Fire was the first force of nature tamed for cooking. Yeast was second.
In the early days of ancient Egypt, around 3100 B.C., there lived a ruler named Scorpion, who probably did not look like The Rock. When Scorpion died, pyramids had not yet been invented, so he was buried in a broad, low tomb that today we call a mastaba (the Arabic word for "bench").
Inside the mastaba, German archaeologists discovered 700 jars that had originally contained 4,500 liters of resinated wine. The resin was from the terebinth tree, which was used in ancient times to slow the wine's inevitable oxygen-induced march toward vinegar, and to mask any off-flavors. Fresh grapes and figs had also been added, along with mint, coriander, and sage.
The contents would probably not seemed much like modern wine, but perhaps something akin to the Greek wine still made with resin today, retsina. In addition to all the fruit and herbs, scientists who analyzed the residue found one other chemical signature: fragments of the DNA of Saccharomyces cerevisiae. Yeast.
We know that wine was made long before the time of Scorpion. Tartaric acid residues from a 7,100 year old potsherd from the mountains of northern Iran tell us winemaking is at least that old, and the earliest known winery was discovered in a cave in Armenia and dates to about 6,000 years ago. About the same time, people discovered how to make leavened bread and beer, too.
Clearly, we've been harnessing yeast for a long, long time to make some of the most celebrated foods in the world. But most people are probably still as clueless as the Egyptians were as to the strange substance turns wheat, barley, and grapes into bread, beer, and wine. What is it, anyway? It's not simply the tan, granulated contents of the Fleischmann's packet. S. cerevisiae, the stuff made by Fleischmann's whose traces were also found in the tomb of King Scorpion is indeed a yeast. But yeast is not S. cerevisiae. Not by a long shot.
Yeasts are fungi. In particular, they are fungi that grow as individual or small clusters of cells instead of in the long, tubular chains of cells called hyphae that compose most fungi.
Here is what fungi normally look like under the microscope:
And here are what yeast can look like (this is S. cerevisiae, but other yeasts may look different):
Some yeasts live almost exclusively this way, but some alternate between single-celled yeast forms and multicellular filamentous forms. Even S. cerevisiae will, under starvation conditions, form filaments to forage for food.
Here is the yeast Candida albicans (of yeast infection fame) appearing in both yeast and filament-like forms.
In short, yeast is the name for a lifestyle -- NOT for a related group of organisms. That is, it's not a taxonomic designation like "Birds" or "Vertebrates". About 1,500 species of fungi -- some closely related, some not -- have adopted this lifestyle in whole or part over the course of Earth's history in response to the similar environmental pressures. We call this convergent evolution, and it is one of biology's dominant themes.
Yeast often reproduce by budding, which in S. cerevisiae appears like a cell blowing up a balloon (the daughter cell). Yeast have also adopted a form of growth that mycologists have dubbed "the shmoo" in reference to a character that appeared in the L'il Abner comic strip post-World War II. It's a look they adopt briefly before budding, but also when they are reaching out to look for yeast mates in response to pheromones released by the opposite sex (which mycologists have quite clinically dubbed a and α).
Here are yeast budding and then "shmooing":
Yeasts are naturally found floating in air and on just about every surface on Earth, including every opened cheese in your fridge (upon which they will form small cream-colored colonies if left long enough) and on grape skins. It's not a long stretch to go from grape juice to wine if said juice is left sitting around for a bit, which is no doubt how the first wine got going.
Yeasts are good at making bread, beer, and wine because they are good at harvesting energy from sugar without oxygen, a process called fermentation. In addition to energy, it produces two by-products: carbon dioxide, which puts the lift in leavened bread and the bubbles in beer; and the alcohol called ethanol, which adds interesting but well-known properties to wine and beer, but evaporates in the bread oven. The process is essentially the same for all three foods: fermentation by friendly microbes, a form of controlled spoilage. During fermentation, the yeast also produce many enticing flavors not originally found in the wheat, barley, or grapes, as anyone can attest who's been intoxicated by the smell of fresh yeast bread, or appreciated how a glass of fine wine differs from a grape.
So there you have it. Yeast are a huge group of (usually) single-celled, fungi that (usually) reproduce by budding. They're not all closely related, and some of them moonlight as filamentous fungi, or only briefly flirt with being yeast. Some are good at turning sugar into ethanol (yeast pee), a few of which we have domesticated. For all their hard work, we often reward them by death in a fiery oven, or by drowning in their own poisonous waste. But without them and their revolutionary food products, it's safe to say the history of cooking -- and history, period -- would be a radically different tale.