Because of climate change and drought wich caused the fall of the mongol empire thats also how it change over time
There are several pieces of evidence that exist that show that Indus Valley civilizations traded with Sumer, but the most significant is in the layers of stone under the ground.
Well, the spaniards spread the smallpox virus to the Native Americans so I would say that the <em>Spaniards </em>were better at spreading it.
