Many viruses including HIV and influenza A code for proteins with channel properties. These channels have auxiliary functions in infection and replication of these viruses. The viral proteins share generally no structural similarity with channels from pro- and eukaryotes. The exceptions are the large dsDNA viruses, which infect algae. They code for K+ channels with all the structural hallmarks of pro- and eukaryotic potassium channels. The results of the present project show that these viruses are not only the source of channels; surprisingly they also code for functional orthologs of K+ transporters and Ca2+ ATPases known from pro- and eukaryotes. The complete genomic sequencing of the hosts of one of the viruses reveals the presence of six putative genes for K+ channels in the alga Chlorella NC64A (now called Chlorella variabilis). A bioinformatics comparison of viral and host channels shows that they are clearly separated on different branches of a phylogenetic tree. These data support the view that the viral genes were not acquired by molecular piracy from the host. The same conclusion can be drawn for the K+ transporter genes. The long evolutionary separation between the Chlorella viruses and the Ectocarpus virus EsV-1, which all code for very similar K+ channels, stresses that the respective viral genes must have a very old evolutionary history. Collectively the data support a recent idea in evolutionary biology, which considers viruses as important elements in evolution. They are probably not only important as a shuttle for lateral gene transfer but also as a source of genes. The K+ channels in prokaryotes and eukaryotes could be among these genes.