Viruses can be defined in short as "non-cellular organisms that cannot survive on their own but reproduce themselves by using surrounding environments."
Where and how did viruses come into being? Experts have established three main hypotheses based on genetic analyses.
The first hypothesis -- the regressive hypothesis -- asserts that viruses stem from normal cells that have atrophied to genes and coat protein. This hypothesis is based on the fact that just like the human DNA, the genes of poxviruses and herpes viruses have double-stranded DNA and that the viruses have 80 to 100 genes.
However, this hypothesis cannot explain the origins of RNA viruses that account for more half of the entire viruses. While cells use just DNA to store genetic information, some viruses use RNA.
Hence the progressive hypothesis. According to this hypothesis, some viruses may have evolved from bits of DNA or RNA that "escaped" from the genes of a cell and became capable of obtaining coat protein that they need to protect themselves from the outer environment. The poliovirus that causes poliomyelitis, the comovirus that uses beans and plants as hosts, and the sindbis virus that moves between mammals and mosquitoes, seem to have totally different hosts and activity areas. However, the fact that their genes` structures very much resemble the messenger RNA (mRNA) in cells.
The coevolution hypothesis views that viruses and cells have different origins but have mutually influenced their evolution. For example, the retroviruses, without exception, have reverse transcriptase, an enzyme encoded from the genetic material of retroviruses that catalyzes the transcription of retrovirus RNA into DNA. In existing cellular organisms, the activity of reverse transcriptase is extremely limited. Most of them originate from retrotransposon, a relative of retrovirus.
Although the origin of viruses has yet to be fully discovered, what deserves our attention is that the human and viral areas of activity are getting mixed increasingly rapidly due to the influence of human activities.
"As humans put plants and animals in one place that would otherwise be unable to be in close contact, and as humans enter the activity areas of new viruses, these areas began to get mixed," said Jeong Yong-seok, a professor of biology at Kyung Hee University. "We will see the advent of new viruses more often."