[Albion Wilde]

I wonder, with plentiful food, has anyone ever investigated the possibility that southern humans add fat as a buffer to the heat and humidity? What if people in subsaharan Africa had plentiful food, would they also become fat as a buffer?

I'm not following your logic -- a buffer against what? Can you explain a little more? Example: in many hot cultures, from Mexico to India, the food is very spicy. This is thought to have evolved before the days of refrigeration and rapid transport because food spoils more quickly in hot climates, and the spices counteract the "off" taste.

[MHGinTN]

Fat holds a lot of water, which is a necessity for perspiration. Latent heat of evaporation cools the body.

Fat around the vital organ region acts as an insulator to moderate temp increase and decrease rate.