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