Using written symbols such as hearts, arrows and hands, the ancient Aztecs maintained an arithmetic system that was far more complex than previously understood, scientists said on Thursday.

The Aztecs, an empire in central Mexico toppled by Spanish invaders in 1519, has long been recognized for its sophistication in architecture, engineering, astronomy and other fields. And the new research confirms arithmetic can be added to the list.

The researchers examined hundreds of drawings in two manuscripts dating back to between 1540 and 1544 that were used to document agricultural properties by the Aztec people in the city-state of Tepetlaoztoc, near modern Mexico City.