Though Africa was the cradle of humanity, it would seem the big rise in Homo Sapiens' growth during the last 20,000 years coincided with creation of/relocation to the temperate zones (grasslands, forests, more conducive to farming). As you said, tropical areas (ie equatorial Africa, Amazonian S America) have poor soils, full of parasites/predators, deficit of clean water, etc. Meanwhile, Fertile Crescent, N America, Steppes offer huge farming opportunities (as do Argentinian Pampas, parts of Australia, but S Hemisphere not so land rich). Thus, I believe that living in tropical zones may be great for reptiles and ferns, not so great for humans (who have to succeed w one hand tied behind their backs). This may get very interesting if global warming allows tropical areas to start reclaiming temperate zones.