My biggest problem is that this hypothesis does not explain why farming started so much later in the Americas than in Eurasia. From Wikipedia "approximate centres of origin of agriculture and its spread in prehistory: the Fertile Crescent (11,000 BP), the Yangtze and Yellow River basins (9,000 BP) and the Papua New Guinea Highlands (9,000–6,000 BP), Central Mexico (5,000–4,000 BP), Northern South America (5,000–4,000 BP), sub-Saharan Africa (5,000–4,000 BP, exact location unknown), eastern North America (4,000–3,000 BP). " and American civilisation were about 4 thousand years behind Eurasia when it come to technology.