Approximately 100 million years ago (mya), within the yeast lineage there was a whole genome duplication (WGD).  A majority of Crabtree-positive yeasts are post-WGD yeasts.  It was believed that the WGD was a mechanism for the development of the Crabtree effect in these species due to the duplication of alcohol dehydrogenase (ADH) encoding genes and hexose transporters.  However, recent evidence has shown that aerobic fermentation originated before the WGD and evolved as a multi-step process, potentially aided by the WGD.  The origin of aerobic fermentation, or the first step, in Saccharomyces Crabtree-positive yeasts likely occurred in the interval between the ability to grow under anaerobic conditions, horizontal transfer of anaerobic DHODase (encoded by URA1 with bacteria), and the loss of respiratory chain Complex I.  A more pronounced Crabtree effect, the second step, likely occurred near the time of the WGD event.  Later evolutionary events that aided in the evolution of aerobic fermentation are better understood and outlined in the section discussing the genomic basis of the Crabtree effect.