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September 03, 2013 | By:  Julia Paoli
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Newly Found Pandoraviruses Hint at a Fourth Branch of Life

If you were to guess right now what kinds of organisms have driven some scientists to believe that another branch of life exists, what would they be? Hideous, green aliens or maybe man-eating flies? Actually, the organisms in question are neither alive nor visible to the naked human eye. Recently identified giant viruses - literally just uncommonly large viruses - are the organisms responsible for this new line of thinking.

Giant viruses appear to have been hiding from us in plain sight (figuratively speaking of course). In 2003, a group of French researchers identified the first mega viruses in a cooling tower, where they had been living off of amoebas. Since then, other giant viruses have been found in swamps, contact lens fluid (!), a pond in Australia, and off the coast of Chile. The recent identification of giant viruses doesn't mean that they have suddenly appeared on Earth in the last ten years. On the contrary, it signifies the changing notions of what it means to be a virus. Scientists have long thought that viruses are small and fairly simple. Seeing that giant viruses are neither, other scientists who may have found them before now probably assumed that they were just bacteria.

Giant viruses are unique because of their immense size and vast number of genes. In comparison to other viruses, giant viruses are like lions amongst mere house cats - larger, and more complex. Since the discovery of the first few giant viruses, such as Mimivirus and Megavirus chilensis, two extremely large new ones have been identified. Known as Pandoraviruses, for their box like shape, these two particular specimens are the largest viruses ever found. How large are they? So large that they can be seen under a light microscope. Pandoraviruses are even bigger than some eukaryotic cells, the ones humans are made of, which is radical since they are after all viruses.

The number of genes Pandoraviruses have is especially noteworthy. Their genomes range from 1,900 to 2,500 genes. In comparison, the influenza virus has a mere 13 genes while humans have roughly 24,000. What's more, only 7% of their genes have been seen before in other organisms, meaning that the other 93% is foreign. According to Jean-Michel Claverie and Chantal Abergel, who are researchers on Pandoraviruses, "the lack of similarity of most of their genes with other life forms might be an indication that they originated from a totally different primitive cellular lineage." As of now, three domains of life are recognized: Bacteria, single-celled Archaea and Eukaryotes, the domain plants, animals, and humans belong to. Pandoraviruses' foreignness alludes to the possibility of a fourth branch of life distinct from the other three and greater biological diversity.

Hand in hand with the theory of a fourth domain is the idea that Pandoraviruses and other giant viruses descended from primitive, living cells separate from bacteria, archaea, and eukaryotic cells. If so, then ancient cells must have embarked on two different evolutionary paths: some gave rise to "modern life" while others evolved into viruses.

Giant viruses, as far as we know, are harmless to humans. They have been found lurking in amoebas inside the human body but don't appear to cause any sickness. Only time and more research will tell if they actually can wreck havoc in the body. There is even some evidence that they are useful, not to humans but to the oceans. Pandoraviruses found in water regulate some populations of phytoplankton, which contribute significantly to the world´s supply of oxygen, by preying on them.

The French scientists who discovered Pandoraviruses are hopeful that future studies on the viruses will lead to further "biomedical and biotechnical innovations". Overall, I think that the discovery of giant viruses shows that much biological diversity on earth is still unknown to humans and worth discovering.


Dell´Amore, C. "Biggest virus yet found, may be fourth demain of life?" National Geographic. July 18, 2013

Ledford, H. "Megavirus claims ‘biggest genome' crown." Nature. October 10, 2011.

Lemonick, M. "Viral Beast: Meet the Supersized Pandoravirus." Time. July 19, 2013

Sheridan, K. "Huge viruses may open 'Pandora's' box: French study." July 18, 2013

Yong, E. "Giant viruses open Pandora's box." Nature. July 18, 2013

Zimmer, C. "Changing View on Viruses: Not So Small After All." The New York Times. July 18, 2013

Image Credits:

Chantal Abergel (via Wekimedia Commons).

Chantal Abergel and Jean-Michel Claverie (via Flickr).

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