In the abstract below they refer to people who built something in the 400-650 AD time frame as 'ancient humans' what is the commonly accepted dates for determing if a culture is ancient or not? ...Fall of Rome?

W.J. Rink & J. Bartoll. 2005. "Dating the Geometric Nasca Lines in the Peruvian Desert," Antiquity 79: 390-401

Abstract: The Nasca lines are geoglyphs-- arrays of stones forming geometric shapes constructed by ancient humans, the largest ones occupying areas of more than 1 km^2. The authors used optically stimulatd luninescence dating of quartz buried when the stone lines were constructed to give a new date for contexts associated with geoglyphs on high mesetas near Palpa. They conclude that the stone lines at sites at San Ignacio and Sacramento were constructed between AD 400 and 650. this suggests that they were made in the late part of the Early Intermediate Period of the people of the Nasca culture.