You'll have to get a copy of Eaton's study as I don't have a copy here, but
you can order it through Medline.  Generally speaking Eaton is
well-regarded among researchers in this area.

Berries are loaded with fibre.  Nuts have fibre too.  Wild fruits in
general are higher in fibre than the stuff we buy in supermarkets.  If
about 50% of your diet is meat and the other 50% is nuts, berries, and a
few succulents, I imagine you'll be getting quite a lot of fibre.
Remember, once you eliminate things like rice and potatoes and ultra-sweet
fruits and breads and such, almost any non-meat food has fibre in it.