How do you think people -thousands of years ago discovered that the roots, bark, leaves etc of some plants have certain health benefits?

Yes like finding which mushrooms are edible and which ones are magic.

Yes or digging up the root of a plant (ginseng) and find it gives one energy to keep going before starving to death. 

Laboratory testing -probably funded by Big (capitalist) Pharma- shows that many herbs, roots et al have no kind of benefits at all. This post was edited by Kittigate at November 15, 2017 11:10 AM MST

Actually, willow bark has salicylic acid in it, which is a pain reliever and a starting reagant for aspirin.