I'm sure you will get a historical answer in good time, but I think that the fundamental premise of your question may be a little off.

Speaking with my 'avid bread baker' hat on rather than my 'historian' hat, you don't need to add extra sugar to dough for yeast to do its thing. It will certainly kick things off, but yeast in the presence of water and flour will start doing what it does best and breaking down the flour by chowing down on the sugars already present in the flour (and producing alcohol and carbon dioxide as its waste products).

You make sourdough by simply mixing together water and flour and topping it up with fresh flour regularly, no sugar required. Before cultivated yeast was mass produced this was a perfectly good way of making bread with no sugar involved.

You might wish to rephrase your question a little to ask about bread baking processes more generally (though still with a specific time and place in mind will help you get focused answers) if nothing else pops up here.