If you could send one piece of knowledge back to the Stone Age, what would it be? Assuming you’re making this decision for the good of humankind rather than scheming to make yourself rich, your answer will likely depend on your definition of human progress.

You’ve got options. You can right a historic wrong. Introduce life-saving technology centuries early. Expedite a social revolution of your choice. You should try to offer something concrete and practical, but forget about everyday gadgets: What’s the point of a smartphone or ventilator if the tools and knowledge required for production are still millennia away?¹ Better than a product is an idea. Press fast-forward on humanity’s greatest discoveries, and offer a head start on flight, radio or antibiotics. Or you can introduce ...