Probably the best non-fiction book I read in the last 10 years. It puts forward many reasoned arguments for the Eurasian historical supremacy (the Guns, Germs and Steel of the title), but as you say, the crux of it is that Eurasia is the continent with the largest contiguous area of similar climate. In other words, Eurasia is wide, while North America is narrow, and South America and Africa are both tall and narrow, resulting in very large climatic diversity, and smaller ecosystems adapted to each of those climates.