The other thing that I think you have to consider when you speak of the rapid changed in a human's appearance in the last 100,000 years is that as better tools were developed, and societies became increasingly more organized, and food was easier to gather, just having more nutrients, or even enough nutrients on a more regular basis could have has an extreme impact on the way humanity apeared, in terms of making us taller, more thickly muscled, etc. just by giving our existing genetic programming more raw materials to work with.
Now, that change might have, in turn, effected who got to breed, who got to be in charge, etc. etc. which would, in turn, effect our genetics.
I guess what I'm saying is that I don't think it would be necessary to alter the genes themselves to see a significant change in the appearance of humans. So I don't necessarily think that we are way off on our ideas of not having adapted to eating a carbohydrate rich diet.
Also, while animals do adapt and change relatively quickly, that often is a response, in my understanding, to a radical change in their environment. Such as a bird who's food source changes entirely and within a few generations, those traits best adapted to eat from that source now dominate.
If humanity was suddenly faced with the prospect of eating nothing but carbohydrates, then I would espect that we would see much of the same thing happen to us. Within several generations those of us who were best adapated to eating all carbs would be, genetically, more successful than others who were not. Most likely, those ill-suited would sicken, die, and not reproduce.
Because agriculture has always included animals as well as plants, and high-carb foods have been incorporated into our diets rather slowly, until very recently, it's likely that we haven't adapted, for the simple reason that we haven't needed to. There has always been a mix of dietary nutrient sources available. This sort of falls in line with the idea stated above of a two-stage dietary change.
That's just out of my arse, but what do I know?
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