Tuesday, 23 September 2014

Yes, viruses are alive. Even computer viruses.

The one where I get to define what Life is...



Definition as distinction with a difference that matters

I try to avoid getting into purely semantic debates that turn on nothing more than subjective preference for one arbitrary definition over another.  It might be thought that the Definition of Life question is one of these semantic arguments, but is it?

I am going to think of a definition as the set of rules governing a category.  It fixes the principles by which things can be judged to either fit the category or not.

For my money, a reasonable definition has at the very least to avoid making a distinction without a difference that matters.  By which I mean, one should not fix a category boundary between two things which are not distinguished for a good reason.

So while definitions are subjective, I am putting constraints on which subjective definitions I allow.  The category of board-games should not turn on whether the candidate activities involve throwing dice.  The definition of beverages should not require that the candidate liquids are transparent.  The distinction made by a definition has to match a difference that matters.

By what principle can one decide whether a proposed distinction distinguishes between things for a good reason, whether it matches a difference that matters?  I think I'll leave that question for now!  Maybe after putting it into practice on the Definition of Life question I will have an idea what the principle is.

Definitions of Life and the Virus

I feel strongly that viruses should be regarded as living organisms.  I think the definitions that exclude them only do so unreasonably by erecting distinctions without differences that matter.

I will proceed by taking a traditional, restrictive definition and show that, for each rule governing the category, either viruses satisfy it or the rule is unreasonable.

The list collated on Wikipedia, which I have re-ordered, includes:
1. Organization: Being structurally composed of one or more cells — the basic units of life.

2. Reproduction: The ability to produce new individual organisms, either asexually from a single parent organism, or sexually from two parent organisms.

3. Adaptation: The ability to change over time in response to the environment. This ability is fundamental to the process of evolution and is determined by the organism's heredity, diet, and external factors.

4. Metabolism: Transformation of energy by converting chemicals and energy into cellular components (anabolism) and decomposing organic matter (catabolism). Living things require energy to maintain internal organization (homeostasis) and to produce the other phenomena associated with life.

5. Homeostasis: Regulation of the internal environment to maintain a constant state; for example, electrolyte concentration or sweating to reduce temperature.

6. Response to stimuli: A response can take many forms, from the contraction of a unicellular organism to external chemicals, to complex reactions involving all the senses of multicellular organisms. A response is often expressed by motion; for example, the leaves of a plant turning toward the sun (phototropism), and chemotaxis.

7. Growth: Maintenance of a higher rate of anabolism than catabolism. A growing organism increases in size in all of its parts, rather than simply accumulating matter.
Let's consider these definitional conditions in turn, whether viruses satisfy them and whether they are reasonable requirements.

1. Organisation: Being structurally composed of one or more cells — the basic units of life.

I would argue that it is unreasonable to require a priori that life be structured by cells.  The definition of life should focus on functional requirements, i.e. requirements about what the candidate thing does.  If a virus can do everything reasonably required of a living organism, but without being a cell, then it would be arbitrary to tag on the rule that it must also be a cell.  This would be like defining the word "home" to require that the object be a house.  But if somebody lives and keeps their possessions on a canal-boat, and thus it satisfies all the functional requirements of being a home, then it would be arbitrary to deny it was their home just because it was not a house.

Note that some scientists theorise that at least some viruses evolved from cellular life:
According to proponents of this hypothesis, autonomous organisms initially developed a symbiotic relationship. Over time, the relationship turned parasitic, as one organism became more and more dependent on the other. As the once free-living parasite became more dependent on the host, it lost previously essential genes. Eventually it was unable to replicate independently, becoming an obligate intracellular parasite, a virus. Analysis of the giant Mimivirus may support this hypothesis. This virus contains a relatively large repertoire of putative genes associated with translation — genes that may be remnants of a previously complete translation system.
(Alternatively, but less preferably, I might argue that a virus is composed of cells, since parts of its reproductive cycle must take place inside a cell.  Thus the virus-in-cell complex would fit the cellular organisation requirement, and we might regard extra-cellular viruses as merely its reproductive expression.)

2. Reproduction: The ability to produce new individual organisms, either asexually from a single parent organism, or sexually from two parent organisms.

Viruses reproduce, generally asexually.  They require a cell to do it, but we just disarmed the cellular organisation requirement.

3. Adaptation: The ability to change over time in response to the environment. This ability is fundamental to the process of evolution and is determined by the organism's heredity, diet, and external factors.

Since they reproduce with mutation and under natural selection, viruses evolve and adapt.

4. Metabolism: Transformation of energy by converting chemicals and energy into cellular components (anabolism) and decomposing organic matter (catabolism). Living things require energy to maintain internal organization (homeostasis) and to produce the other phenomena associated with life.

Viruses are pieces of genetic material that hijack cellular machinery.  This includes hijacking cellular metabolism for the purpose of self-replication.  I already argued that if viruses can do everything required of a living organism without being a cell, then they should not be required to be one.

5. Homeostasis:  Regulation of the internal environment to maintain a constant state; for example, electrolyte concentration or sweating to reduce temperature.

