Introduction To Viruses - Origins

Origins

Further information: Virus#Origins

Viruses co-exist with life wherever it occurs. They have probably existed since living cells first evolved. The origin of viruses remains unclear because they do not form fossils, so molecular techniques have been the most useful means of hypothesising how they arose. However, these techniques rely on the availability of ancient viral DNA or RNA but most of the viruses that have been preserved and stored in laboratories are less than 90 years old. Molecular methods have only been successful in tracing the ancestry of viruses that evolved in the 20th century. Three main theories speculate on the origins of viruses:

Regressive theory
Viruses may have once been small cells that parasitised larger cells. Over time, genes not required by their parasitism were lost. The bacteria rickettsia and chlamydia are living cells that, like viruses, can reproduce only inside host cells. They lend credence to this theory, as their dependence on parasitism is likely to have caused the loss of genes that enabled them to survive outside a cell.
Cellular origin theory
Some viruses may have evolved from bits of DNA or RNA that "escaped" from the genes of a larger organism. The escaped DNA could have come from plasmids—pieces of DNA that can move between cells—while others may have evolved from bacteria.
Coevolution theory
Viruses may have evolved from complex molecules of protein and DNA at the same time as cells first appeared on earth and would have depended on cellular life for many millions of years.

There are problems with all of these hypotheses: the regressive hypothesis does not explain why even the smallest of cellular parasites do not resemble viruses in any way. The escape hypothesis does not explain the structures of virus particles.The coevolution, or virus-first hypothesis, contravenes the definition viruses, in that they are dependant on host cells. But, viruses are recognised as ancient and to have origins that pre-date the divergence of life into the three domains. This discovery has led modern virologists to reconsider and re-evaluate these three classical hypotheses.

Read more about this topic:  Introduction To Viruses

Famous quotes containing the word origins:

    “Compare the history of the novel to that of rock ‘n’ roll. Both started out a minority taste, became a mass taste, and then splintered into several subgenres. Both have been the typical cultural expressions of classes and epochs. Both started out aggressively fighting for their share of attention, novels attacking the drama, the tract, and the poem, rock attacking jazz and pop and rolling over classical music.”
    —W. T. Lhamon, U.S. educator, critic. “Material Differences,” Deliberate Speed: The Origins of a Cultural Style in the American 1950s, Smithsonian (1990)

    “The origins of clothing are not practical. They are mystical and erotic. The primitive man in the wolf-pelt was not keeping dry; he was saying: “Look what I killed. Aren’t I the best?””
    —Katharine Hamnett (b. 1948)

    “Grown onto every inch of plate, except
    Where the hinges let it move, were living things,
    Barnacles, mussels, water weeds—and one
    Blue bit of polished glass, glued there by time:
    The origins of art.”
    —Howard Moss (b. 1922)