Magic Forums

This may sound silly, but i was thinking about scary movies the other day. Then i thought to myself, why are we so scared of things in the shadows, and beings with sharp teeth?
Then i was in anatomy class learning about dreams, more specifically nightmares, and why we have them. A man in the documentary i was watching said that he believed the visions we have during a nightmare are the ones of our ancestors. For example lets say we are being chased by some strange being, our brain cant really imagine up new beings, just like every person you see in your dreams you've already seen them before.
It just makes me wonder what is out there, or what was out there in the time of our ancestors, what do you all think?