How can evolution change something as essential as the cell division cycle that is responsible for controlling the division of one cell into two? This is simple a function that cells can't just not have for a while. But, clearly things changed, and they changed especially drastically in the transition to the fungal lineage where the G1/S control system is dominated by components not found in animals, plants or protists. Edgar Medina and Nick Buchler found that the transition to fungi was particularly interesting, as the basal fungi retained the animal components, Rb and E2F, but also maintain the newer fungal system of Whi5 and SBF. This is the first evidence for a hybrid intermediate network, in which both systems could provide for some redundant functions and thereby facilitate more drastic evolution in this essential network. But, then, where did the newer fungal components come from? We guess viruses. For more speculation, and all the data, read the paper in eLife!
https://elifesciences.org/content/5/e09492