Although the Inca civilization created a vast empire in Peru, they lacked basic skills and knowledge without which European civilization could not have advanced. Two of these were writing and the use of iron. What notable invention was also unknown to the Incas?

Although the Inca civilization created a vast empire in Peru, they lacked basic skills and knowledge without which European civilization could not have advanced. Two of these were writing and the use of iron. What notable invention was also unknown to the Incas?