The continents of Earth have some important physical differences: most of them are wider from north to south than they are from east to west, so one could say that their “major axis” is the north-south axis. The differences in the shapes of the continents result in some big differences between civilizations.
Chapter 10 is largely about one of the most basic and obvious differences between the regions of the world—their length from east to west. As such, it’s a particularly lucid (and also perhaps over-simplified) illustration of how geography influences society.
In world history, agriculture arose in certain areas (Mesopotamia, the Americas, China) and then spread to other areas. The ease with which agriculture could spread around the world varied greatly. It’s estimated that agriculture spread east and west far quicker than it spread north and south. For example, agriculture spread from Mesopotamia to the Indus Valley at a rate of almost a mile a year (but in the Americas, it spread north from Mexico at a rate of only 0.3 miles a year). In general, innovations in food production, from agriculture to the domestication of large mammals, spread more slowly through the Americas than through Eurasia. Why?
We begin with the raw data: the rate at which agriculture has spread around the world, both to the east and west, and then to the north and the south. Diamond will offer a theory for why there’s a measurable difference between the rates of diffusion of agriculture for the two different sets of directions.
Consider how agriculture spread from Mesopotamia to Europe. While it’s been argued that Europeans simply learned how to domesticate wild plants that grew in their regions by imitating their neighbors to the east, the truth is that Mesopotamian agriculture transplanted specific crops, not just farming techniques. Europeans began to grow wheat and barley because they acquired Mesopotamian wheat and barley seeds through trade and travel.
Just as Mesopotamia was the first region of the world to foster human agriculture, it was also the first (known) region of the world to spread agriculture to its neighbors. Mesopotamian agriculturalists spread their techniques and their literal crops, diffusing (spreading) wheat and barley to Europe.
The key point is that latitude (the measure of how far a location is from the equator) is a better determinant of climate than longitude (the measure of how far east or west a location is). This is because Earth rotates on a north-south axis, meaning that the sun’s heat warms places with the same latitude equally. Two areas that share the same latitude will tend to have very similar climates, even if they’re on opposite sides of the world, whereas two areas with the same longitude often have very different climates.
The reason that agriculture, which is dependent on climate, diffuses east and west faster than it diffuses north and south, is that regions with a similar latitude (i.e., either to the east or the west) generally have a more similar climate than regions with the same longitude (i.e., to the north or the south). A person whose homeland’s climate is similar to that of a farmer’s is more likely to adopt techniques from the farmer.
Eurasia is unique among the continents in that it's longer from west to east than it is from north to south. Therefore, there were by definition more regions in Eurasia that shared latitude than there were in Africa or the Americas. Mesopotamian agriculture spread quickly to the many areas that shared Mesopotamia’s climate, benefitting many different peoples. By contrast, Mesopotamian agriculture never spread very far south into Africa because of “climate barriers” like the Sahara Desert. Similarly, the Incas never transported their domesticated animals north into Mexico—again, changes in climate made such travel difficult.
In this section, Diamond explains why he prefers talking about “Eurasia” to talking about Europe and Asia. Eurasia should be considered as one solid landmass, not two continents, because the modern boundary between Europe and Asia didn’t exist thousands of years ago—instead, agriculture diffused from the Middle East into Europe and Asia. Similarly, agriculture doesn’t diffuse if there’s a geographic barrier in the way. Because of this, longitude isn’t the only barrier to diffusion, and latitude doesn’t always mean consistent diffusion.
Some caveats: although latitude is an important determinant of similar climate, it’s not necessarily true that areas with identical latitude have identical climate. Crop innovations in the American southwest never reached the American southeast, even though the two regions had the same latitude—this is because most of the area in between (i.e., present-day Texas and the Great Plains) couldn’t support agriculture.
Obviously, agriculture can never emerge in a region where the soil can’t support agriculture. But, somewhat more subtly, agriculture rarely spreads through such a region and moves on to others. Thus, even though both coasts of the U.S. share a similar, fertile climate, agriculture didn’t spread from one side to the other because of the deserts in between.
Diamond hypothesizes that one could study “latitudinal diffusion” of ideas in general, not just crops or domesticated animals. For example, writing was developed in Mesopotamia and “diffused” (spread) to Rome and India (same latitude), whereas writing developed in Mesoamerica never diffused to the Andes (same longitude).
Diamond will study the diffusion of other things—not just literal crops, but ideas and inventions—later on in the book. By and large, he argues, ideas and inventions “travel” east and west more effectively than they travel north and south, suggesting that ideas and inventions are passed along the same trade networks that arose though agriculture in the first place.