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.[196]