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Very Old, Very Big Viruses

Recent studies on phylogeny and evolution suggest that the tree of life may have a fourth branch. Some of this evidence, says Carl Zimmer at The Loom, comes from a newly-discovered group of viruses, called "giant viruses" because they are about a hundred times bigger than typical viruses and can have more than 1,000 genes. If the fourth domain exists, Zimmer says, it might mean that these giant viruses are part of the oldest lineages on Earth, and that researchers could learn about early evolution by studying them. In April, French researchers published a study on a giant virus that infects amoebae, and that giant virus has genes that don't even exist in other known giant viruses. The team concluded that this virus pointed to the existence of a fourth domain. Recently, however, UK researchers published a study that calls that conclusion into question, and developed an evolutionary model that eliminates the possibility of a fourth domain, Zimmer says. Uncertainty over the fourth domain remains, and researchers are waiting for definitive proof of where these giant viruses fit on the tree of life. "One way to cut down on the uncertainty would be to fill in more branches on the tree of life," Zimmer says. "It's easy to forget that for all the millions of species scientists have discovered, there are millions — maybe tens of millions — more that are waiting to be found. Right now, scientists are forced reconstruct the tree of life by comparing species that are separated by hundreds of millions or billions of years of evolution. The more species scientists add to the tree of life, the closer those comparisons will become."

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