In the keto and carnivore communities, it isn’t believed that fiber is as essential for many as we’ve been taught to think.
I personally don’t believe that man, at any point in his development, ever ate *only* animal protein, but was probably often omnivorous. But many people on animal products only claim that they see no ill effects.
You are literally being eaten alive without it. There are even strains of gut bacteria and probiotics that will use the mucus lining as a nutrient source without sufficient fiber. Hundreds of studies indicating plant based diets as the cornerstone of healthy aging and reduced mortality. I would seriously love more high carb foods to get a healthy makeover similar to high protein pretzels but right now fruits and veggies are the best prescriptive tool we can use.