CHAMPAIGN, Ill., Sept. 13 (UPI) -- A study suggests viruses are ancient living organisms, shaking up the currently assumed configuration of the "tree of life," U.S. researchers say.
Research led by the University of Illinois confirming giant viruses as living organisms and not inanimate molecular remnants run amok could mean adding a fourth major branch to the three most scientists agree represent the fundamental domains of life.
The researchers' analysis of the 3-D structural domains of proteins offer clues to ancient evolutionary events in viruses, which previous studies of the relatedness of all living things tended to leave out, the lead researcher said.
"We've always been looking at the Last Universal Common Ancestor by comparing cells," UI genomic biology Professor Gustavo Caetano-Anolles said. "We never added viruses. So we put viruses in the mix to see where these viruses came from."
Giant viruses are large and complex, with genomes that rival -- and in some cases exceed -- the genetic endowments of the simplest bacteria, Caetano-Anolles said in a university release Thursday.
"The giant viruses have incredible machinery that seems to be very similar to the machinery that you have in a cell," he said. "They have complexity and we have to explain why."
The researchers mapped evolutionary relationships between hundreds of organisms to build a new universal tree of life that included viruses, resulting in four clearly differentiated branches, each representing a distinct "supergroup."
The giant viruses formed the fourth branch of the tree, distinct from bacteria, archaea and eukarya (plants, animals and all other organisms with nucleated cells.)
