Here’s Dr. Greger talking about fiber:

I pulled out that chart he included from this study…
Studying the Human Gut Microbiota in the Trans-Omics Era – Focus on Metagenomics and Metabonomics, Current Pharmaceutical Design, 2009

… that shows our fiber intake from Paleolithic times to present:

Greger: “We’ve been eating 100 grams of fiber every day for millions of years.“