Roman sculpture began as imitations of Greek styles but developed into distinctive Roman genres like portraiture and narrative relief. Republican portraits emphasized stern, dignified features to depict prominent citizens. Imperial sculptures evolved from realistic portraits of emperors into idealized, god-like images. By the 3rd century AD, as the empire faced crisis, portraiture took on more emotional, mystical qualities, reflecting the spread of mystery cults. Under Constantine, the first Christian emperor, sculpture became more symbolic and used older works as decoration.