Sara Reardon in Nature:
Evolutionary biologists have never known what to make of viruses, arguing over their origins for decades. But a newly discovered group of giant viruses, called Klosneuviruses, could be a 'missing link' that helps to settle the debate — or provoke even more discord. In 2003, researchers reported that they had found giant viruses, which they named Mimiviruses, with genes that suggested their ancestors could live outside of a host cell1. The discovery split researchers into two camps. One group thinks viruses started out as self-sufficient organisms that became trapped inside other cells, eventually becoming parasitic and jettisoning genes they no longer needed. Another group views viruses as particles that snatched genetic material from host organisms over hundreds of millions of years. A study2 published on 6 April in Science provides evidence for the latter idea, that viruses are made up of a patchwork of stolen parts. But it has already sparked controversy and is unlikely to settle the raucous debate.
After the Mimivirus discovery, some researchers developed a theory that put viruses near the root of the evolutionary tree. They proposed that viruses comprised a ‘fourth domain’ alongside bacteria, eukaryotes — organisms whose cells contain internal structures such as nuclei — and bacteria-sized organisms called archaea. Mimiviruses, which at 400 nanometres across are about half the width of an E. coli cell and can be seen under a microscope, were unique in that they contain DNA encoding the molecules that translate RNA messages into proteins. Normal viruses make their host cells produce proteins for them. The team that discovered Mimiviruses thought the virus' ability to make their own proteins suggested that these viral giants descended from ancient free-living cell type that may no longer exist2. “They reinitiated the debate about the living nature of viruses, and of their relationship with the ‘cellular’ world,”