So far, the only terrestrial organisms we have examined have been cellular life-forms. Opinion is divided on whether acellular life-forms such as viruses should be considered to be alive. On the account that I have defended, certain arguments against viruses being alive - the fact that viruses do not respire, display irritability, move, grow or excrete - are irrelevant, as these features are not constitutive of life as such. However, if one looks at the arguments advanced by defenders of the status of viruses, they tend to focus on viruses' ability to replicate, which, I have argued, is at most a necessary criterion for life, not a sufficient one.
Edward Rybicki, a professor of virology (University of Cape Town, 1998) rejects traditional textbook definitions of life, based on what he refers to as the "classical" properties of living organisms: reproduction, nutrition, respiration, irritability, movement, growth and excretion). He objects that such definitions are biased in favour of animals and plants, and argues that "the only real criterion for life is: [t]he ability to replicate".
Rybicki quotes two definitions of life to justify his characterisation of viruses as living things:
An organism is the unit element of a continuous lineage with an individual evolutionary history (Quoted by Rybicki from Luria S. E., Darnell J. E., Baltimore D. and Campbell A. 1978. General Virology, 3rd Edn. New York: John Wiley & Sons. p.4).
In other words,
an organism is merely the current slice in a continuous lineage; the individual evolutionary history denotes the independence of the organism over time (Rybicki, 1998).
According to this definition, viruses qualify as alive because they replicate, have an evolutionary history and do not depend on any particular host (or even species of host) in order to replicate. However, by the same token, a computer virus would also qualify as being alive.
Another definition, quoted by Rybicki, suggests that viruses are temporally non-contiguous organisms, which are alive when their inbuilt program, contained within their nucleic acid, is activated, and dead while the program is inactive:
Life can be viewed as a complex set of processes resulting from the actuation of the instructions encoded in nucleic acids. In the nucleic acid of living cells these are actuated all the time; in contrast, in a virus they are actuated only when the viral nucleic acid, upon entering a host cell, causes the synthesis of virus-specific proteins. Viruses are thus alive when they replicate in cells, while outside cells viral particles are metabolically inert and are no more alive than fragments of DNA (quoted by Rybicki from Dulbecco R. and Ginsberg H.S. 1980. Virology, pp.854-855).
This definition sharpens the rationale for regarding a virus as a living organism: a virus possesses a program of sorts, which is activated when it invades a host. This sounds like Aristotle's formal cause. A virus may even be said to be "designed" in a way to enable it to easily penetrate its host, take over the host cells and force them to make more copies of itself - in other words, it appears to possess intrinsic finality. In addition, a virus, unlike its computer analogue, possesses a material substrate: the nucleic acid strand of which it is composed.
It may be objected that the simple structure of a virus precludes it from possessing a nested hierarchy of organisation, one of the defining properties of living things. However, from the following description by Spencer, Nibert and Sgro (1994), it would appear that the proteins enclosing the nucleic acid in a virus interact in a co-ordinated fashion to promote the well-being and replication of the "whole" to which they belong:
A virus is a submicroscopic parasite that must infect a host cell in order to replicate, i.e. make copies of itself. The genetic information - the viral genome - is encoded by nucleic acid, either DNA or RNA. The genome is enclosed in one or more layers of protein and, if the virus is enveloped, lipid as well. In many cases, the protein layers are highly symmetrical and are composed of many copies of a few viral proteins, arranged in shells or "capsids," in which repetitious protein-protein interactions are found....
Each virus encodes its own collection of viral proteins, acting in concert, each with specific roles that enable or enhance viral replication. The function of each viral protein is inherent in its tertiary structure (three-dimensional conformation). Some viral proteins function as components of the virus capsid. Others act as enzyme catalysts of chemical reactions that are essential to viral replication, such as RNA synthesis or proteolysis. Some viral proteins may actually do both, participating in forming the capsid and acting as an enzyme catalyst (italics mine).
From the foregoing description, it seems unreasonable to deny that viruses are genuinely alive, as they appear to satisfy the formal, final and material requirements for being alive, as well as instantiating most of Koshland's seven pillars: a program (DNA or RNA), improvisation (they mutate), compartmentalisation (a coating), energy (supplied by the host cell), regeneration (they reproduce), and adaptability (they can hide inside their host until conditions become more favourable). (The satisfaction of the seclusion criterion is more doubtful.) Despite the inadequate rationale ("They reproduce") offered by most people who believe that viruses are true life forms, there are sound Aristotelian reasons for viewing them as bona fide organisms, unlike, say, computer viruses.
This does not mean that we can put viruses on a par with other living things. The life they have is a borrowed one, as they rely on their host to provide all of their metabolic functions and raw materials:
Without a host cell, viruses cannot carry out their life-sustaining functions or reproduce. They cannot synthesize proteins, because they lack ribosomes and must use the ribosomes of their host cells to translate viral messenger RNA into viral proteins. Viruses cannot generate or store energy in the form of adenosine triphosphate (ATP), but have to derive their energy, and all other metabolic functions, from the host cell. They also parasitize the cell for basic building materials, such as amino acids, nucleotides, and lipids (fats) (Davidson, M. and Florida State University, 2002).
Using Platonic terminology, we may say that a virus participates in the life of its host, in which it "lives, and moves and has its being" (Acts 17:28, originally from Epimenides' poem, Cretica, in honour of Zeus). In other words, a virus is a life-form, but only in a secondary sense of the word.
Some viruses, according to Rybicki (1998), are associated with satellite viruses: for instance, the tobacco necrosis satellite virus, which depends for its replication on the presence of the tobacco necrosis virus. These satellite viruses could be described as "third-tier" or tertiary life-forms.
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