Guns Germs And Steel is certainly an interesting book. And while I may not completely buy all his arguments, it certainly is an interesting concept. The connection to native grain seed size in particular, I don’t think I would have considered that aspect otherwise. Additionally the east west axis of migration versus the north south, as a means of factoring in the relative technological advancement of Eurasia versus the Americas, is one that he goes to great lengths to explore.
The tropical aspects are perhaps the bit I’m least sold on. As it flows a bit too closely to some nasty tropes about certain ethnic groups. Not to say it’s inherently wrong, just that I’m not as sold on the evidence he presents, as it is, by nature, less quantifiable than things like climatographic similarity and seed sizes.