I clearly remember an argument between my history teachers at the university, if flour milling has evolved prior to blacksmithing or not. The fact is that "The origins of bread-making are so ancient that everything said about them must be pure speculation" (Encyclopaedia of Food Microbiology, Michael Gaenzle).
Prior to sourdough, bread was flat. And then some baker, somewhere, around 6,000 years ago, noticed that the flour and water mix he’d left lying around forgotten was bubbling, and expanding and it smelt odd. He stuck it in the oven nonetheless and was amazed: his bread was chewier, it had an interesting flavour. This, of course, is speculation, but however it happened, the new baking technique caught on, was developed, and gradually spread all around Europe and the Middle East.
This is the process known today as sourdough, in which a “starter” of combined flour and water is fermented over several days with regular additions of flour and water by the wild yeasts and lactobacilli naturally present in ground grain: this starter is then added to the baker’s dough, which is left to rise for several hours, and produces delicious bread full of holes, with a firm springy crust. Bread production relied on the use of sourdough as a leavening agent for most of human history. And even if yeast took its privileged place some 150 years ago, sourdough gets back its glory nowadays. I personally eat only sourdough breads and as there is only one bakery which bakes exclusively with sourdough and it is 30 km away, I started to bake all sourdough breads and cakes. Truly, sourdough became my obsession.
Why Sourdough Is a Health Food
Why should sourdough bread be the only option? Maybe you heard about so called phytic acid, the principal storage of phosphorus in seeds, found in the bran part of the grain. In humans, and animals with one stomach, this phytic acid inhibits enzymes which are needed for the breakdown of proteins and starch in the stomach. These phytic acid molecules bind with other minerals, such as calcium, magnesium, iron and zinc, which make these important nutrients unavailable to us. It is this lack of enzymes which results in digestive difficulties. The wild yeast and lactobacillus in the sourdough neutralise the phytic acid as the bread proves through the acidification of the dough. This prevents the effects of the phytic acid and makes the bread easier for us to digest. The lactic acid bacteria present in sourdough enhance acidification, which lead to increased magnesium and phosphorus solubility. Simply put the phytase enzymes released by the yeasts as the dough acidifies effectively pre-digests the flour, which releases the micronutrients and in turn reduces bloating and digestive discomfort. Sourdough bread also takes longer to digest. Lactic acid bacteria produce beneficial compounds: antioxidants, the cancer-preventive peptide lunasin, and anti-allergenic substances, some of which may help in the treatment of auto-immune diseases. 
Sourdough is very rich in vitamins and minerals - it contains vitamins B1-B6, B12, folate, thiamin, niacin, riboflavin, vitamin E, selenium, iron, manganese, calcium, magnesium, phosphorus, zinc and potassium. Most commercially produced breads maintain only a fraction of their original nutrient content after all the processing they undergo.
Moreover, sourdough tastes amazing! And if you haven't tried it yet, check these recipes and give it a try as a New Years' Resolution!