The theories are linked. Diamond's assertion that Europeans conquered the world because they had an easier time domesticating animals makes sense if you're tal

The theories are linked. Diamond's assertion that Europeans conquered the world because they had an easier time domesticating animals makes sense if you're talking about the Americas, but not so much when talking about Africa, which was in constant contact with Europe and in theory should have been able to trade for any technology invented there. Simply saying that Africa didn't have domesticable animals isn't good enough because, certainly, they could have imported them along with other technology from Europe. The continental axis hypothesis is his answer to this. He says crops (and later animals) could be more easily moved east to west rather than north to south because the climate will change less. In his mind, this explains why Africans didn't adopt European advancements in agriculture and animal domestication. You don't need a study to debunk this theory within the context of Diamond's overall hypnosis; you need a globe and some measuring tape. Africa's east-west axis (4600m) is only 8% smaller than its north-south axis (5000m), which in turn is almost identical to Eur-Asia's (4,900m). The only time this isn't true is on a distorted map like the Mercator projection he uses in the book. Sub-Saharan Africa is not some wasteland that couldn't support these animals and crops. And this is obvious when you consider that colonial empires brought their crops to Africa without the need to fundamentally change them for the climate. Colonial holdings in Africa often produced a good chunk of the grain for their empires as a whole. Axis theories aside, Diamond is just factually wrong about the origins and timelines of domestication. Many of the crops and animals that supposedly helped Europe dominate the world were widely present in Africa centuries before European invasions. Romans explored into Sub-Saharan Africa in the 1st century BC; Turks and Arabs had established trade routes deep into Africa by the 6th century AD; the Indian Ocean, including the coast of Africa was continually interconnected through trade going back to the bronze age at least, and trade caravans across the Sahara have been recorded in ancient Egyptian records. This is covered well in Diamond in the Rough: Reflections on Guns, Germs, and Steel https://www.jstor.org/stable/24707701 https://www.jstor.org/stable/24707701 You could fill a book disproving his theories, but I'll point out one last thing: trying to explain European conquest through the lens of continents is super weird. Africa, let alone any other 'continent,' is a vast and diverse place. The idea of continents in and of itself makes less and less sense the more we learn about geography. You could as reasonably draw continental boundaries in dozens of other ways that would make more sense than the ones Diamond presents in his books.