William Shatner

From imdb trivia

Amongst the names originally mooted to play the two human leads were James Caan, Lee Majors, Elliott Gould, Jon Voight and James Franciscus. However these all proved to be either unavailable during the filming dates or too expensive to hire. The then-42-year-old William Shatner (Captain Kirk from Star Trek (1966)) was also seriously contemplated by the studio but the producers struggled with ideas as to who to cast alongside him. They eventually decided to cast two younger relative unknowns so as to make sure Yul Brynner remained the star name in the film. Coincidentally, Star Trek creator Gene Roddenberry's wife Majel Barrett appears in this film.

Peter Graves and Leslie Nielsen were both considered for the role of John Blane (alongside William Shatner as Peter Martin) before it was decided to cast younger actors. Coincidentally, all three would appear in Airplane! (1980) and its sequels.