The radiocarbon work of Jonathan Haas "et al.", found that 10 of 95 samples taken in the Pativilca and Fortaleza areas dated from before 3500 BC; the oldest, dating from 9210 BC, provides "limited indication" of human settlement during the Pre-Columbian Early Archaic era. Two dates of 3700 BC are associated with communal architecture, but are likely to be anomalous. It is from 3200 BC onward that large-scale human settlement and communal construction are clearly apparent. Mann, in a survey of the literature in 2005, suggests "sometime before 3200 BC, and possibly before 3500 BC" as the beginning date of the Norte Chico formative period. He notes that the earliest date securely associated with a city is 3500 BC, at Huaricanga, in the Fortaleza area of the north, based on Haas's dates.