The origin of viruses Steven Pelech, Kinexus Bioinformatics 25 September 2012 The existence of giant viruses has been known for a while now, including the fact that they can sport genomes that feature over 1 million base pairs and encode over a thousand proteins, as exemplified with the Megavirus chiliensis. Most of the largest viruses target unicellular organisms such as protozoa and algae. By contrast, some of the smallest bacteria have ten-fold smaller genomes with 5-fold less encoded proteins than these giant viruses. Consequently, it is not hard to envision that all viruses originally evolved from invasive prokaryotes that eventually become mobile parasites that utilized the proteins encoded by their hosts to facilitate their own replication. The giant viruses discovered to date probably represent more recent transitions of parasitic prokaryotes into viruses. With very short generation times relative to even bacteria, the earliest viruses underwent much faster rates of evolution to the point that now some viruses, such as retroviruses, have as few as three genes. Presumably, the smaller genomes arising from the loss of non-essential genes in evolving viruses conferred a selective advantage, because they could enter their host, reproduce and package their genomes faster than larger viruses. The total number of different species of bacteria on the planet is believed to be in the order of 100 million. The number of different viruses may well be in the billions. However, it is not necessary to invoke the existence of a fourth branch of life that predated or co-existed with the three known superkingdoms to explain their ubiquitous presence. Competing interests I have no competing interests on this matter.