I read somewhere and would love to give credit, but I'm not sure where I read it. The tropical is personal addition however. I won't recreate this theory as eloquently but the general premise is:
in the spring/summer meat was plentiful, we hunted and ate meat, to stay fit enough and athleticly light enough to hunt effectively, we burned the fat as fuel to have the energy to keep hunting and because we had more calories available soon, fat caveman doesn't hunt well
late summer/fall fruit bloomed we ate it and stored it as fat(as we know fructose does)and that gave us both warmth and fuel to survive the winters animals were scarce but coupled with scrounged veggies, fermented whatever, stored meat etc we made it thru winter. As spring hit we were lean from burning the weight over the winter and ready to hunt once more
the more tropical climates were winter was less a concern have fatty fruits like coconut and avacado..?? Coincidence or were they our way of staying trim year round with energetic sat. Fats?
Opinions? (please note original source of this theory if you can)
This "theory" is idle evolutionary speculation that everyone falls prey to at some point or another. It fails to hold up to scrutiny, but is intuitively satisfying if you don't really think about it.
1) What is "winter" in east Africa like?
2) What was the seasonality of fruits in the rift valley?
3) Do existing HG tribes "fatten up" for the winter? (recall that a core founding observation for paleo is a lack of fatness in HG tribes)
4) How much fruit do you think we ate during Ice Ages?
5) When was the last time you hibernated?
Point 5 is more snark than genuine point, but it's worth remembering that most animals which fatten up at a certain time do so in preparation for a months long fast. Humans are active year round, have access to animals year round, and hunt year round. We don't follow the pattern.
All that said, it's possible that the presumed fat-storage enhancing effect of high blood sugar is an adaptation to a rare food like fruit - better to store and save it then burn it off - but then again maybe it's just a last ditched effort of your metabolism to prevent glycation damage. Speculation breeds speculation!
I haven't seen any real evidence that humans evolved to fatten on specific foods because of their rarity. It strikes me as implausible given that HGs tend to do well all over the planet, from places with zero fruit to places with abundant plant food sources. If we actually were adapted to fruit rather than just being generalists, I think it would play a much more important role in our health than being so obviously optional.
A lot of people who practice the bestselling Paleo™ diet (as opposed to a style of personal nutrition one could call the Paleolithic diet) seem to have some assumptions of Paleolithic humans based on stereotypical imagery of cavemen in Ice Age Europe. First, we must dispel this by noting that humans did not become humans during the Ice Age or in Europe, and that even during the glacial maximum (18,000 BCE, the peak of the Ice Age) most of the world was pretty warm, tropical and temperate.
Humans did not evolve into the species we are now in the Arctic like animals that have evolved body fat for warmth. The existence of "blubber" on Arctic mammals is an evolved necessity, as much as the cold blood of reptiles is for keeping cool in the desert. Winter, as a cold and snowy event, is unknown to those in warm climates, which are the type of climates in which most of our evolution occurred. The move into cold climates is relatively recent, and humans already had the tools available to adapt to it without the need for any kind of biological mechanism. The use of fur and fire kept humans warmer than they would be otherwise.
Fruit is not the cause of obesity. You need only look at the members of the website 30 Bananas a Day wouldn't be so thin. How could fruit make you fat, as I hear often, yet at the same time, the same people will call these fruitarians "emaciated"?
We all learn about USDA "myths" but then it seems like I can't read anything on paleo sites without being confronted with new "truths" that to me appear to be myths as well. Questioning these is like questioning some holy religion. If the paleo diet is going to become more respected it's not by simply being a repackaged version of the Atkins Diet.
Another "truth" I can't seem to stop hearing: humans only ate the fatty portions of wild animals and threw away the rest of the meat to dogs. First of all, wild animals like deer are so lean that there would be no point in hunting them if we did not eat their lean muscle meat. And after all that we gave most of it to dogs, well then dogs really are the top of the food chain and the master species of the world, not humans. My dog barks and barks and barks begging for some food when I'm eating (quite annoying actually!), and I usually give her some, but I don't give her most of it!
This is the best argument that I've sen on the subject: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2917125/
...but I still disagree with it completely. The loss of ascorbic acid synthesis occurred while we were still frugivorous apes living in tropical climates. No seasonal fattening would have occurred, nor would it have been particularly useful.
Fruits and vegetables in paleo times would only have been used at medicine cause they were all bitter, except maybe wild berrys which were probably the sweeter varietys. Fruits now adays are hybridized to taste sweeter, thus being sugar bombs we need pesticides to keep insects away. Have you ever eating a wild banana? Try it.. And wild apples? Well they tasted more like crab apples if you ever tried one...
It is a myth started by some temperate-climate thinker who has obviously never hunted, gathered, stored, or grown his/her own food. Many fruits and vegetables keep well on their own, or are easily fermented or dried. Most hunters know it is better in the winter. Most farmers know that fruits and vegetables ripen at many times of the year, not just the fall. It is more likely that people survived the winter eating fatty meats killed throughout the fall. Fat cavemen could have sat around even more making nets and small traps, and catch all the fish and small game they could use.