Hunter-gatherers didn't just eat meat. It is likely that fruit made up a very large portion of calories. It's easy for people living in cities as many of us do to forget that food exists outside in the wild, whether in animal or plant form.
Most vegetables do not have much in the way of calories. The botanical definition of fruit also includes nuts. Fruit and nuts both have a fair amount of calories.
That was the concern. Getting more calories with more nutrition. They didn't have a culture of processed food, nor did they have a diet book culture. Humans in the wild devote their lives to food. Food is primal, and few things are more primal than fruit.
I read about wolves ravaging watermelon farms. Yep, they were eating the watermelons. Some people think humans are carnivores, but not even carnivores like the wolf can resist sweet fructose temptation.
Recently there has been reports of a study done where tumors were fed glucose and then fructose and from this they concluded that tumors "thrive" on fructose. Well, the vast majority of fruits are far from having 100% fructose like in those studies. Did they also feed these cancer cells protein or fat? They had a hypothesis (fructose makes tumors fat) and sought to replicate that. And maybe they didn't care whether it made sense or was logical or not. It fed into the current talk of fructose.
The fructose scare is doing nothing to make America healthy. All it will make people do is avoid HFCS, which is laughably small way to make a change unless one consumes sugary drinks regularly. Diet drinks have made a huge success in the market already. I think you have to consider the food that's being eaten, in what quantities, and the person's activity level. And because they think fructose is bad they are going to avoid healthy fruit.
Remember the low-fat craze, then there was a low-carb craze. And between all this we got lots of mini crazes based on avoiding this or that ingredient. HFCS (and all fructose by association) becomes a scapegoat for America's gluttony and sloth.
I would like everyone to consider the Kitava who consume lots of carbs including fruit and consider whether they are unhealthy, they're not. And obesity is rare. Most other hunter-gatherers also consume fruit. Only when humans ventured into cold climates (or perhaps also very hot desert climates) did we have to adopt very low carb diets simply because there wasn't much fruit to be found. Would it be close to the diet of much of our evolution? No. Would it get us through the harsh winter? Yes. Humans find ways to survive.
I just don't see how early apes would go from eating lots of fruit as they most likely did (our evolutionary heritage is arboreal, just look at all the other primates) to getting to a point where consuming it would be bad for our health. It seems contrary to evolution for such a nutritious and energy dense food source to become bad for the human species.
Recently I read a post on a low-carb blog extolling gluconeogenesis as proof that humans actually don't need any carbs. Our livers produce glucose anyway. Why would our bodies produce something we don't need in the first place for use in our body's systems? If sugar is so scary, why is glucose, abundant in our blood, a form of sugar?