Isn't it quite naive to use a calculation of the path of least resistance to account for travel costs which the whole argument relies upon? Early civilizations would settle close to rivers and travel on th rivers and along the coasts. It seems like this model does not take this into account.

So when they calculate differences in environmental barriers across continents, it seems important to take water ways into account, which would lead to eurasia having an upper hand over at least NA and Africa. I'm not saying that Diamonds original argument holds, and its great that they try to test the hypothesis.