Apologies to anyone who has already heard my thoughts on this – because this isn’t the first time.
Years ago when I read Playboy magazine, and I won’t lie here, I didn’t read it just for the articles, though I did often read the articles, in 1977, if memory serves, they had an article entitled Kill Them and Eat Them.
If you’ve read this far, then you probably already know the gist of the article. It was a sort of tongue-in-cheek-but-sort-of-serious essay to answer this very question, “What should we do if we discover alien visitors to the planet Earth?” The answer was contained in the title of the essay, but the essay described why that would be the best policy.
In essence, as others have already noted, any civilization with the technology to A) discover that we actually exist and then B) mount a “manned” (for varying definitions of “man”, obviously) mission to visit and contact us and C) actually make that contact in some unambiguous way as to say, “Here we are, bitches. Talk to us,” (I’m extrapolating on the language a bit; we didn’t speak like that as a rule in 1977, even in Playboy magazine.) would be so far in advance of our technology that there would be nothing for us to offer in the way of commerce and trade. (What do we trade with ants, for example?)
So the only benefits, if any, would be one way: We might be food or we might be pets, but we would not be equals. So the best response that we could make to this advanced civilization, in whatever numbers they chose to send our way (because we have to assume that they would not mount some kind of interplanetary mass colonization effort right off the bat) would be, specifically, to kill them and then eat them.
The point we would have to make to survive as a species (and presumably as the “master” representative of other Earth species) is to be so entirely violent and disgustingly, unwaveringly unwelcoming that we would not even be marked for later follow-up; we’d just be crossed off the list of places to visit, marked as “dangerous, do not attempt landing” on the charts, and bypassed for eternity (we might hope).
I’m thinking that future human interplanetary explorers should be given a copy of the essay, too, as a sort of warning of what kind of reception they might expect from a sapient and capable alien culture on their home planet, in case the roles are ever reversed.
It might help if they brought along a copy of Playboy, too.