 The first large Maya cities developed in the Petén Basin in the far south of the Yucatán Peninsula as far back as the Middle Preclassic (c. 600–350 BC), and Petén formed the heartland of the ancient Maya civilization during the Classic period (c. AD 250–900).The 16th century Maya provinces of northern Yucatán are likely to have evolved out of polities of the Maya Classic period.From the mid-13th century AD through to the mid-15th century, the League of Mayapán united several of the northern provinces; for a time they shared a joint form of government.