Time Undetermined: Alien travelers make frequent visits to Earth to study the local population and are greeted as gods by the primitive inhabitants of that world. On initial visits, these travelers have free contact with the natives, allowing them to see their landing craft and interact with the crew.
When subsequent visits begin to report a contamination of Earth culture from these visits, with inhabitants building standing stone circles, pyramids, geometric lines and other objects visible from a great height, apparently to encourage more contact, the visitors begin to use a more circumspect approach.
Some investigators who know of these visits believe the aliens altered humanity to have the capacity for psychic ability. While others maintain the ability itself is the reason for contact.
There is definitely ancient visitation in Prometheus, and even the Grover's Mill incident gets a mention.