Michelangelo often sculpted himself. The one that stands in the Duomo of Siena is a very early work of his. How can you tell? In his later self sculptures, he always put a dent in his own nose because he broke it at some point in his life. This one is obviously early because he hadn't broken his nose yet!
Being one of the best sculptors in the world at the time, Michelangelo was always getting dragged places to carve sculptures and make beautiful art for important people. During his career, he was always trying to go home to Florence, but the Pope kept dragging him back to Rome and telling him to "finish what you started." Poor guy! He was always getting shipped places and he always owed someone money.