This month, the pathogen experts at the University of the Mediterranean in Marseilles announced that their new discovery, a virus so big it can be seen with an ordinary microscope, is not likely to be hazardous to human beings. It will, however, almost certainly create confusion in biology.
For most of their history, viruses have been viewed as self-replicating machines, automatons that hijack the cellular machinery of the host organism in order to make copies of themselves. They are not alive, but are bits of chemical machinery that use biology's raw materials, so the story goes, because they don't have their own. It's true that most viruses have only a dozen functioning genes at most; it was always hard to think of them as biology.
Until we discovered the giant viruses, that is. In 1992, we found Mimivirus in the water in the base of a hospital cooling tower in Bradford. Mimivirus contains more than 900 genes. Some of these genes are far more specialised than you'd expect from a chemical machine: they can repair damage to their DNA, for example. Even more surprising was the discovery that a handful of Mimi's genes are found in every living organism - even the most rudimentary forms of life - suggesting that Mimi evolved from the first complex cells.
It was always a controversial idea. According to the orthodoxy, viruses ought to have evolved much later than this, as parasites could not have survived when there was nothing much around to parasitise. The late-evolving Mimi, it was thought, must simply have stolen those genes from a cell that it infected.
That was just about plausible when Mimi was the only viral carrier of these genes. But now we have the genome of Megavirus chilensis, found in seawater off the coast of Chile. According to a research paper published on 10 October, it has 1,000-plus genes - including some that also belonged to our common ancestor.
It would be too much of a coincidence to see the same set of genes stolen twice over. Much more plausible now is that viruses are a stripped-down evolutionary offshoot of the first complex cells. These giant viruses, it seems, evolved at pretty much the same time as the nucleus-carrying cells that make up our bodies. We have, more or less, confirmed the existence of another branch on the tree of life.
In recent years, biologists have laid bare a phenomenon called horizontal gene transfer, which involves the acquisition of new genetic material from other organisms. This may have shaped life far more than what was always seen as the main source of variation: inherited mutations and in-species shuffling. Then we discovered that environmental conditions can induce lasting change in the genome. Now the giant viruses - which were not even thought to be alive - are putting another twist on the story of life. Biology is fast becoming
a fascinating mess.
By the way, the researchers said Mimivirus couldn't infect human beings - but it then gave one of their technicians a bout of pneumonia and the lab's safety procedures had to be upgraded to Biosafety Level 2. Whatever their public pronouncements, the Marseilles researchers are almost certainly treating their new giant with rather more care than they are letting on.