Hi Cyrene
Sorry it has taken so long to respond. At first I did not have enough time to respond and then I spent too much time thinking about the response.
Are viruses living?
I think that a very conservative view would be that living organisms contain DNA and reproduce, or have parents that reproduce, using that DNA. Using this conservative definition, I think that, at a minimum, the DNA family of viruses would have to still be considered a life form. (It does however exclude the RNA family).
At one point ToWanderIsToWonder drew the distinction between reproducing and being reproduced. I think it does not matter since some organisms that are generally thought to be living can not reproduce with out outside aid. The Jack Pine comes to mind.
If we look at the definition provided by ToWanderIsToWonder we have:
“Essential criterium for life:
· Environmental exchange (this one reflects the far from equilibrium nature of the system -a steady state- which is essential for the emergence of complexity).
Non-essential criteria for life:
· Reproduction.
· Mutation.
· Growth.
· Sensory input.
· Movement.”
If we consider viruses to be dormant parasites outside their hosts, then despite ToWanderIsToWonder comments, we can see that all viruses can be classified as living.
My current definition, which is subject to change, carries no particular weight, and is therefore meaningless, is below.
Some of the odd things that I have pondered prior to making this post are:
The universe
Androids
Ecosystems - as a whole not simply their constituent parts
And Subatomic particles. (I know that Feynman would smile if he knew how I interpreted his diagrams)
Random Thought:
I think that drawing the line is arbitrary, but regardless of the definition that a person chooses, assuming the choice is a thoughtful one, that choice reflects on the nature of the person.
Amino Acids and Proteins
I can not say why, but I do not think of amino acids as living, however at least one protein probably can fit many definitions of life. Prions (the reputed cause of Mad Cow’s Disease) are very interesting and I would like to know more.
Thanks Ed
Standard Definitions:
Choices from The following definitions of life have been taken from a 1973 edition of the Encyclopedia Britannica: with the exception of the prion* example which I inserted:
Physiological: eating, metabolizing, breathing, moving, growing, reproducing, and responsive to external stimuli. A problem is that some machines can be thought of as examples.
Metabolic: boundary, continually exchanging its materials with its surroundings without altering its general properties over some period of time. Exceptions are seeds and spores that can be dormant over 100’s or even 1000’s of years. A flame in a closed room can also be thought of as a life form.
Biochemical: contain reproductive hereditary information within nucleic molecules. Exceptions include prions* & implies that theatrical organisms on different models could not be defined as living.
Genetic: Large molecules carry multiple genes sufficient to reproduce succeeding generations. These genes are responsible for various traits and can change over time. This emphasizes replication but there are many examples of living organisms such as hybrids that do no replicate.
Thermodynamic: Living systems can be defined as localized regions where there is a continuous increase in order. Counter examples have not been provided.
My definition (and comments):
My first condition would closely align with the Thermodynamic definition given by the Britannica but it would be modified to read:
Living systems can be defined as localized regions where there has been an increase (or increases) in Order.
Modifing the condition of continuous allows for dormancy, which many organisms, that are currently considered living, exhibit.
I would move Reproduction to an essential criterion but phrase it as reproduction of a parent (this is actually a fairly standard strategy) in order to include hybrids.
Mutation, Growth, Sensory Input, or Movement would not be in the definition because, while I see these as elements of a successful life strategy, I would not like to remove what I view as “Last of their kind” life forms.
So my criterion would be:
Living systems can be defined as localized regions where there has been an increase (or increases) in Order.
AND
Where Parental Reproduction has taken place.