It's been a good while since I read Guns, Germs & Steel (and Diamond's equally excellent Collapse), but my remembrance is that physical obstacles in the north/south-oriented Americas and Africa -- e.g., deserts and extremely dense jungles -- inhibited the relatively "easy" transfer (given time) of technology, culture, etc. that happened in Eurasia. Climate would have affected the exchange of domesticated animals and plants to some extent, but such animals and plants or variants seem to have adapted to different climates in Eurasia. (It's been a while and I haven't read the Wikipedia article, so take what I say with a grain of salt!)