If you are talking feudalism, the only way a peasant could rise to nobility was to marry into it. In the Byzantine Empire, it would depend on what you called a peasant. A slave, the lowest in the social hierarchy had no chance at rising, unless they bought their freedom. Farmers in the Byzantine Empire could also marry into nobility. Justinian rose to power through ties with his uncle, who was a military commander. Justinian received a high quality education because of this, was able to influence his uncle, until he was appointed coemperor. Justin 1, Justinian's superior, was soon after killed, and Justinian then became emperor. So, yes. Justinian did rise from being a peasant.