According to gladiator documentary on the “Gladiator” DVD, it was originated by some early gladiator film director (specific person not named) and his impression of the signal from the emperor (or other games sponsor) for the victor to kill or spare a yielding fighter. The gesture was described in Roman literature, but not very well.
Although early gladiator flicks brought the thumbs up-thumbs down motif widespread fame, it was preceded by (and believed derived from) French painter Jean Leon Jerome’s 1874 gladiator painting “Pollice Verso” (literally, “Thumbs Down”).