Marian reforms
The "adsidui" were, as opposed to the "proletarii", eligible to serve in the legions. However, along the structural evolution of the Roman army, the "census" levels necessary to be an "adsiduus" were gradually lowered. In 107 BC, Gaius Marius enrolled the landless capite censi in his army. These Marian reforms are generally taken as the last step in the evolution of the army, allowing any Roman citizen to become a legionnaire, though some have argued this was a one-off.