Sure, what can be more natural than eating vegetarian diet... and helping oneself to a handful of supplements daily. Stuff that can be picked off the land easily includes grubs, worms, snails, insects, shellfish, bones left behind with marrow etc, things that were easy to pick and that supplied protein. The steady hypothesis is that what actually led to the humans deviating from great apes is protein and fat eating that provide the surplus to grow brain. Makes sense. Chimpanses stuck mostly with veggie diet, they are only 2% different from us in genetic make up, and they didn't get the boost to the brain size the way the human branch of chimps did.