The botany of desire,? M.PollanCan't remember the title, but it was a book that also covered the history of potatoes and tulips.
At the time, no one had found the "reason" for THC or resin on cannabis, not knowing its function in how it helped or protected the plant. The conclusion was that the plant produces it because people like it. Because we like it, we grow more than it would on its own. Evolutionary purpose of THC: human pleasure.
Something like that could probably never be scientifically validated--or denied.