It's Not an Accident that Aspirin Kills Pain

 So, salicylic acid, which is the plant-based equivalent to aspirin, is produced by most plants. And now we know why.

Plants produce more salicylic acid when they are under environmental stress.

And we're starting to think plants may in fact feel pain.

So, plants evolved a natural chemical to regulate reactive oxygen species (things like sunburn) and now we copy it to treat our own stress-related aches and pains.