In the long history of bread-making, commercial yeast is a relatively new invention (evidence of breadmaking was found in Egyptian pictures from 4000 years ago, while commercial yeast was invented just a couple centuries ago). 

Bread used to be fermented, using wild yeasts captured from the air (still the case with sourdough breads). This method is slower than simply buying a yeast at the supermarket, but there is now evidence that naturally-leavened breads not only last longer, but are more digestible and likely healthier.

In this video, I- with the help from my 2 young girls- capture wild yeast from the air (in our Barcelona kitchen) and proceed to bake a couple loaves of spelt bread with this sourdough starter. And now that we've caught the yeast, we will keep it as our "pet", feeding it once a week in our refrigerator.

For more on how to make a sourdough starter and naturally-leavened bread, see our DIY: Ferment your own bread.