When agriculture began to spread around the globe, farmers had an easier time expanding along east-west routes than along north-south ones. This is because locations along the same latitude generally share similar climates, amounts of sunlight and rainfall, and changes in season. These factors allowed farmers in Europe and Asia to domesticate a few crops and grow them along the entire stretch of land from France to China.
By comparison, the climate varies greatly when traveling from north to south. Just imagine how different the weather is in Florida compared to Canada. You can be the most talented farmer in the world, but it won’t help you grow Florida oranges in the Canadian winter. Snow is a poor substitute for soil. In order to spread crops along north-south routes, farmers would need to find and domesticate new plants whenever the climate changed.
As a result, agriculture spread two to three times faster across Asia and Europe than it did up and down the Americas. Over the span of centuries, this small difference had a very big impact. Increased food production allowed for more rapid population growth. With more people, these cultures were able to build stronger armies and were better equipped to develop new technologies. The changes started out small—a crop that spread slightly farther, a population that grew slightly faster—but compounded into substantial differences over time.