Yeast are eukaryotic, single-celled microorganisms classified as members of the fungus kingdom. Yeast The first yeast originated hundreds of millions of years ago, Yeast and at least 1,500 species are currently recognized. Yeast  They are estimated to constitute 1% of all described fungal species. Yeast  are single-celled microorganisms that are classified, along with molds and mushrooms,  is a single-cell organism, called Saccharomyces cerevisiae, which needs food, warmth, and moisture to thrive. as members of the Kingdom Fungi