At their essence, eukaryotes are combinations of bacterial, archael and viral genomes. Eukaryotic evolution has been one of parasitism, mutualism, endosymbiosis, and frequent horizontal gene transfer. While this began with endosymbiotic events between Bacteria and Archaea, it progressed into endosymbiotic events between Eukarya and Bacteria, as well as Eukarya and other Eukarya. In many ways, eukaryotes can be viewed as the products of the coordination (and sometimes competition) between two or more genomes. Eukaryotic evolution has been the result of endosymbiotic events that have mixed large numbers of genes into the progenote cells followed by mutation and selection that has led to combinations of genes and gene products that have resulted in successful cells and multicellular organisms. Additionally, horizontal gene transfer events are common in Eukarya. They have occurred often in the past and continue to happen daily, although all of them do not lead to successful evolutionary events. These can be the transfer of a single gene or multiple genes by viruses, Bacteria, or other Eukarya. Alternatively, entire genomes can be transferred in the form of entire organisms, as was the case in the evolution of the organelles in eukaryotes.