The Aztec Native American Human civilization existed in the southern regions of North America (an area that would later include the nation-state of Mexico) on Earth from the 14th, to the 16th century.
Kukulkan, a benevolent alien being, visited ancient Earth civilizations and influenced their architecture and agricultural development. Kukulkan was worshiped as a god and knowledge of his visit was passed on to the Aztecs, who still remembered him in their culture. Kulkulkan was discovered by the crew of the USS Enterprise in 2270. (TAS: "How Sharper Than a Serpent's Tooth")