We know this thanks to archaeologists who excavated many places and discovered that many early civilizations gathered around large rivers where they could develop agriculture. This was seen in places such as Mesopotamia, India, China, Egypt, etc.
We know this based on the fact that there are a lot of remains of ancient civilizations which had no contact with each other, but at the same time were starting to settle in into particular places as they grew plants and these plants gave them produce.
