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POPSThe evolution of drug abuse "All this shows “our ancestors were regularly exposed to plant neurotoxins,” they added, so the view of our brains as unsuspecting victims of the new chemical threat is untenable. " " One possibility, the scientists suggested, is that animals co-opted some plant toxins and used them for their own defenses against parasites. If this is true, then evolution, the process by which species adapt and change to meet environmental demands, might have designed our brains to encourage some drug use. This could involve shaping our brains to associate drug intake with feelings of reward."