What I wanted to point out is that when one looks at our "outfit" from a biochemistry point of view we have clearly and simultanenously the abilities to
- metabolize fructose and glucose or other sugars and take advantage of them in various ways if they are present in our diets
What I mean is, given the example of the giant panda, fruit-eating chimps, and other evidence, I think that's suggestive, not conclusive.
If our ancestor's diet had never included fruit...
I think that everyone agrees that our ancestors' diets included fruit at various times in the past, to greater and lesser degrees. That doesn't seem to be a matter of dispute. It also doesn't guarantee that fruit is an optimal part of our current diet, though it suggests it
might be. Clearly, the example of caries-prone frugivorous chimps in the wild suggests that too much fruit can be a bad thing (at least for people who don't want caries, dental plaque, gingivitis, etc.), and I think we're in agreement on that also.
So this strongly suggests that we are probably quite capable to adapt both to different diets
Yes, where I am still unclear is whether we are facultative faunivores who can digest plants better than most or all of the other facultative faunivores, or omnivores who are better equipped to digest meats than most or all of the other omnivores. We seem to be on the edge of the two categories, which is part of what makes for so much interesting debate. I think our ability to straddle both categories may be part of what enabled humans to survive and eventually overpopulate. Maybe scientists will eventually even create a new category just for homo sapiens and proto-humans that lies in-between the other two.
Where some confusion tends to come in, I think, is many people seem to assume that carnivore/faunivore automatically means zero plant foods, and therefore any animal that eats some plant foods in addition to fauna, or has some ability to digest them, is automatically an omnivore. Facultative carnivores/faunivores do eat some plant foods, and one has reportedly been eating 99% plant foods or thereabouts since the dawn of the species (and before in its ancestor). So simply showing some ability to digest glucose or starch does not appear to prove omnivorous physiology. It looks like it's more complicated than that, though I'm admittedly no expert on physiology.
Even if it were known absolutely for sure that humans have an omnivorous physiology and are best classified as omnivorous, I don't think that guarantees that everyone or even anyone will do best on a diet rich in plant foods, for we could be on the meat-heavy edge, physiologically, of omnivory and it could also be that today's plant foods have become too degraded to provide the same level of nutrition as in the past (and the same could be true for the meats, too, but I suspect less so).