He invented a new species.
No one had ever played an alien like this before. Nimoy didn’t do it exactly ex nihilo, there was a script by Roddenberry and a few predecessors to the humanoid alien type. But only a few. Most aliens were either monsters or humans. Nimoy played a human-looking, but very subtly (but somehow definitely) alien.
There are shades of Vulcan in The Day The Earth Stood Still. Michael Rennie was distant and detached, much like Spock. But he was really stepping down to be on the human level. Spock wasn’t quite as superior.
Nimoy’s tenure as Spock is noteworthy. He was the only actor to appear in the original pilot episode of Star Trek and go on to the regular series (excepting Majel Barret who played a different character). He was also the only of the original series’ cast members to appear in the reboot movie from 2009 (again, excepting Majel Barret, who played Nurse Chapel in the show and the computer voice in the movie).
Spanning 45 playing the same character, Nimoy not only invented a new species, he sustained it. There’s something quite poignant about a man incarnating his art for a lifetime. He entered the story, eventually writing/directing/plotting the best chapters of the film series (having a hand in the plotting of Wrath of Khan, writing/directing Search For Spock and The Voyage Home, plotting The Undiscovered Country.)
Hats off to Nimoy and Happy Birthday.