Guns, Germs and Steel
Farming was a big change. In a farming society, more people could live in the same area (population density) than a hunter-gatherer society. This is why they could easily take over the lands of hunter-gatherers, if their crops could grow there. Diamond notes that it was usually easier for crops to spread east or west than north or south, because crops could only grow in a similar climate. Humans from farming societies spread to regions where their crops could grow, such as from the Fertile Crescent to Europe, North Africa and South Asia and from China to the rest of East Asia. Eurasia (Europe and Asia) became home to far more farming societies than the rest of the world, and they had better crops. Meanwhile, these crops could not spread from North Africa into the middle of Africa, and therefore they did not spread to South Africa (where the crops could grow). Meanwhile, until Europeans discovered the Americas, crops from Central America did not spread to South America, and crops from South America did not spread to Central America.