2001: A Space Odyssey's monolith. I think the way the monolith is so clearly an artifact where there should not be one is part of the relationship it has in each encounter.
It is a communication device in both the scene with the hominids and the uncovering it of it on the moon. It is not clear what was imparted to the hominids, but it sends a traceable signal to the outer solar system after the moon discovery. Whatever its purpose, it is acting as a lure some kind to both groups.
In case of the moon discovery, it is also a 'motion sensing' device. Informing its maker that the project was showing results.
To the extent that encounters with the monolith has 'made' us into something, the crisis with the AI named HAL show us another collision of the natural with the artificial.