Many snakes have almost perfect camoflage and without movement are very difficult to detect. I have not heard about human visual acuity not being able to pickout snake movement but there does seem to be an almost instinctual fear of snakes.

Certainly early 'religious' symbology seems to be concentrated on dangerous animals but I have wondered about the facination with snakes.

If we look at the array of predators in middle stone age Africa there were numerous species that could easily kill early man but most of those species actually benefited early man because they left a carcass which could be scavanged. The python (and crocodile) were different because they left no carcass for humans to scavange.