Viruses in their extra-cellular stage have a protective covering called the capsid.  This is "a stable, protective protein shell to protect the genome from lethal chemical and physical agents".  I think it is reasonable to say that the capsid shell forms an internal environment that safely bears the virus' genetic material inside it, i.e. it performs "regulation of the internal environment to maintain a constant state".

I also suppose that viruses during the inter-cellular stage have the power to regulate the conditions inside cells in order to promote their own propagation.  This would naturally arise by natural selection.  Here is an example.

Alternatively, and even though I argue that viruses meet this condition, I might consider removing the homeostasis condition from the definition of life.  Imagine an organism that reproduced and propagated itself wherever its internal and environmental conditions were apt to it, but which was powerless to regulate those conditions.  Would that incapacity really deny it the title of Life?

6. Response to stimuli: A response can take many forms, from the contraction of a unicellular organism to external chemicals, to complex reactions involving all the senses of multicellular organisms. A response is often expressed by motion; for example, the leaves of a plant turning toward the sun (phototropism), and chemotaxis.

Viruses respond to the stimulus of the presence of cells by attaching to them then infecting them.  Also, since viruses regulate the internal conditions of cells, they must necessarily respond to changes in those conditions, i.e. stimuli.  Here are some ways in which viruses affect cells.

7. Growth: Maintenance of a higher rate of anabolism than catabolism. A growing organism increases in size in all of its parts, rather than simply accumulating matter.

It is unreasonable to demand of a living organism that it grow.  What if it happens to spend all its available energy replicating itself instead of growing?  This is a perfectly functional survival strategy.  So I reject this requirement.

To sum up:
  1. It is unreasonable to require that life be cellular, if an entity can perform all the functions reasonably required of a living organism, and yet not be cellular.  Viruses are such entities.
  2. Viruses reproduce.
  3. Viruses adapt.
  4. Viruses use host cells to metabolise.  They have a way to perform the metabolism they need without being cells.
  5. Viruses perform homeostasis, both inside and outside of cells.  I am not sure this is a reasonable requirement anyway.
  6. Viruses respond to stimuli, both inside and outside of cells.
  7. It is unreasonable to require a living organism to grow, when it might just as functionally use all its energy to reproduce itself.
Therefore, viruses appear to be living organisms by a reasonable definition.

That definition requires: reproduction (and thus adaptation), metabolism, and response to stimuli.  Homeostasis is arguable.  Cellular structure and growth are unreasonable demands.

Artificial life

What about computer viruses?  Stephen Hawking has said he thinks they are alive.  Based on my three-rule definition of reproduction, metabolism, response to stimuli, are computer viruses living organisms?

They certainly reproduce.  They are essentially lines of code that copy themselves through wires and sockets onto other items of host media.

Do they metabolise?  Do they transform energy and matter so as to sustain their existence?  Clearly they do.  They are lines of code that program computers to sustain them in existence and copy them.  So what if their matter is made up of binary switch-positions and the energy they use to preserve and replicate themselves is electrical?

Do they respond to stimuli?  Surely some of them do.  Is not installing in response to a click a response to a stimulus?  Is not detection of the insertion of a USB stick a stimulus to which the virus' programmed response is to copy itself onto the new medium?

Computer viruses reproduce, metabolise, and respond to stimuli.  They are artificial living organisms.

My definition of life

In the end, I am not so sure that response to stimuli ought to be a condition of being a living organism.  Imagine an organism that simply performed ad infinitum the same processes, and thereby survived and replicated for a length of time, even without its responding to changes in its environment.  This would not be very adaptive in terms of survival in changing conditions, but who says conditions must change?  Perhaps an organism could survive in deep intergalactic space where the conditions are so stable that response to stimulus is unnecessary.

Therefore I am striking response to stimuli from my definition.

I'm down to reproduction and metabolism.

I have pared down the traditional conditions until I have reached Hawking's definition:
One can define Life to be an ordered system that can sustain itself against the tendency to disorder, and can reproduce itself.
Frankly, I'm not sure about reproduction either.  If we do not require a living organism to grow, why should we require it to reproduce?  What if it simply exerted energy in order to sustain itself in existence?  Why should making copies of itself be a requirement?

If we equate survival with maintaining an existing physical order, and strike reproduction, then we are left with this:
Life is an ordered system that can sustain itself against the tendency to disorder.
This requires the use of energy and matter, i.e. metabolism.

Life is order-sustaining metabolism.

This is the only definition that encompasses the whole of the history of life on Earth, from the first hypothetical self-replicating molecules through to the elephant.

Finally, can we come to a conclusion as to what sorts of differences support reasonable distinctions, so that we can determine which definitions are reasonable in general?  I'm not sure I'm there yet!

2 comments:

  1. Just glanced over your essay so forgive me if I didn't catch your full thesis.
    If a virus cannot sustain itself without a host its metabolism would not sufficiently meet the criteria required for a self sustaining organism. Perhaps the attributes of 'life' are on a continuum from ultra basic to super complex and that all we are debating are bits of a generally accepted definition.
    Nevertheless you've got me thinking. I'll read it more closely tomorrow.

    ReplyDelete
  2. I criticised the criteria. That was kind of my point! :)

    ReplyDelete

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