Tanya Woyke, Frederik Schulz, and their team from the Joint Genome Institute were looking for ammonia-processing bacteria in a sewage treatment plant in Klosterneuburg, Austria, when they stumbled across the jumbo virus, which they named Klosneuvirus. After analyzing its genome, they discovered that it contains the genetic information required to make proteins with 19 of the 20 amino acids used by other life forms. The virus’s size wasn’t its only unusual trait—Klosneuvirus also might not need a host cell’s machinery to thrive.

The possibility of self-sufficiency might suggest that viruses hail from a fourth domain of life (the other three being bacteria, archaea—another type of single-cell microorganism—and eukaryotes—multicellular organisms like humans). The fourth-domain theory was first floated back in 2003 when evolutionary biologist Jean-Michel Claverie and his team discovered another giant virus, Mimivirus. Claverie believes there’s a good chance that viruses were once full-fledged, single-celled organisms capable of living on their own. They evolved into modern viruses, he says, by flushing out unnecessary bits of genetic code as they became more entrenched in the parasitic lifestyle.