Eurasia is an enormous landmass, but it also has the longest east-west axis of any other landmass, especially compared to Africa and South America (just think of their shapes.) Because of this, enormous expanses of land in Eurasia lie along similar latitudes, which means that they have relatively similar climates. Thus domesticated crops, like cereals first raised in Mesopotamia, could spread fairly quickly along these axes, far more than it could from, for example, north to sub-Saharan Africa. The spread of crops and domesticated animals couldn't spread through the tropics, which bisected these continents. To cite another example, the cultivation of maize, beans and squash, staples in Mesoamerica and the American Southeast, could not spread through the tropics into the Andes because of the the extreme changes. Thus the fact that their landmass was oriented along an east-west axis was a major relative advantage for Eurasian peoples in developing food, technology, and civilizations.