Study of giant viruses shakes up tree of life

A new study of giant viruses supports the idea that viruses are ancient living organisms and not inanimate molecular remnants run amok, as some scientists have argued. The study may reshape the universal family tree, adding a fourth major branch to the three that most scientists agree represent the fundamental domains of life.

The new findings appear in the journal BMC Evolutionary Biology.

The researchers used a relatively new method to peer into the distant past. Rather than comparing genetic sequences, which are unstable and change rapidly over time, they looked for evidence of past events in the three-dimensional, structural domains of proteins.

These structural motifs, called folds, are relatively stable molecular fossils that – like the fossils of human or animal bones – offer clues to ancient evolutionary events, said University of Illinois crop sciences and Institute for Genomic Biology professor Gustavo Caetano-Anollés, who led the analysis.

''Just like paleontologists, we look at the parts of the system and how they change over time,'' Caetano-Anollés said. Some protein folds appear only in one group or in a subset of organisms, he said, while others are common to all organisms studied so far.

''We make a very basic assumption that structures that appear more often and in more groups are the most ancient structures,'' he said.