lolz, lovely story. THC is not the only cannabinoid. Endocannabinoids, those which are produced in the body, are the ligands with which our cannabinoid receptors developed through evolution. Not to mention, cannabinoid receptors are common to many mammals, birds, fish, and reptiles. So as much as I would like to believe it, Mary Jane didn't have that significant a role in human evolution.
Maybe not a huge impact, but I think it had some. It makes sense for us to not be the only ones that have it, since we share some (yes, some) genes with every living thing on this planet. Maybe Cannabis evolved THC because we, or some humans, already had the ability to use the Cannibinoids it produces. When humans discovered fire they began experimenting with burning different things, and with this came smoking. They(the first humans) discovered cannabis plants, which I mentioned earlier may have coincidentally had trace amounts of THC that our brains could use. They liked it so they began picking and chosing the types of Cannabis with higher THC content. Of course they didnt know that there was a chemical called THC, but they knew that certain plants got you higher quicker and for longer. So we basically began to "naturally select" the Cannabis plants that had higher THC content(as we do now). So THC got stronger and our ability to use it did as well. THC induced visions that could have helped those early religions get off the ground, or even helped produce those basica animistic traditions I was talking about earlier. Man began spreading its seeds everywhere. So THC is very beneficial for the Cannabis plant as a species or it would have been tossed aside like other "non-useful plants" that man has caused the extinction of over the years. This is just the kind of shit I think about when I am stoned, so there you go. And this doesn't just apply to Cannabis, although I think it applies more to it than any other plant. Take corn for instance, Corn is not a naturally occuring plant in the way we know it today, Native Americans practically genetically engineered it(through generations of selective breeding). Thats not co-evolotion but it is human caused evolution. The same way with all the dog breeds that are in existence now, but this is getting off subject and I am getting sleepy.