>what our biologically identical ancestors ate for a huge multiple of time longer than the modern western diet.

Why is there a belief that our ancestors had a singularly common diet? Humans have lived in all areas of the earth with vastly different diets. Some were high in fat, some were high in protein, some relied on plants/fruit, others were more balanced, etc.

Yes we probably on average have a higher carb intake than most humans in the past. But that doesn't mean there was some standardized diet that all pre-historic humans ate. They ate whatever was around to survive.