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.” That’s a lot! I can’t imagine eating that much fiber. I think I do better than that 12-18 grams of most Americans though, because everything I eat is a plant. Only plants provide fiber.
Fanatic Cook 
I’e been surprised to find that many greens–arugula, lettuce mixes, etc.–actually don’t have much fiber at all. Maybe 1 or 2 grams per serving at most. Hardy greens like kale or collards probably have more, but I haven’t checked.
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I’ve been wondering … What foods is a person following a Paleo diet eating? That provides that 100 grams of fiber a day? They don’t eat any grains or beans. I don’t have an answer.
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