I guess they should have the same goal that they are all willing to accomplish. That might get all of the different people to work together despite there differences.
Answer:
Because people found an extra source of energy with an incredible capacity for work.
The Maya, Inca, and Aztecs built great civilizations in Mexico and in Central and South America between 1,800 and 500 years ago. The first of these was the Maya civilization. About 2,800 years ago, people known as the Maya lived in farming villages on the Yucatan Peninsula and the highlands to the south.
The plague of Justinian plagues was a pandemic that afflicted the eastern Roman Empire especially its capital Constantinople the Sassanid empire and port cities around the entire Mediterranean Sea
How long it lasted was for 541 AD - 542 AD