There's no substantial evidence that poor, lower class grunts were ever promoted to the rank of Centurion. More often they were middle class people, or upper middle class, which seems to make sense, considering these people could have afforded some education, thereby being better able to grasp tactics, discuss tactics with superiors before battles, and display said tactics to their men.
So despite the common image, in reality they were much more like the junior officers than sergeants, in their role and their background.
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