We may have eliminated our dogs' ability to scavenge somewhat in the
following way. Although you didn't note any problems in your dogs, some dogs
did die from eating food with bones, etc. in it. Autopsies on dogs were
rarely done in the old days, and the dog's sudden death probably wasn't
attributed to the food. A dog that couldn't tolerate bones, etc. would have
died young, since these diets were given at an early age. So these dogs went
out of the gene pool in a hurry. Today, these dogs aren't eliminated from the
gene pool, since modern dog diets allow these dogs to survive.
The ability to digest garbage without harm isn't lost in all breeds. My two
Basenjis have eaten the most horrifying things before I could grab them out
of their greedy little mouths, and I've never seen any ill effects other than
diarrhea. (They haven't eaten chocolate, onions, or large amounts of garlic,
though). Other Basenji owners have reported similar experiences. Basenjis are
relatively recent imports from Africa, where they are scavengers. The
villagers don't feed their dogs much, and Basenjis who can't live on rubbish
and bones die very quickly and disappear from the gene pool.