American Diffusion vs. Afro-Eurasian Diffusion

"The distance between Mesoamerica and South America   say, between Mexico's highlands and Ecuador's   is only 1,200 miles, approximately the same as the distance in Eurasia separating the Balkans from Mesopotamia. The Balkans provided ideal growing conditions for most Mesopotamian crops and livestock, and received those domesticates as a package within 2,000 years of its assembly in the Fertile Crescent. The rapid spread preempted opportunities for domesticating those and related species in the Balkans. Highland Mexico and the Andes would similarly have been suitable for many of each other's crops and domestic animals. A few crops, notably Mexican corn, did indeed spread to the other region in the pre-Columbian era." 

"But other crops and domestic animals failed to spread between Mesoamerica and South America. The cool highlands of Mexico would have provided ideal conditions for raising llamas, guinea pigs, and potatoes, all domesticated in the cool highlands of the South American Andes. Yet the northward spread of those Andean specialties was stopped completely by the hot intervening lowlands of Central America."   Guns, Germs, and Steel by Jared Diamond

When I first read this, I was floored. The idea that Fertile Crescent crops diffused throughout Afro-Eurasion crops from a single ancestral region while American crops largely failed to is astonishing! The most interesting fact was that, although most crops did not diffuse between the two regions, both Mexican highland and Andean highland peoples cultivated many of the same crops   with a few exceptions. This means that the people of the respective regions domesticated some of the same plants independently. In contrast, many of the crops in Afro-Eurasia were only domesticated once and then diffused from their ancestral origins through trade and travel. Diamond explains that major reason for this strange disparity is the difference in climates along the major axes of the landmasses. Climate is greatly influenced by latitude and altitude, and, therefore, traveling east-west is much less difficult than traveling north-south. As well, differences in altitude provide differences in climate similar to how latitude does with lower altitudes resembling regions closer to the equator and higher altitudes resembling regions farther from the equator. This is why the Mexican highlands and Andean highlands generate similar climates despite the variation in latitude and therefore support similar crops. However, between the two mild highlands are tropical lowlands. Different climates create different barriers. Different climates generate different diseases which is a major factor in trade; you can't trade if the people succumb to disease before reaching their destination. As well, tropical rainforest is very difficult to traverse because of the thick vegetation. In contrast, travel along the Eurasian landmass generates much less drastic climate variation. This facilitated the transport of crops and livestock throughout north Africa, Europe, and Asia west of the Himalayas. However, African diffusion faced similar issues that American diffusion did. Although southern Africa was capable of supporting Fertile Crescent crops, it never acquired them. The drastic climate variation between Ethiopia and southern Africa generated devastating diseases and difficult terrain. Therefore, it was an unreasonable feat to attempt to traverse it.