Ha, teach me to disappear for so long. What a good thread. There's lots that's been touched on, and not too much flaming. I, naturally, have a lot of opinions on this subject, some of it based on science, some on personal experience, and some of it of more of an environmental/political flavour.
The one thing that is very clear to me is that humans are omnivores from an evolutionary POV, and that we can thrive on a very broad range of diets. But diet is only one aspect of health. I am not diabetic, nor pre-diabetic, and that after around half a lifetime as a vegan who ate more carbs than anything else. As an ex-bodybuilder, I am also keenly aware of the effects of different diet and exercise for me. I have tried the high protein, moderate carb. low fat pre-competition diet, as well as the cyclical ketogenic diet. I have spent many hours in metabolic chambers having my RER measured under fasted and non-fasted states, and when I had a type I diabetic boyfriend, I used to also measure my blood glucose under a variety of conditions. To me, the MOST important factor for how MY body handles carbs is my exercise/fitness activities. Whether on a CKD or ordinary diet in a metabolic chamber, I quickly (as in overnight) enter into a ketogenic state even after a high carb dinner the night before. My blood glucose never really 'spikes' no matter what I eat. Why? I can only speculate. Most likely a combination of relatively decent muscle mass combined with daily (sometimes twice daily in the past) weight training and sprinting, as well as regular endurance activities, means all the glucose entering my blood stream had somewhere to go besides straight to fat or stuck in my blood due to poor insulin sensitivity. So really, and this is just opinion, I think activity is more important than diet, assuming you are otherwise getting enough nutrients for good health. Sure, our diet has changed dramatically since the paleolithic period, but obesity didn't become an epidemic until very recently when we no longer had to get off our butts and actively do something to earn our food.
Women's fertility has always been associated with buxomness until recently, and for good reason. A woman need an absolute minimum of around 18% bodyfat to be fertile, 22-25% is probably optimal, AND it didn't used to be very easy for a woman to get enough calories to achieve this fertility in the past. The same is not true for men, so the dichotomy of the lean and muscular male versus the round and voluptuous female as being the ideal is well grounded in our evolution.
I would also say that it s near impossible for any of us to achieve a diet that really resembles what our pre-agricultural ancestors ate, due to the nature and pace of agriculture itself. We've had over 10,000 years of intensive and selective breeding of livestock, fruits, vegetables and grains, and where ever 'modern' man is found we also have a pretty monocultural diet of foods grown on soils that bear no resemblance to what our ancestors ate. Of course, there are still foods like fish and wild game around, but not enough to feed the current hoards. I feel particularly strongly about the fishing industry, as it is so far below sustainable that I hesitate to recommend seafood as something that we should eat a lot of. Not because it's bad for us personally, but because it is bad for us collectively, however, I know that for many of you this is irrelevant and you are really only interested in what is good for you personally, right here and now, and let the future be someone else's problem.
Bringing some science back into this, it seems that focusing on one particular part of your diet as being 'bad' for you is easy to do because of the vast amount of wrong or mis-information that we have been fed. Cholesterol is not 'bad' for you, saturated fats are not 'bad' for you, and the glycemic index is a load of hogwash for most of us, unless you are really going to sit and eat just a bowl of rice, with no other carbs, proteins or fats in the meal. However, the concept of glycemic load is important, as any IDDM patient will tell you. But even this pales in importance if you have a healthy pancreas, insulin sensitive cells and a neutral calorie balance.
What DOES seem to be 'bad' for us, is combining lots of saturated fats with lots of sugars. This combination is bad news on all levels of health, so limiting one of these nutrients in your diet seems sensible, as does limiting sodium and non-nutrient dense calories. The one thing I will agree on is that sugar is bad in excess, and was a rare treat for our ancestors, often reserved for children and sick people. Yes, this includes fruit, especially modern fruit that is bred and carefully tended for high sugar content and large size.
It is easy for many people to assume that someone like me who has spent much of their life as a vegan is a nut-case. Well, I may be a nut-case! But I approached veganism with a solid background in nutrition, plus I am no longer a vegan. I eat eggs laid by our happy hens in the back yard, and kindly accept offers of wild game that has had a humane life and quick death (in NZ, pretty much all wild game is considered a pest, so wild game is in season all year round). I grow as many of my own organic fruit and veg as I can, and the rest of it *mostly* comes from local sources. I am also a firm believer that if it needs a food label, you shouldn't eat it, unless the label is merely there to tell you that it is/is not locally produced, or is/is not organically grown and humanely raised. But this is in the area of ethics rather than nutrition, so doesn't relate to the OP.
I also know that there is no one "right" diet. Some folks have allergies/intolerances to certain foods, and some ethnicities are less able to cope with modern foods. Why, for instance, do the Pima indians have such a high rate of diabetes? It is also really a lot easier for some of us to say no to certain foods/drugs that others have strong addictions to? I personally can't stand the taste of sweet foods, naturally occurring or otherwise. This is probably the hand of genetic luck, as much as I would like to say I just have better will power or restraint. Some of us are just more unlucky than others (I'm guessing Miguel was pretty unlucky to end up with IDDM). So find what works for you and be happy that you found something. But try not to feel too religious about it, we are too diverse a species to have a one-size-fits-all approach to nutrition.