Cellulose, a complex carbohydrate, or polysaccharide, consisting of 3,000 or more glucose units. The basic structural component of plant cell walls, cellulose comprises about 33 percent of all vegetable matter (90 percent of cotton and 50 percent of wood are cellulose) and is the most abundant of all naturally occurring organic compounds. Nondigestible by man, cellulose is a food for herbivorous animals ( e.g., cows, horses) because they retain it long enough for digestion by microorganisms present in the alimentary tract; protozoans in the gut of insects such as termites also digest cellulose.
I think due to our herbivorous primate ancestry, we retain the relic metabolic ability to metabolize carbohydrates of simpler molecular structure than cellulose. But as a family/species we gave up the guts necessary to metabolize cellulose to redirect the energy required by that big gut to feed our brains. We evolved over the course of several million years to eat primarily the nutrient dense fat and protein of other animals instead of nutrient dilute plants.
Yes, we retain the ability to utilize glucose to fuel cells that do not contain mitochondria. But we have evolved an elegant mechanism to produce all the glucose we require for those cells: gluconeogenesis. We do not need to eat it. In fact, I think gluconeogenesis is the mechanism we evolved to provide that small amount of required glucose during millions of years when eating it was impossible. It saved us from going extinct during the Pleistocene when plants were low in digestible carbohydrate and high in cellulose. If our ancestors had to rely on eating carbohydrates to get the glucose necessary to survive, we would not be here. That’s just the way it was.
You might argue that we were ‘not meant to be’ one thing or another. But until I see evidence that Pleistocene flora had a lot more digestible nutrients than it did (or I’ve seen so far), my bet is that our ancestors spent a lot more of their lives in ketosis than not. I think that’s how we were ‘meant to live’ to attain health and well-being.