Instead of eating fiber, it may be better to eat the dirt the plants grew in.

Sure, it might be. Is there evidence that it actually is? Other than the speculation that other generations of toddlers ate more dirt than toddlers today, and might have benefited from it?

I guess we could have non germ-free mice that are exposed to natural soil etc. and others in more sterile cages. The natural experiment of paleo/traditional peoples versus um, us, seems too varied.

Such studies have already been done. Kifs that play in the dirt are healthier. Have a stronger immune system.

My kids played outside. In the woods, in the dirt, out with dogs, feeding the cats, fishing, basically exposed to everything. No alergies. General rule was wash hands before eating....Im SURE they ingested much while out and about as little ones, and still do as teens. Felling trees, chopping wood, and digging inthe orchard and gardens.

This is the same environment that can create sour dough, and pickles.....

My kids are healthier than most. Rarely miss a day of school.