If it's true that the aliens were created by the Space Jokeys, then it means that those eggs came from something, and that something must have been a "pure" alien, I think. I there was a "pure" alien, but after that, there were only hybrids. By the way, the facehugger isn't a parasite, because, he doesn't depend on the host. The chestburster, I agree to be a parasite, because it can't live without a host, but, I think, when a facehugger "grabs" a human, the human becomes, a little like a parasite, because, a human once hugged by the facehugger, will die if it will be removed