Imagine a single living cell, its tiny organs and brain-like nucleus protected from the world by a strong cell wall. Suddenly, a virus touches down on the outside of the cell like a spacecraft landing on the moon. Similar to a parasite, the virus injects its genetic material through the cell wall, taking control of the host cell.
The cell is forced to make copy after copy of the invading virus, and soon the cell dies a violent death as the new viruses burst forth.
This example of a virus “life” cycle is staggeringly common; in the oceans, a viral infection occurs 1023 times per second! Despite their ubiquity, the exact nature of viruses remains somewhat mysterious. Early virus research in the 1930s described viruses as completely non-living packages of biochemicals. Today, scientific opinion varies over whether viruses are alive or not. The debate revolves around several key issues:
When a virus is not inside a host cell, it exists as an inert particle called a virion—otherwise known as the “virus particle”—and generally consists of genetic material (RNA or DNA) and a protective protein coating. In this state, a virion is similar to an acorn, which is inactive, but has a latent potential for life.An acorn, however, has the ability to use energy and to grow into something else entirely, whereas a virion only possesses the genetic instructions for how to grow another virion.
A virion needs the resources of a living cell to put these genetic instructions into action, which is why it must take control of a cell. And if a virus can’t self-replicate or convert food to energy, is it a living thing?
Viruses and the environment
Some viruses have influence over their surroundings on a worldwide scale. For example, marine algae called Emiliania huxleyi periodically produce massive blooms, which are subsequently killed off by viruses named EhV viruses.
The destruction of the blooms releases chemicals into the atmosphere that induce cloud formation and rain. Intriguingly, the EhV viruses seem to lie in wait inside the algal cells, and then kill all of them when the total size of the blooms reaches a critical mass. Does such control over the environment and climate mean viruses deserve a place on the “tree of life?”
What came first, the virus or the cell?
Viruses may play an important role in the evolution of other biological organisms. When viruses replicate, they splice fragments of their genes in with those of the host cell. The host cell then replicates; successive generations of cells contain and express the genetic characteristics of the virus as their own. Gene transfers between cells create more variations amongst members of the same species, possibly influencing the evolutionary process.
But do viruses themselves evolve? Since viruses undergo random mutation and gene exchange much more often than other organisms, it is difficult to trace back their lineages in time. Proposed theories for virus evolution include:
First, the “escape theory” that long ago some genetic material somehow escaped from the nuclei of living cells. The genetic material became doomed to search, zombie-like, for host cells in order to replicate.
Second: cells evolved from viruses. Some early viruses came together, gathered some structure around them, and evolved into cells.
And finally: ancient viruses were possibly much more complex, like cells, and have since devolved into simpler organisms.
This third option is supported by the case of giant viruses, called giruses. Discovered relatively recently, many giruses have more genetic sequences than some single-celled organisms. Strangely, some of these sequences are instructions for processes which cells routinely perform, like gene translation, but which are useless to modern viruses.
Could it be that these genes are remnants of a more complex, cell-like virus ancestor? If viruses used to be alive, but have evolved into non-living structures, then we are forced to reconcile the difference between living and non-living states.
So, are viruses alive?
It is too soon for a scientific consensus on this question – many of the issues outlined here are still being investigated. Clearly, viruses blur the line between the living and the non-living. Today’s young virologists can look forward to taking part in a fascinating debate.