Leonard Nimoy’s death in February brought to a close his unusual career continually playing a single role for half a century. Between 1966, when the television show Star Trek premiered, and 2013, when the movie Star Trek Into Darkness hit the screens, Nimoy portrayed the franchise’s beloved first officer, Mr. Spock, in two TV series and eight films.

As he acknowledged, the key to Star Trek’s longevity and cultural penetration was its seriousness of purpose, originally inspired by creator Gene Roddenberry’s science-fiction vision. Modeled on Gulliver’s Travels, the series was meant as an opportunity for social commentary, and it succeeded ingeniously, with episodes scripted by some of the era’s finest science-fiction writers. Yet the development of Star Trek’s moral and political tone over 50 years also traces the strange decline of American liberalism since the Kennedy era.