The Maya were once considered one of the greatest civilizations in North America. They built pyramids and temples to honor their gods and to preserve their religion. The cultural and educational achievements of the Maya came centuries before other cultures. These achievements still exist today along with the Mayan culture, which has spanned over two thousand years. The Mayan people of today still hold these traditions sacred and want to preserve them. Only about two million Mayan Indians exist today, but their culture reflects that of their ancestors, along with the Spanish, who invaded the Maya in the sixteenth century. Archeologists who have studied many Mayan sites trace the Mayas back ten thousand years when their ancestors migrated from Asia to the Yucatan peninsula and northern Central America. The history of the Maya is divided into three major time periods: preclassic (2000 BC - AD 300), classic (AD 300 - AD 900), and postclassic (AD 900- AD 1500). Early Mayan settlements date back to twenty four hundred BC, but few traces of Mayan culture before AD four hundred have been found. In the preclassic era of Mayan history, corn was farmed and the early Mayans laid a base for their culture, which was believed to have been influenced by the Olmec Indians. The very first hieroglyphics were written, and cities started to appear. The early Mayan economy was based on agriculture and the exchange of farm goods. The Maya grew Indian corn, or maize. It was a staple food of many Indians in Central America for centuries. The Mayans developed the slash-and-burn farming method. A Mayan farmer would clear the cornfield by cutting bushes and girdling trees, and then he would allow the piled brush to catch fire under the sun. The ashes were then scattered among the stumps of the trees, and a sharp stick called a mattock was used to poke holes in the ground for the seeds to be laid. This method was used for centuries and it made farming the basis of the Mayan economy.