Around 2.4 billion years ago, the ancestors of bacteria took over the oceans and began photosynthesizing, creating massive amounts of oxygen where before there had been barely any. This was the Great Oxygenation Event, which made all subsequent life on Earth possible. All life except . . . yeast.
Yeast do depend on oxygen, so these tiny fungus species really should only have popped up when oxygen took over the planet 2.4 billion years ago. The weird thing is that we have fossils of steroid molecules - which can definitely only exist in the presence of oxygen - going back to 2.7 billion years ago, and that 300 million year difference represents a bit of a mystery to scientists. Somehow, there must have been exactly the right amount of oxygen for these molecules to be produced in the oceans while leaving no trace of the gas in the ancient atmosphere.
The answer to this mystery appears to be yeast, which could have manufactured these steroid molecules while working with only trace amounts of oxygen. Researchers at California Institute of Technology placed yeast in water whose oxygen levels were 30,000 times less than what's found in oceans today. Even at those oxygen-starved levels, the yeast had no trouble putting together the steroid molecules. And the researchers say they gave the yeast a more difficult challenge than they absolutely had to - there was probably at least four times that much oxygen in the oceans before the Great Oxygenation Event.
The researchers say that early oxygen-producers, such as the ancient bacteria relative cyanobacteria, produced more minute amounts of oxygen in the days before the Great Oxygenation Event, and a lot of that oxygen ended up combining with other gases in the atmosphere. What oxygen remained in the oceans was then devoured by the yeast, who put it to use building steroid molecules and other basic tasks.
At the 2.4 billion year mark, the ancient Earth must have reached a tipping point, and a slight imbalance in the amount of oxygen produced quickly became a full-on oxygen revolution. But it looks like yeast had a crucial role to play in keeping life going in the oxygen-starved eons.