Among the many claims that Diamond makes in Guns, Germs, and Steel is the argument that the development of agriculture occurred first in Eurasia because those continents lie on an east-west axis instead of a north-south one. He directly addresses this issue in Chapter Ten. Looking at a world map or a globe, it is easy to see what Diamond means by this:
The Americas span a much greater distance north-south (9,000 miles) than east-west: only 3,000 miles at the widest . . .That is, the major axis of the Americas is north-south. The same is true, though to a less extreme degree, for Africa. In contrast, the major axis of Eurasia is east-west.
Diamond argues that this had major consequences for the spread and development of agriculture. Because Eurasia has a longer east-west axis than north-south, much of its expanse lies within similar latitudes that therefore have similar climates. Therefore they had similar wild flora and fauna (Diamond cites the example of flax, which grows wild from Britain to North Africa.) More importantly, crops that will grow in one area will also grow in other areas at similar latitudes. This enabled crops to spread throughout Eurasia, making it easier for settled societies to grow there.
In Africa and the Americas, on the other hand climate and geographic features, like deserts and mountain ranges, tended to isolate one region from another. This made it more difficult for agricultural practices based on certain crops to spread. So, in short, the orientation of the continents was very significant in human history according to Diamond.