Scientific opinion regarding human antiquity in Mesoamerica has reflected larger trends in conceptualizing human antiquity in the western hemisphere in general. Within that foundational topic, the establishment of sites demonstrating human antiquity in Mesoamerica has centered on the association of remains and artifacts with geologic strata (context) and on the reliability of the dating of the remains and strata (methodologies).
At the beginning of the twentieth century, humans were believed to be very recent post-glacial immigrants to the western hemisphere. Although there was believed to have been anywhere from 20,000 to 60,000 years of post-glacial time in which such immigration could have occurred over the Bering land bridge, the antiquity of human presence in the western hemisphere was popularly fixed at about 5,000 B.P. (3,000 B.C.). William H. Holmes and Ales Hrdlicka led this school of thought. The Folsom and Clovis discoveries of the 1920s and 1930s revised the minimum time frame for the initial occupation of the New World. The Folsom lithics dated to 10,000 - 11,000 B.P., and the Clovis lithics dated to 12,000 - 12,500 B.P., allowing for an original immigration date of about 14,000 B.P.
In Mesoamerica, the time period from about 9,000 B.P. back to the earliest occupations is referred to as the "Paleoindian" period. Evidence of human occupation in Mesoamerica consistent with that 14,000 B.P. original occupation date has been presented, debated, and accepted. Fluted points have been found north of Mesoamerica in the states of Sonora and Durango as well as in central Mexico, with proof of a mammoth hunt being uncovered at Santa Isabel Iztapan. Pleistocene-age bone artifacts have been found at Los Reyes La Paz. Human presence during this time period has been further documented by cranial finds at Peņa, Xico, Tepexpan, Santa Maria Astahuacan, and San Vicente Chicoloapan. A variety of methods were used to determine the antiquity of the cranial remains, including chemical bone analysis (nitrogen and fluorine tests), geological analysis (stratigraphic, carbon-14, and volcanic ash composition tests), contextual association with faunal remains, and contextual association with lithic artifacts dated by obsidian hydration.
The 14,000 B.P. immigration date as a temporal maximum has been challenged. Human presence has been claimed in northwestern North America by 30,000 B.P. Claims have been made for human presence during the 20,000 - 30,000 time frame at Pennsylvania's Meadowcroft Rock Shelter and in California's Yuha Desert. Claims of occupation during the 20,000 - 30,000 period have also been made regarding sites in South America, Central America, and Mesoamerica.
A bone artifact from Tequixquiac may come from a pre-projectile point horizon. Evidence from Tlapacoya suggests human occupation dating to 23,000 B.P. Valsequillo has five sites that appear to date from at least 20,000 B.P. Based upon increasing evidence for an earlier antiquity for human presence in the western hemisphere, in 1976 Irving Rouse and Richard S. MacNeish independently published proposals revising western hemisphere lithic stages, allowing for human occupation as early as 30,000 B.P. and leaving open the possibility of an even earlier initial arrival.