However there is a significant amount of truth that humans were opportunistic omnivores. We ate meat, vegetables, nuts, berries, but no significant amounts of grain. we got fructose in our diet from fruit, but it wasn't constant and it was most likely an episodic binge. When we learned agriculture, our life spans, cranial capacities and overall measures of health decreased. The source of this information is the very fascinating book: "Paleopathology at the Origins of Agriculture (Bioarchaeological Interpretations of the Human Past: Local, Regional, and Global)" it's recommended reading.
Clearly we didn't evolve to eat so much grain. A more reasoned approach to the problem can be found in the work of Dr. Peter Attia, who was an obese and pre-diabetic surgeon turned nutrition researcher who, I think quite rightly shows that people confuse acute with chronic toxicity. He explains the issue quite well here.
Personally I was a vegan (for moral, not health reasons) and was very obese. I have lost 65 pounds consuming more calories than I ate before on a slow carb diet. I'm still losing, but now I'm just fat, not obese. I resumed eating meat (grass fed, organic and also game meat. (tip: wild boar makes a great burger)) and all of my blood markers are better than they were whilst vegan. You can chalk that up to anecdotal evidence. But I do believe all calories are not equal.
Stay healthy out there. Take what I say with a grain of salt, but do look into the concept of caloric difference.