Imagine if the ability to program customized self-reproducing bacteria becomes as ubiquitous as computer programming.
The gray goo scenario. Or worse. Actually, the gray goo scenario starts to look pretty cheerful compared to what could happen.
Lucky Neanderthals. They only had to worry about being eaten by a sabre-tooth tiger. Look at what we can worry about in the modern era. Puts a whole new aspect on "proliferation."