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Geography
Meditate on geography for a minute. Climates allow certain foods to grow. Some areas even of similar climate have a greater range of native flora. The local topography of some areas creates seasonal and temperature variations which maximize the harvest potential of that wide range. And latitudinal (east-west) migration of those useful grains is easier than longitudinal or north-south migration--at the simplest level, because of bands or zones of climate. As you travel on a latitude, you stay in roughly the same climate. As you travel on a longitude, you encounter radically different zones of climate. The axes of continents are important for food production.
Migration along a latitudinal rather than a longitudinal axis is also easier for humans and for animals. As we'll see when we look at Diamond's argument about domesticated animals, the Fertile Crescent (and points west: Greece, Rome, Western Europe) began with an array of animals unmatched elsewhere. Of the five animals it's easiest to domesticate, the Fertile Crescent had four--and the fifth was native just to the east. Other continents were not so fortunate.