text: It is believed that a major driving force in the origin of aerobic fermentation was its simultaneous origin with modern fruit (~125 mya). These fruits provided an abundance of simple sugar food source for microbial communities, including both yeast and bacteria.  Bacteria, at that time, were able to produce biomass at a faster rate than the yeast.  Producing a toxic compound, like ethanol, can slow the growth of bacteria, allowing the yeast to be more competitive.  However, the yeast still had to use a portion of the sugar it consumes to produce ethanol.  Crabtree-positive yeasts also have increased glycolytic flow, or increased uptake of glucose and conversion to pyruvate, which compensates for using a portion of the glucose to produce ethanol rather than biomass.  Therefore, it is believed that the original driving force was to kill competitors.  This is supported by research that determined the kinetic behavior of the ancestral ADH protein, which was found to be optimized to make ethanol, rather than consume it.
  Further evolutionary events in the development of aerobic fermentation likely increased the efficiency of this lifestyle, including increased tolerance to ethanol and the repression of the respiratory pathway.  In high sugar environments, S. cerevisiae outcompetes and dominants all other yeast species, except its closest relative Saccharomyces paradoxus. The ability of S. cerevisiae to dominate in high sugar environments evolved more recently than aerobic fermentation and is dependent on the type of high-sugar environment.  Other yeasts' growth is dependent on the pH and nutrients of the high-sugar environment.