Recent excavations in the Soconusco region of the state indicate that the oldest civilization to appear in what is now modern Chiapas is that of the Mokaya, which were cultivating corn and living in houses as early as 1500 BCE, making them one of the oldest in Mesoamerica. There is speculation that these were the forefathers of the Olmec, migrating across the Grijalva Valley and onto the coastal plain of the Gulf of Mexico to the north, which was Olmec territory. One of these people's ancient cities is now the archeological site of Chiapa de Corzo, in which was found the oldest calendar known on a piece of ceramic with a date of 36 BCE. This is three hundred years before the Mayans developed their calendar. The descendants of Mokaya are the Mixe-Zoque.