Here is an interesting article on a theory by Geoffrey Miller that the brain evolved as a sexual display, similar to a peacock’s tail.  His idea is that we evolved the brain to help us attract a mate.  He has some interesting experiments showing that men are more likely to make conspicuous purchases (such as fast cars) or donations when they’re in a romantic mood and that women are more likely to volunteer (but only in visible settings).  Apparently this means “that what women want in a partner is material support while men require self-sacrifice”.  It’s certainly interesting that these behaviors may have evolved to help us attract mates.  But making conspicuous purchases to attract a mate doesn’t mean the entire brain evolved for that purpose.