Transcript What is Life?

Is it Alive?
By Will Pierce
I400
Spring 2007
A difficult Question

Many possible definitions
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Each seems to exclude some living things
while including non-living ones
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Fundamentally limited by our lack of
experience with life outside Earth
• Carbon-and-water-based life is all we know
Conventional Definition
(Wikipedia)

Homeostasis
• Self-regulation of internal temperature
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Organization
• Composed of one or more cells
• Parts are interdependent
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Metabolism
• Consumption and conversion of non-organic
material into energy
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Growth
• Both individual organisms and species
population will grow in response to an
abundance of energy
Conventional Definition
(Wikipedia)

Adaptation
• Individual organisms adapt to circumstances
and environment
• Population as a whole evolves due to natural
selection
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Response to Stimuli
• Ranging from very simple to very complex
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Reproduction
• Species population is able to autonomously
expand via sexual or asexual reproduction
Problems

Not all living organisms conform to all of
these specifications
• Classic example: The Mule, a genetic hybrid, is
clearly alive but not able to reproduce
• Viruses are unable to reproduce without a host
• Certain ‘castes’ of insect species (i.e. worker
ants) are sterile
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Some non-living phenomena could be
encompassed by these criteria
• Fire can grow, reproduce, adapt to its
environment, convert surrounding material
into energy, maintain its internal temperature,
and basically do everything else on the list
• Computer programs can be made to perform
most of the above functions as well
Farmer and Belin

Life is a Pattern in Space-Time
• Rather than a set of physical characteristics

Autonomous reproduction, if not in the
organism itself, in a related organism
• Covers mules, viruses, and worker ants

Storage and interpretation of a Self-image
• Example: DNA (genotype) is interpreted by
RNA to define an organism’s characteristics
(phenotype)

Metabolism
• Organisms such as viruses can make use of a
host organism’s metabolism
Farmer and Belin
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Functional Interactions with the
Environment
Interdependence of Parts
Stability Under slight Changes in
Condition
Evolution
Alternative Definitions

Schroedinger: Negative Entropy
• Life maintains a low level of entropy by
‘exporting’ entropy
• Entropy, as defined by the Second Law
of Thermodynamics, states that in a
closed system, all objects’ energy will
move towards equilibrium
• Living things derive a net increase in
energy from their environment
Alternative Definitions

Stuart Kauffman’s “Systemic”
definition
• Autonomous agent or multi-agent
system capable of completing at least
one thermodynamic work cycle

Self-organizing and self-producing
(autopoietic)
Still Problematic

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All of these definitions, while
insightful and generally workable,
leave something to be desired
A mule, worker and, or virus would
be “less alive” than an intelligent
robot, by some criteria
• Forces us to attempt some sort of
physical definition (i.e. carbon chains)
• Or we can adapt our definition to
include certain forms of artificial or
man-made life
More Problems

Some criteria are too specific
• Example: “genotype is separate and different from
phenotype”


Applies to most but not all organisms
Example: Prions, a special type of virus, are mad up of a
single protein, which carries both their genotype and
phenotype
• This can be misleading

Biochemical or thermodynamic definitions tend to
exclude automata or computer intelligence, which
are considered by some to be alive
• Also excludes theoretical or extraterrestrial life that we
have not yet encountered
Highly Subjective

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Dependent upon individual values
and schemas
Can man-made entities truly be alive
• Computer intelligence
• Robots
• Clones or genetic hybrids

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Or are these just simulations of life?
Heavily dependent on personal
values and prejudices
Middle Ground

Some properties can be universally agreed
upon
• Response to stimuli, adaptation, selforganization

Not all criteria satisfied by all life forms
• But we can all agree that a mule or a worker
ant is alive despite inability to reproduce
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Many non-living things satisfy some of our
criteria
• But we can all agree that fire is not alive,
despite its ability to grow, adapt, maintain
homeostasis, metabolize non-organic material,
etc.
Intuitive
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For lack of a better terminology, we
could say that we “just know” if most
things are alive
Obviously there are grey areas
Hence the study of “theoretical life”,
“artificial life”, etc.
Study the Fringes
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Most biological life can be easily
categorized and defined by reasonable
criteria
Theoretical life studies the oddities,
exceptions, and borderline cases
• These cases help to further our understanding
and expand our thinking about what
constitutes life
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If we can assimilate these difficult cases
into a unified definition of life, we can
eliminate some of the confusion
Conclusion
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It seems nearly impossible at this
point to come up with an allencompassing definition of life that
everyone can agree on
Best to agree on the most common
characteristics, which define most life
as we currently know it
• Deal with the exceptions on a case-bycase basis