You're sitting around your fire at night, and you see something moving. It might be just a shadow, but it might also be a predator. Your brain says "Predator!", puts you into fight-or-flight, and your eyes resolve the shadows into the shape of a panther or whatever. And you get up and grab your spear.
There are two possibilities. One is, it's a panther, you stab it, you win, you get to pass along your genetic material because your brain saw "shadows" and made your eyes perceive "enemy." Or it's a shadow, but you saw it as a panther, so you're awake but no skin off your nose.
But if your brain says "Shadows, never mind, going back to sleep," and it's really a predator...well, you're fucked. And that's why we hear voices in the wind, see monsters in closets, and Jesus in tortillas. We're genetically designed by evolution to see patterns, specifically faces.
So when people tell you that They look human, just keep that in mind.
curse the darkness