Who professionalized the Roman army?

Gaius Marius is credited with introducing the professional army to Rome. Before his reforms the army consisted of part time volunteers with most of the officers from the wealthier classes because they were the only ones who could afford to equip themselves. These officers also brought their clients into the army as a duty to their patrons. Marius did away with this and opened up the army to all citizens, paid them, equipped them and changed their fighting divisions.