From Diana Kwon at The Scientist:
While analyzing genetic material found in a wastewater treatment plant, scientists uncovered the genomes of four new species of related giant viruses. These newly discovered specimens, dubbed Klosneuviruses, challenge the notion that giant viruses evolved from a fourth domain of life, researchers wrote in a study published today (April 6) in Science.
Some scientists believe that rather than having a common ancestor, these giant viruses began as small viruses and gradually accumulated host genes over time. Analysis of the Klosneuvirus genome reveals evidence supporting the latter theory, according to the authors of the present study. More.
These viruses have up to 1.57 million base pairs, with many genes encoded for components of translation machinery.
Not everyone agrees that this study provides sufficient evidence to support the theory that giant viruses evolved from smaller ones. “We are at the beginning of the history [of the study of giant viruses], and it’s very early to make generalizations about this,” said Didier Raoult, the director of the Research Unit on Infectious and Emerging Tropical Diseases at Aix-Marseille University, France. Raoult, one of the scientists who discovered Mimivirus, was not involved in the current study. “I don’t believe that you can make any conclusion on the origin of this giant virus just because you find that amino acid transferases are subject to lateral gene transfer—these are the genes that are most commonly exchanged,” he told The Scientist.
Actually, we hardly know what viruses are at this point. One thesis kicked around is that giant viruses may be degenerate cells (devolution). Possibly, all the origins stories will turn out to be true, but different ones for different giant viruses. That’s what happen when evolution becomes a history, not dogma.
Lots more about giant viruses. Are they a fourth domain of life?:
Meanwhile, here is a site on giant viruses.
Horizontal gene transfer: Virus carries DNA of black widow spider toxin
Largest virus genomes hint at fourth domain of life?
The French vs. Darwinism
Mimivirus discoverer doubts Darwin, banned from publication in France
Megavirus has largest currently known genome, mimics cell genome
New Nature paper challenges conventional boundaries between kingdoms of life
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One theory has viruses as the earliest life of sorts: