The United Mexican States, commonly known as Mexico is a federal constitutional republic in North America. It is bordered on the north by the United States; on the south and west by the Pacific Ocean; on the southeast by Guatemala, Belize, and the Caribbean Sea; and on the east by the Gulf of Mexico. Covering almost two million square kilometres, Mexico is the fifth largest country in the Americas by total area and the thirteenth largest independent nation in the world. With an estimated population of over 112 million, it is the eleventh most populous country and the most populous Spanish-speaking country. Mexico is a federation comprising thirty-one states and a Federal District, the capital city.
The earliest human remains in Mexico are chips of stone tools found near campfire remains in the Valley of Mexico and radiocarbon-dated to ca. 21,000 BCE. Around 9,000 years ago, ancient indigenous peoples domesticated corn and initiated an agricultural revolution, leading to the formation of many complex civilizations. Between 1,800 and 300 BCE, many matured into advanced pre-Columbian Mesoamerican civilizations such as: the Olmec, Izapa, Teotihuacan, Maya, Zapotec, Mixtec, Huastec, Purepecha, Totonac, Toltec and Aztec: Mexica, which flourished for nearly 4,000 years before the first contact with Europeans.
These civilizations are credited with many inventions and advancements in fields such as architecture, mathematics, astronomy, medicine and theology. The Aztecs were noted for practicing human sacrifice on a large scale. At its peak, Teotihuacan, containing some of the largest pyramidal structures built in the pre-Columbian Americas, had a population of more than 150,000 people. Estimates of the population before the Spanish conquest range from 6 million to 25 million.
At the time of Spanish contact, Teotihuacan was no longer occupied, although the site was well-known; the population of the Aztec Empire and its immediate predecessors had become centered on Lake Texcoco, also in the Valley of Mexico, where the island city of Tenochtitlan was founded in 1325.
In the early 16th century, from the landing of Hernán Cortés, the Aztec civilization was invaded and conquered by the Spaniards. Unintentionally introduced by Spanish conquerors, smallpox ravaged Mesoamerica in the 1520s, killing millions of Aztecs, including the emperor, and was credited with the victory of Hernán Cortés over the Aztec empire.[citation needed] The territory became part of the Spanish Empire under the name of New Spain. Mexico City was systematically rebuilt by Cortés following the Fall of Tenochtitlan in 1521. Much of the identity, traditions and architecture of Mexico were created during the colonial period.