The Olmecs were the earliest known civilization in Mexico, known for giant carved stone heads. They flourished from 1400 BC to 400 BC before disappearing. The Aztecs ruled central Mexico from 1100-1522 AD, building cities like Tenochtitlan. They had a complex religion involving human sacrifice. They were conquered by the Spanish in 1521. The Mayas occupied the Yucatan peninsula from 1800 BC to 900 AD, developing an accurate calendar, hieroglyphic writing, and advanced math. They built great stone cities before their decline.