Nor did the aborigines remain hunter-gatherers because they had strong cultural taboos against agriculture or were some kind of ecologically noble savages who preferred to live in harmony with nature, as romantics like to claim. From this initial head start in agriculture, further advantages followed. Agricultural societies can produce surplus food to feed extra mouths. These can then specialize as craftsmen, to invent and produce better tools and weapons, as administrators who organize more efficient uses of manpower and resources or as warriors. From this states and empires grow, again a more efficient arrangement than small tribes, who are often at war with each other.
Writing allows the keeping of records and better transmission of knowledge over generations.
The main obstacle for the expansion of a given agricultural system is climate. The limits are most severe in the direction North-South where temperature and day lengths vary rather quickly. The Americas extend mostly North-South, as does Africa. Thus plants domesticated in one part of those continents could not easily be transplanted to other parts. Eurasia, on the other hand, extends mostly in an East-West direction.