19th
The genes are so different, the scientists argue, that giant viruses represent a fourth domain of life
“Charles Darwin pictured evolution as a grand tree, with the world’s living species as its twigs. Scientists identify 10,000 new species a year, but they’ve got a long, long way to go before finding all of Earth’s biodiversity. So far, they have identified 1.5 million species of animals, but there may be 7 million or more in total. Beyond the animal kingdom, our ignorance balloons. Scoop up some sea water or a cup of soil, and there will likely be thousands of new species of microbes lurking there. (…)
The tree is, in some ways, more like a web. Genes sometimes slip from one species to another, especially among microbes. (…)
Cell fusions and horizontal gene transfer are probably best portrait by interconnected branches, rather than diverging ones. The base of the tree seems especially tangled, more like a mangrove rather than an oak. With all those caveats in mind, here’s a rough picture of the tree of life that Norman Pace of the University of Colorado offered in a scientific review he published in 2009. It shows life divided up into three domains: eukaryotes (that’s us), bacteria, and archaea.
There’s a lot of debate about whether eukaryotes actually split off from within the archaea, or just branched off from a common ancestor. Nevertheless, the two forms of life are quite distinct. For one thing, the common ancestor of living eukaryotes acquired oxygen-consuming bacteria that became a permanent part of their cells, called mitochondria. They’re keeping you alive right now.
A lot of scientists wonder how all the new species that scientists are discovering are going to change the shape of this tree. Will its three-part structure endure, with each part simply growing denser with new branches? Or have we been missing entire swaths of the tree of life? (…)
Giant viruses also explode a lot of conventional ideas of what viruses are supposed to be. Not only are giant viruses monstrously big, but they are overloaded with genes. A flu virus has just ten genes, for example, but a number of giant viruses have well over a thousand. Giant viruses even get infected by viruses of their own.
For years, researchers have been finding that the diversity of genes in viruses is tremendous. It turns out that giant viruses are particularly bizarre, genetically speaking. Didier Raoult and his colleagues compared one set of genes in giant viruses to their counterparts in other lineages. Here’s the evolutionary tree they came up with. (The giant virus genes are shown in red.)
The genes are so different, the scientists argue, that giant viruses represent a fourth domain of life. Here’s an impressionistic figure they created to show how the four domains emerged from the web of gene-trading early on in the history of life (from left to right, archaea, bacteria, eukaryotes, and giant viruses).
Jonathan Eisen of UC Davis and his colleagues publish still more evidence for a possible fourth domain. (Some of the evidence can be found in a paper in PLOS One; the rest is in a shorter note at PLOS Currents.) Their evidence comes from a voyage Craig Venter and his colleagues took in his yacht, scooping up sea water along the way. They ripped open the microbes in the water and pulled out all their genes. The advantage of this approach is that it allowed the scientists to amass a database of literally tens of millions of new genes. The downside was that they could only look at the isolated genes, rather than the living microbes from which they came. (…)
That discovery might show how this possible fourth domain got its start. Did it start out as ordinary cellular life, and then some of its genes ended up in viruses? Or is the fourth domain another sign that life as we know it actually originated as viruses?”
— Carl Zimmer, Glimpses of the Fourth Domain?, Discover Magazine, March 18th, 2011.
See also:
☞ GiantVirus.org
☞ A Tree of Eukaryotes (infographic)
