Ancient Roman spectators in the Coliseum did use their thumbs to show their decisions on whether a losing gladiator should live or die, but not in the manner we see expressed today.
It was the movies that gave us the simple “thumbs up or thumbs down.”
The thumb symbolized the weapon of the victor.
“Up” meant “lift your sword and let him live.”
But if the verdict was death, then the thumb was thrust forward and downward in a stabbing motion.