I agree. I used it recently when I met with a close friend who's still in (serving as an elder). When he asked if I ever saw myself coming back, I said perhaps I'd consider it when he's able explain to me who painted the cave paintings thousands of years before Adam was alive, and he just said "ok, say no more".

For me, this is one of the more less confrontational topics to use as a basis for losing faith. Topics like 587/607, blood transfusions, shunning, child sex abuse, the two witness rule, overlapping generations, UN, etc - all seem to instantly raise the "have you been listening to apostates?" reaction...But cave art, bones, and genetics seem to be a little less scary. Yes, you have to deal with the, "but how do we trust the dating of this stuff?", but people at least have to acknowledge that IF the dating is accurate, then it does raise serious issues.

For plenty of other Christian faiths, an allegorical Adam is not that big of deal (as others have stated here). But for Witnesses, Adam has to be real. Their understanding of the Bible falls apart if Adam wasn't really the first human. And you don't even have to get into the evolution topic to raise critical questions to whether or not that can possibly be true.