Have you seen those 'Ancient Aliens' shows? Bear with me, I'm not a believer in that stuff, but it's SO interesting. (I do totally love alien movies, though.) According to the people on that show, these aliens basically started civilization for whatever reason, and there are all these carvings and murals and stuff showing "aliens" giving humans technology and knowledge, etc. What if aliens gave us this knowledge of agriculture? Or, I don't know... sometime in the future, we come up with a way to travel back in time, and we introduce it to our ancestors? They're not "wands of power" the aliens are handing to the humans, but stalks of wheat. Way unlikely, but fun to think about at any rate.
I'm afraid I'm going to get down-voted for my response, but it's just something I've sort of daydreamed about. :/
And the grains back then were different from what we've got going on today, and people probably soaked and sprouted those grains to make them edible, so they didn't have as big of an irritating effect that grains do now. And no, eating a piece of wonderbread isn't going to outright get you HIGH, but it does have a sort of calming, relieving effect. Ever notice how much in a daze people are when they're eating that crap? Or do you remember when you zoned out eating something with gluten in it? It's obviously not like heroin; it's very, very subtle..but it's still chemically addicting, not just psychologically because you 'know you shouldn't eat it'. There are many reasons why we FIRST started eating grains, and one reason we KEPT eating grains is because they're addicting.