2001: A Space Odyssey (1968)
The final script removed this explanation, but it is hinted at when HAL asks David Bowman if Bowman is bothered by the "oddities" and "tight security" surrounding the mission. After Bowman concludes that HAL is dutifully drawing up the "crew psychology report", the computer makes his false prediction of hardware failure. Another hint occurs at the moment of HAL's deactivation when a video reveals the purpose of the mission.
In an interview with Joseph Gelmis in 1969, Kubrick stated that HAL "had an acute emotional crisis because he could not accept evidence of his own fallibility".
Something that had been intentionally ignored is that the earth's atmosphere is extremely thin and would not diffuse the image of the surface. If the planet were the size of a basketball the atmosphere would only be as deep as two sheets of paper. As a result, wherever there is no cloud cover, the details on the surface would be exceedingly sharp, even from the moon. In the context of this film, that was not desirable, as it would call undue attention to the earth whenever it is in the background. This is why the planet was painted in gradients of solid colors.
From that point, at which time we discover the next object, which would indicate our readiness to go interstellar and meet the source of the monolith. The strange ending, in Bowman's living quarters, indicates that the level of development of the aliens was so advanced that the only way to interact with them was in the context of familiar objects and surroundings to which humans were able to relate. Not only were they four million years beyond us, but even back then their advancement was so great that they were able to travel between the stars and place the "sentinels";
As the premise developed into this movie (on which the 2001 novel would be based), the monolith came to represent a metaphor not unlike that of a toddler who grows to the point that it can touch a doorbell. Its presence serves to compel the toddler to seek the next level of development, although it is not known what that next level may be. Clarke himself likened The Sentinel to an acorn and 2001 to a tree that was nurtured from multiple sources.