The answer to this question can be found in Chapter 10 of Guns, Germs, and Steel. Having already established that the development of agriculture is dependent on climate as well as the availability of domesticable plants and animals, Diamond turns in this chapter to showing what factors facilitated or inhibited the spread of food production. Diamond argues that food production spreads easiest through similar climatic zones, and that these zones exist over much greater spaces on the Eurasian landmass, because it is oriented along an east-west axis, unlike the Americas and Africa, where the greatest distance is from north to south. Eurasia also has fewer natural barriers, like mountain ranges and deserts, that isolate one region from another, slowing down the diffusion of agriculture. Much of Eurasia is along a similar climatic zone to the Fertile Crescent, so the food production packages that emerged there spread quickly throughout the landmass.