Viruses share a common ancestor that existed over 3 billion years ago and may even have preceded cellular forms of life, according to a report in the May 3 PNAS by George Rice and colleagues at Montana State University.

Based on a comparison of known virus types and an icosahedral virus isolated from a hot spring in Yellowstone National Park, the team found that coat proteins in all viral types that inhabit the three domains of life—Eukarya, Bacteria, and Archaea—have conformational similarities even though the genetics underlying them is quite different.

Nearly all of the Yellowstone virus' 36 predicted open reading frame products showed no significant similarity to proteins in public databases, and so basic structural and assembly principles in this virus were compared instead, revealing an “astounding” similarity with all virus types, according to the authors.

“This suggests that this type of coat protein arrangement preceded the split of the three domains of life over 3 billion years ago,” Mark A. Young, the team's leader and coauthor of the paper, told The Scientist.

Virus genes evolve so quickly that it is very difficult to trace relationships over long evolutionary distances on the basis of sequence, said David Prangishvili, from the Department of Microbiology at Regensburg University. To investigate evolutionary considerations, more sophisticated criteria than simple sequence similarity are required. “There is this pressure upon the conservation of the structure, although the sequence is changing so very much,” said Prangishvili, who was not involved in the study.

Young said that this kind of evidence supports the notion that viruses are indeed very old and likely were present at the time of first life, if not before. He said that he believed that viruses—or progenitors of viruses—preceded the formation of cellular life on this planet.

“I think there's a huge evolutionary consequence, the realization that viruses are everywhere and life has been everywhere for a long time,” said Young, a professor in the Department of Plant Sciences and Plant Pathology at Montana State University. This fact, combined with the information from genome projects indicating that viral signatures are very common in many genomes, suggests that viruses play a major role in driving evolution of life on this planet, he said.

The research was partially supported by the US National Aeronautics and Space Administration (NASA) and by the National Science Foundation. Astrobiology often takes researchers into extreme environments so that they can understand how life may have begun. “We use Yellowstone as a laboratory to develop techniques and to find interesting novel viruses that essentially replicate in boiling acid,” said Young.

“To a certain degree, [this work] helps NASA and other agencies to think about how to look for life when you don't really know what you're looking for,” Young said.

For example, Young said he thought it highly likely that lifelike or organic molecules could have been delivered to Earth from non-Earth based bodies. “I don't think it's science fiction any more, to believe or to investigate that forms of life itself or pre-life molecules like viruses maybe could have come to the Earth from space. What's really impressive about life is how it can adapt to just unbelievable environments, and I think the ability for life to travel through space is possible.”

“It's a new world which is opening,” said Marie-Claude Serre, at the Institut de Génétique et Microbiologie. “As the authors point out, there are very few viruses from Archaea that have been isolated, and there are already a lot of new families of viruses that have been defined, so it's just like a new world to explore. It's really nice work, and very exciting to see something in 3D,” said Serre, who was not involved in the study.