The Origins of Viruses
Viruses were first recognized as entities distinct from bacteria and other disease-causing organisms between 1886 and 1898. In 1886, a German scientist by the name of Adolf Mayer was studying a disease of tobacco plants, known as tobacco mosaic disease, which he believed to be caused by a bacteria of some kind. To his surprise, however, repeated tests and attempts to isolate and culture a bacterial agent failed. In 1892, a Russian scientist by the name of Dmitri Ivanovski ruled out the possibility of a bacterial agent. In a clever experiment, he demonstrated that the disease was either caused by a toxin or, as he thought more likely, by a life form far smaller than any organism previously described. Six years later, Martinus Beijerinck, a scientist in the Netherlands, replicated Ivanovski's experiment and convincingly showed that the disease was caused by an infectious life form of some kind. Far smaller than any known life form, the agent could be diluted many times in solution and still cause disease. Unlike bacteria, however, which can reproduce independently, this life form could only reproduce by infecting tobacco leaves. This mysterious life form became known as tobacco mosaic virus - the first virus to be recognized.
Animal viruses were soon discovered via the isolation of the virus responsible for a disease of cattle, foot and mouth disease, in 1898. The discovery of the first human virus followed in 1900 with the isolation of the yellow fever virus. But it was not until the 1930's, however, that it was possible to first get a glimpse of the elusive viruses, for they are far too small to be seen under a conventional microscope - most viruses are in fact smaller than the wavelength of visible light. The introduction of the electron microscope eventually made it possible to increase magnification from 1,000 to 2,000 diameters to
around 300,000 diameters - and to see viruses for the first time.
The true ancestry of viruses is a mystery, and perhaps always will be, for viruses have left no fossil record behind them. They are so small that it is unlikely that any record of them has survived for very long, and they have only been known to science for about a hundred years - scarcely long enough to learn very much about their evolution.
Some scientists believe that viruses evolved out of cells, gradually losing so much of their genetic information that they became dependent on other cells for their reproduction, or alternatively that they arose from bits of genetic material within the cell that acquired a life of their own. Other scientists believe that viruses originated and evolved along with the most primitive forms of life, the simple molecules that gained self-replicating abilities. Some of these took the form of cells - others evolved into the viruses which parasitized those same cells.
There are a number of complex molecular life forms that blur the boundaries cells and viruses. There are pieces of self-replicating genetic material found in bacteria, called episomes, which evolve independently of their hosts, and can even move from host to another - but carry genetic information that is beneficial or even essential to their host. Many bacteria would be unable to reproduce at all without them. Episomes are in many ways quite similar to viruses - except for the fact that they only reproduce themselves when their hosts do, whereas viruses reproduce themselves hundreds of times, causing disease.
Viroids and virusoids are the smallest and simplest form of all recognized viruses and self-replicating molecules. Viroids are composed of nothing more than a single, circular strand of genetic material, and cause disease in plant cells. Replicating in the nuclei of plant cells, they often cause striking diseases in their host plants. Lacking even a protective shell of protein, viroids do not even spread easily from one cell or plant to another. Virusoids, like viroids, are small, circular molecules of genetic material. Virusoids "infect" other viruses, using the replication processes of the host virus to replicate themselves instead.