Yeast originally originated on the lush tropical planet of Yeastonia, some 47,000 light years from Earth. Just before Yeastonia exploded, their top scientists launched a single baby yeast cell into space, aboard their most advanced spacecraft. The orphaned spore hurtled at the speed of light squared through our galaxy, until, in 137 BCE, it finally plunged screaming through Earth's atmosphere and plopped into a bowl of instant waffle batter. The little yeastling cried tears of joy at finding itself immersed in a warm sticky ooziferous substance, whereupon it multiplied exceedingly and instantly yeastiformed the entire lump of batter into a delicious slice of Yeastonian paradise.
Later, the fledgling colony of yeast sent out scout parties and successfully infiltrated all available Earth foods, with the sole exception of Jewish cuisine (which they couldn't tolerate because of dangerously high levels of lethal radiation and chopped liver).
Yeast helps to feed the multitudes
Yeast is well known for its magical ability to expand even the tiniest morsels of foodstuffs to gargantuan proportions. It is written that Jesus Himself miraculously fed thousands and thousands of hungry fanatics by injecting a tiny handful of loaves and fishes with yeast cultures (allegedly acquired from Mary Magdalene's secret hiding place), which instantly transformed them into a fully-staffed McDonald's restaurant with valet parking. The Resturant was destroyed years later during the crusades and the company went bankrupt until its revival in the 20th century.
Yeast are the party animals of the known Galaxy
Utilizing the latest results from genome sequencing, electron microscopy and Hubble telescope imagery, exobiologists have recently discovered that the typical yeast cell is equipped with a cylinder of hypercompressed carbon dioxide, a biomatter replicator, tiny beer goggles, a surprisingly large supply of miniature pornographic magazines, and a distinct lack of contraceptives; thereby conclusively proving that the humble yeast is truly the party animal of the known Galaxy.
Seriously Don't Steal Yeast's Porn!
A Professor at Harvard University responsible for the genome sequencing of yeast was victim to the first yeast related murder. Upon one of his colleagues relaying to him that the yeast cells were in possesion of miniature pornographic magazines he ordered that a cell be isolated for investigation into the content. On the following monday the Professer was found dead with his willy in his hands and his eye on a microscope. Further investigation revealed that he had separated the porn magazines from the isolated cell and was masturbating to the Cell reproduction photography when he was consumed by yeast.