That was a stronger variant of the same hypothesis. And indeed it explains the nearly uniform agriculture/husbandry recipes in Eurasia. But it doesn't explain

That was a stronger variant of the same hypothesis. And indeed it explains the nearly uniform agriculture/husbandry recipes in Eurasia. But it doesn't explain why at the dawn of the renaissance all these cultures were producing similar crucible steel (from India) or making gunpowder (China). Obviously the book goes farther still, I wasn't trying to summarize the whole thing. The "germs" angle matters too, the pervasive Eurasian trade networks meant that acquired immunity could be traded like technology, leading to a Eurasian "fortress of disease" that prevented immigration but did nothing to deter colonialism.