“Why are we afraid of things that look so much like us, yet somehow seem not quite right? Evolution has no accidents. The most likely reason we react this way is that, at some point in the past, there was a natural enemy of humanity. They looked almost exactly like us, and behaved no differently from us...”
“Humans who lacked the ability to recognize these creatures were wiped out over the long course of evolution after being hunted in great numbers. The survivors passed down, generation after generation, the genes that etched this fear of humanlike creatures into us...”
“But what is despairing is that a few heart-stopping accidents in my otherwise ordinary life have convinced me that this isn't just a possibility. It is the truth...”
“What's more, humanity's natural enemies don't just exist in the past. They have always been here. They are evolving, becoming more and more like humans, blending into human society, putting down roots and having children. They appear no different from ordinary people, but inside, they are still inhuman...”
“By now, they have evolved to an even more terrifying degree. They look so much like humans that even they believe they are human. At this point, the genes we evolved to recognize outsiders have stopped working against their true targets...”
Remember, you must be wary of those who are plain, dull, stiff, and ordinary. Don't let them notice you. Keep your distance. They are our true natural enemies, humanity's most terrible nightmare, and the humen who are about to devour us all...”
It's great, I'm looking forward to what happens next.
It's well-written and has a great feel to it.