Caetano-Anollés said his team looked at protein folds instead of genetic sequences because these structural features are like molecular fossils that are more stable over time. They assumed the folds that appear more often and in more groups are the most ancient structures.
HSW: West Nile Virus Overview
"Just like paleontologists, we look at the parts of the system and how they change over time," Caetano-Anollés said.
They found that many of the most ancient protein folds in living organisms were present in the giant viruses, which "offers more evidence that viruses are embedded in the fabric of life," Caetano-Anollés said. The tree his team created had four clear branches, each representing a distinct "supergroup" - bacteria, archaea, eukaryotes and giant viruses.
The researchers said the study, which was published in the journal BMC Evolutionary Biology, also bolsters claims that giant viruses were once much more complex than they are now. A dramatic decline in their genomes over time likely reduced them to their current parasitic lifestyle, Caetano-Anollés said.