 Few pictorial manuscripts (or codices) of the Maya, Mixtec and Mexica cultures of the Post-Classic period survive, but progress has been made particularly in the area of Maya archaeology and epigraphy.The presence of people in Mesoamerica was once thought to date back 40,000 years, an estimate based on what were believed to be ancient footprints discovered in the Valley of Mexico; but after further investigation using radiocarbon dating, it appears this date may not be accurate.It is currently unclear whether 23,000-year-old campfire remains found in the Valley of Mexico are the earliest human remains uncovered so far in Mexico.