DNA
With regards to aliens - i see the Bible as exactly historical document of such an unplanned/unauthorized contact by an alien observer gone rogue. Contains all elements, like medical nanobots, [food]replication, specific lessons that violence is the main obstacle of civilization development, blue light sterilizing the cave with decontamination workers in white overalls and some devices on their backs (angels :), etc... Hunt for the lost sample of the alien's DNA - Holy Grail- which also contains those nanobots.
The probes reached their replication limit, then went into hibernation when they phoned home, and subsequently failed to receive a reset signal. Or perhaps they simply propagate too slowly. Imagine if the Morris worm spread only half as fast as the growth of the Internet itself.