is being held back by an approach to researching, testing and approving treatments that sees enhancement as fundamentally different from remedies for disorders and disease.