According to Chapter 10 of Guns, Germs, and Steel, what was it about the Americas and Africa that put these two continents at a developmental disadvantage?
According to Chapter 10, the main problem that the Americas and Africa have is that the long axes of those land masses run the wrong way.
According to Diamond, the people of a land mass will develop civilizations more quickly if they have access to more food. The more they can engage in effective agriculture, the quicker their societies can become large, complex, civilized, and therefore powerful. Diamond says that land masses with axes that run east to west are at an advantage over those, like the Americas and Africa, with axes that run north to south.
In general, areas that are on the same east-west axis tend to have similar climates. The same is not true of areas along a north-south axis. This means that crops can spread from place to place along an east-west axis, but not along a north-south axis. Therefore, crops were not able to spread as easily from place to place in the Americas and Africa as they were able to in Eurasia.
Because crops could not spread as easily, people in the Americas and Africa could not develop civilizations as soon or as easily as those in Eurasia could. This put them at a developmental disadvantage.