Although some Native American languages were similar, many were different, and some groups developed writing systems. The Mayan Indians used and dashes for numbers, and they created a calendar that was more accurate than the systems for the ancient Egyptians, Greeks, or Romans. The Mayan system of writing had become extinct before the Spaniards arrived, but it was an early system for recording events. The picture writing of the Aztec Indians was in use when the Spaniards arrived in Mexico, and the Spaniards learned to read Aztec writing. Because the Indian tribes spoke many different languages, they needed a way to communicate with each other, so sign language, picture, smoke signals, and drum beats became popular.