The earliest complex civilization in Mexico was the Olmec culture, which flourished on the Gulf Coast from around 1500 BC. Olmec cultural traits diffused through Mexico into other formative-era cultures in Chiapas, Oaxaca and the Valley of Mexico. The formative period saw the spread of distinct religious and symbolic traditions, as well as artistic and architectural complexes. The formative-era of Mesoamerica is considered one of the six independent cradles of civilization. In the subsequent pre-classical period, the Maya and Zapotec civilizations developed complex centers at Calakmul and Monte Albán, respectively. During this period the first true Mesoamerican writing systems were developed in the Epi-Olmec and the Zapotec cultures. The Mesoamerican writing tradition reached its height in the Classic Maya Hieroglyphic script. The earliest written histories date from this era.  The tradition of writing was important after the Spanish conquest in 1521.