Most Egyptians were probably descended from settlers who moved to the Nile valley in prehistoric times, with population increase coming through natural fertility. In various periods there were immigrants from Nubia, Libya, and especially the Middle East. They were historically significant and also may have contributed to population growth, but their numbers are unknown. Most people lived in villages and towns in the Nile valley and delta. Nearly all of the people were engaged in agriculture and were probably tied to the land. In theory all the land belonged to the king, although in practice those living on it could not easily be removed and some categories of land could be bought and sold.