Well - I don't discount aliens scouting out this planet, for any number of reasons.
The one that has stuck with me for 20 years was when a work buddy and I happened to see some Piranha's being fed in a downtown Aquarium shop window, there were about 10 of them in the tank and the owner dumped 4 goldfish into the tank and the carnage was pretty extreme. I remember as he was watching, he said in all seriousness, this will be exactly what it will be like for us when the aliens take over.