| Many people are exposed to viruses and become | | | | organism is alive. The two biggest blocks to viruses |
| sick, but for the majority of the population, they have | | | | being accepted as 'living' organisms is their lack of a |
| no idea what a virus actually is. A virus is something | | | | cell membrane and their inability to metabolize on their |
| that causes an illness and it is contagious. At its most | | | | own. Another criteria for living organisms that viruses |
| basic level a virus is just a strand of DNA surrounded | | | | lack is adaptation, or the ability to change and evolve |
| by a protein shell, which is what makes it different | | | | depending on their environment. Viruses lack any hard |
| from other fragments of DNA such as prions and | | | | parts that can fossilize such as bones and they are |
| viroids. A virus cannot reproduce outside of a host cell | | | | too small to leave imprints in sand, like some ancient |
| and is considered by many scientists as not being truly | | | | jellyfish. Thus there are no fossil records of viruses, |
| 'alive'. Viruses can infect plants and animals and some | | | | which makes showing relationships between different |
| are even able to infect bacteria.Humans have dealt | | | | viruses very difficult.Because viruses do not leave any |
| with viral diseases, such as rabies and smallpox, for | | | | fossil remains, the best way to hypothesize their origins |
| many centuries. As early as the 18th century, people | | | | has been to use molecular techniques, that is to study |
| were observed who had been exposed to smallpox | | | | the genetic materials of different viruses and try to |
| or cowpox, a similar virus and had subsequent | | | | determine any relationships by finding similarities |
| immunity to smallpox. In the late 19th century a | | | | between them. There are two main hypotheses |
| porcelain filter, developed by Charles Chamberland, | | | | concerning virus origins. The first hypothesis, involving |
| was used to indirectly study the tobacco mosaic virus. | | | | the small viruses with only a few genes, is that these |
| Dmitri Ivanowski performed experiments on crushed | | | | small viruses may be runaway snippets of a living |
| tobacco leaf extracts that showed infected plants | | | | organisms DNA. These small DNA fragments could |
| were still infectious after bacteria were filtered out. | | | | have come from plasmids or transposons, which are |
| Many others performed experiments that showed | | | | transferable genetic elements and are prone to |
| similar filterable disease causing agents. Felix d'Herelle | | | | entering or exiting genomes. The second hypothesis |
| and Frederick Twort, working independently, found that | | | | involves the larger viruses. These large viruses may |
| bacteria could be infected by viruses, not just animals | | | | have been small parasitic cells at one time and, through |
| and plants. In the 1930s, Wendell Stanley showed that | | | | the process of reverse-evolution, lost any genes not |
| the tobacco mosaic virus was mostly protein and | | | | needed by their parasitic lifestyle. There are a few |
| shortly after, it was further separated into genetic | | | | bacteria, Rickettsia and Chlamydia, that support this |
| material and a protein shell.A virus is not considered | | | | hypothesis because they cannot reproduce outside of |
| truly alive because it is missing several of the main | | | | a host cell, like viruses. |
| qualities that scientists use to determine whether an | | | | |