I didn't say tropical areas were not fertile or that things wouldn't grow there. The opposite may be true. The temperate zones are more conducive to grasslands, which may benefit from surviving as monocultures w less competition from other plants. Thus, the N. American Great Plains, Fertile Crescent, Steppes (S African savannahs, Argentinian Pampas) may be the giant grain belts that have sustained human growth (for better or worse). In a transition to a fruit/vegetable diet, frost free/wetter zones have a big advantage, ideal climates tending to be more tropical. Where will ag go with all this? We'll see.