Mimi, the largest known virus. - redrawing of the tree of life.

A monstrous discovery suggests that viruses, long regarded as lowly evolutionary latecomers, may have been the precursors of all life on Earth

by Charles Siebert, Photography by Jörg Brockmann
From the March 2006 issue, published online March 15, 2006
Discover_magazine


Few things on Earth are spookier than viruses. The very name virus, from the Latin word for "poisonous slime," speaks to our lowly regard for them. Their anatomy is equally dubious: loose, tiny envelopes of molecules—protein-coated DNA or RNA—that inhabit some netherworld between life and nonlife. Viruses do not have cell membranes, as bacteria do; they are not even cells. They seem most lifelike only when they invade and co-opt the machinery of living cells in order to make more of themselves, often killing their hosts in the process. Their efficiency at doing so ranks them among the most fearsome killers: Ebola virus, HIV, smallpox, flu. Yet they go untouched by antibiotics, having nothing really biotic about them.

Now, with the recent discovery of a truly monstrous virus, scientists are again casting about for how best to characterize these spectral life-forms. The new virus, officially known as Mimivirus (because it mimics a bacterium), is a creature "so bizarre," as The London Telegraph described it, "and unlike anything else seen by scientists . . . that . . . it could qualify for a new domain in the tree of life." Indeed, Mimivirus is so much more genetically complex than all previously known viruses, not to mention a number of bacteria, that it seems to call for a dramatic redrawing of the tree of life.
"We have a lot of evidence with Mimivirus that the virus phylum is at least as old as the other branches of life and that viruses were involved very early on in the evolutionary emergence of life."
With the discovery of Mimi, scientists are close to ascribing to viruses the last role that anyone would have conceived for them: that of life's prime mover.