Over the past century, the average human lifespan has doubled. That astonishing statistic is the subject of a new book and PBS series by acclaimed science writer Steven Johnson called “Extra Life: A Short History of Living Longer.” In this episode, he tells Rufus about the renegades who shamed milkmen, spiked public reservoirs, and rode rocket-powered sleds — all in the name of science. They discuss how inventions like vaccines, seatbelts, and sewers made the world a safer place. And they peer into a future where aging might be a thing of the past.