Did a tree's bark really turn into aspirin?

Yes, but it took about 2,400 years and a fair amount of chemistry. Willow bark contains salicin, which the human gut turns into salicylic acid, and people have used willow for its health effects for at least that long. Clay tablets from ancient Sumer and an Egyptian papyrus mention willow medicines, and Hippocrates described a tea to bring down fevers around 400 BC.

In the eighteenth century, an English clergyman named Edward Stone noticed that willow bark tasted bitter, just like the cinchona bark that was used in Peru against many ailments. He tried powdered willow on people in Chipping Norton for five years and reported to the Royal Society in 1763 that it worked as well as the imported bark and was cheaper. That was a kind of home-made experiment: a hunch from a taste, then a test on people.

The trouble was that the usual salicylate medicines irritated. In 1897, chemists at the German firm Bayer were looking for a less irritating version, and Felix Hoffmann, or perhaps his colleague Arthur Eichengrün, made acetylsalicylic acid in a pure, stable form. By 1899 Bayer was selling it worldwide as Aspirin, a name built partly from Spirsäure, the German word for meadowsweet, another plant that holds salicylic acid.

The strange part is the order of events. Aspirin was on sale for about 70 years before anyone knew how it worked. Only in 1971 did John Vane show that it switches off the production of prostaglandins, the chemical messengers behind pain, fever and inflammation. This lesson is where the story of modern medicines starts: a plant remedy, a chemist improving it, and a long wait before the mechanism is understood.