Better farming was just invented in a sweet dank valley right in between these two rivers, and the animals are helping.

Guess what happens next!

More food, and more people who came to buy the food, and you need people to help make the food and keep track of the sales, and now, you need houses for people to live in and people to make the houses, and now, there's more people, and they invent things which makes things better, and more people come, and there's more farming and more people to make more things for more people, and now, there's business, money, writing, laws, power.

Society!