INCA LEVIES Knotted cords created by South America’s Inca empire suggest this mysterious civilization, represented here by ruins of a fortified center in Peru, instituted a tax system shortly before falling to Spanish invaders in 1532. demerzel21/getty images plus Inkawasi khipus express a code of their own, unlike anything seen on knotted textiles from other Inca sites, Urton and Chu conclude. These colored cords may represent Inca record keepers’ credit and debit calculations for communities that stored produce at Inkawasi’s facilities, says archaeologist Jon Clindaniel, a recent graduate student of Urton’s now at the University of Chicago.