“I remember the chain of thought,” he said. “I had to do prehistory, so I have to do some archaeology. But to do it seriously, I’m going to talk about how humans evolved, so, yikes, I’m in biology now. I thought: To do it seriously, I have to talk about how mammals evolved, how primates evolved. I have to go back to multicelled organisms, I have to go back to primeval slime. And then I thought: I have to talk about how life was created, how life appeared on earth! I have to talk geology, the history of the planet. And so you can see, this is pushing me back and back and back, until I realized there’s a stopping point — which is the Big Bang.” He paused. “I thought, Boy, would that be exciting to teach a course like this!”