Colonial records versus carbon
Most of what historians know about the Inca empire comes from Spanish accounts written after the conquest, and from records kept by Inca descendants in the decades that followed. Those sources set out a neat sequence of emperors and campaigns. They were also written by people with reasons to flatter or blame particular rulers.
Radiocarbon dating offers an independent clock. A new campaign reported in this week's issue of Science applied accelerator mass spectrometry to material from many sites along the Qhapaq Nan, the road network that linked the empire from what is now Colombia to central Chile.
What the dates show
The results are not a small correction. As Science summarised the work, the traditional Inca chronology has collapsed. The empire appears to have lasted longer than the colonial record allowed, and it met more resistance as it expanded than the written accounts describe.
The finding follows earlier work at Machu Picchu. A team led by Yale archaeologist Richard Burger dated human remains recovered during early twentieth century excavations at the royal estate of Emperor Pachacuti. Their accelerator mass spectrometry results, published in Antiquity, indicated the site was in use from about A.D. 1420 to about A.D. 1530, at least 20 years older than the accepted record suggested.
Burger argued that the discussion of Inca development based mainly on colonial documents needs revision. In his view, modern radiocarbon methods give a firmer footing than the historical records for understanding Inca chronology.
Why the road system made a good test
Roads are useful for dating because they were built, used and abandoned on a schedule that leaves physical traces. Samples taken at many points along the Qhapaq Nan let researchers compare settlements in different regions instead of trusting a single narrative.
If the network was finished earlier and held under more pressure than the accounts say, then estimates of how the Inca state taxed labour, moved goods and controlled provinces also shift. Archaeologists now have a dated framework to set against the written story, region by region.