Nobody knows how they came into existence, the organisation of the Roman army being shrouded in the mists of time. Polybius talks of the centurions admiringly as men who had proved their worth in battle, were good leaders of men (i.e. leading aggressively by example), and they had to be able to read and write. Hence, they were probably of good Roman citizen stock originally, whether from the city proper or the surrounding countryside. They would have had to be able to afford at least the minimum panoply of legionary arms and armour to qualify as an infantryman in the legion, so this suggests that they could not have risen from the very lowest class. However, with the post-Marian and the Imperial legions the rules change. I would assume that the later centurions would have been a mixture of social types if the army adhered to the ethos of only selecting the best men as described above. Illiteracy would not have been a problem because the later professional army provided a rudimentary education and it is known that very lowly 'peasants' later rose to become generals, so why not centurions.

The Roman army lasted for such a long time that it is difficult to say what the particular conditions were at any one time, or what pertained over much longer periods.