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Meaning of the monolith

For some readers, Clarke's more straightforward novel based on the script is key to interpreting the film. The novel explicitly identifies the monolith as a tool created by an alien race who have been through many stages of evolution, moving from organic form to biomechanical, and finally achieving a state of pure energy. These aliens travel the cosmos assisting lesser species to take evolutionary steps. Conversely, film critic Penelope Houston wrote in 1971 that because the novel differs in many key aspects from the film, it perhaps should not be regarded as the skeleton key to unlock it.

Carolyn Geduld writes that what "structurally unites all four episodes of the film" is the monolith, the film's largest and most unresolvable enigma. Vincent LoBrutto's biography of Kubrick says that for many, Clarke's novel supplements the understanding of the monolith which is more ambiguously depicted in the film. Similarly, Geduld observes that "the monolith... has a very simple explanation in Clarke's novel", though she later asserts that even the novel does not fully explain the ending.

Bob McClay's Rolling Stone review describes a parallel between the monolith's first appearance in which tool usage is imparted to the apes (thus 'beginning' mankind) and the completion of "another evolution" in the fourth and final encounter with the monolith. In a similar vein, Tim Dirks ends his synopsis saying "he cyclical evolution from ape to man to spaceman to angel-starchild-superman is complete."

Humanity's first and second encounters with the monolith have visual elements in common; both the apes, and later the astronauts, touch it gingerly with their hands, and both sequences conclude with near-identical images of the Sun appearing directly over it (the first with a crescent moon adjacent to it in the sky, the second with a near-identical crescent Earth in the same position), echoing the Sun–Earth–Moon alignment seen at the very beginning of the film. The second encounter also suggests the triggering of the monolith's radio signal to Jupiter by the presence of humans, echoing the premise of Clarke's source story "The Sentinel".

The monolith is the subject of the film's final line of dialogue (spoken at the end of the "Jupiter Mission" segment): "Its origin and purpose still a total mystery." Reviewers McClay and Roger Ebert wrote that the monolith is the main element of mystery in the film; Ebert described "the shock of the monolith's straight edges and square corners among the weathered rocks", and the apes warily circling it as prefiguring man reaching "for the stars". Patrick Webster suggests the final line relates to how the film should be approached as a whole: "The line appends not merely to the discovery of the monolith on the Moon, but to our understanding of the film in the light of the ultimate questions it raises about the mystery of the universe."

According to other scholars, "the monolith is a representation of the actual wideframe cinema screen, rotated 90 degrees... a symbolic cinema screen". "It is at once a screen and the opposite of a screen, since its black surface only absorbs, and sends nothing out.... and leads us... to project ourselves, our emotions."