1. Is it Alive? By Will Pierce I400 Spring 2007

2. A difficult Question Many possible definitions Many possible definitions Each seems to exclude some living things while including non-living ones Each seems to exclude some living things while including non-living ones Fundamentally limited by our lack of experience with life outside Earth Fundamentally limited by our lack of experience with life outside Earth Carbon-and-water-based life is all we knowCarbon-and-water-based life is all we know

3. Conventional Definition (Wikipedia) Homeostasis Homeostasis Self-regulation of internal temperatureSelf-regulation of internal temperature Organization Organization Composed of one or more cellsComposed of one or more cells Parts are interdependentParts are interdependent Metabolism Metabolism Consumption and conversion of non-organic material into energyConsumption and conversion of non-organic material into energy Growth Growth Both individual organisms and species population will grow in response to an abundance of energyBoth individual organisms and species population will grow in response to an abundance of energy

4. Conventional Definition (Wikipedia) Adaptation Adaptation Individual organisms adapt to circumstances and environmentIndividual organisms adapt to circumstances and environment Population as a whole evolves due to natural selectionPopulation as a whole evolves due to natural selection Response to Stimuli Response to Stimuli Ranging from very simple to very complexRanging from very simple to very complex Reproduction Reproduction Species population is able to autonomously expand via sexual or asexual reproductionSpecies population is able to autonomously expand via sexual or asexual reproduction

5. Problems Not all living organisms conform to all of these specifications Not all living organisms conform to all of these specifications Classic example: The Mule, a genetic hybrid, is clearly alive but not able to reproduceClassic example: The Mule, a genetic hybrid, is clearly alive but not able to reproduce Viruses are unable to reproduce without a hostViruses are unable to reproduce without a host Certain ‘castes’ of insect species (i.e. worker ants) are sterileCertain ‘castes’ of insect species (i.e. worker ants) are sterile Some non-living phenomena could be encompassed by these criteria 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 listFire 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 wellComputer programs can be made to perform most of the above functions as well

6. Farmer and Belin Life is a Pattern in Space-Time Life is a Pattern in Space-Time Rather than a set of physical characteristicsRather than a set of physical characteristics Autonomous reproduction, if not in the organism itself, in a related organism Autonomous reproduction, if not in the organism itself, in a related organism Covers mules, viruses, and worker antsCovers mules, viruses, and worker ants Storage and interpretation of a Self-image Storage and interpretation of a Self-image Example: DNA (genotype) is interpreted by RNA to define an organism’s characteristics (phenotype)Example: DNA (genotype) is interpreted by RNA to define an organism’s characteristics (phenotype) Metabolism Metabolism Organisms such as viruses can make use of a host organism’s metabolismOrganisms such as viruses can make use of a host organism’s metabolism

7. Farmer and Belin Functional Interactions with the Environment Functional Interactions with the Environment Interdependence of Parts Interdependence of Parts Stability Under slight Changes in Condition Stability Under slight Changes in Condition Evolution Evolution

8. Alternative Definitions Schroedinger: Negative Entropy Schroedinger: Negative Entropy Life maintains a low level of entropy by ‘exporting’ entropyLife 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 equilibriumEntropy, 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 environmentLiving things derive a net increase in energy from their environment

9. Alternative Definitions Stuart Kauffman’s “Systemic” definition Stuart Kauffman’s “Systemic” definition Autonomous agent or multi-agent system capable of completing at least one thermodynamic work cycleAutonomous agent or multi-agent system capable of completing at least one thermodynamic work cycle Self-organizing and self-producing (autopoietic) Self-organizing and self-producing (autopoietic)

10. Still Problematic All of these definitions, while insightful and generally workable, leave something to be desired 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 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)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 lifeOr we can adapt our definition to include certain forms of artificial or man-made life

11. More Problems Some criteria are too specific Some criteria are too specific Example: “genotype is separate and different from phenotype”Example: “genotype is separate and different from phenotype” Applies to most but not all organisms 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 Example: Prions, a special type of virus, are mad up of a single protein, which carries both their genotype and phenotype This can be misleadingThis can be misleading Biochemical or thermodynamic definitions tend to exclude automata or computer intelligence, which are considered by some to be alive 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 encounteredAlso excludes theoretical or extraterrestrial life that we have not yet encountered

12. Highly Subjective Dependent upon individual values and schemas Dependent upon individual values and schemas Can man-made entities truly be alive Can man-made entities truly be alive Computer intelligenceComputer intelligence RobotsRobots Clones or genetic hybridsClones or genetic hybrids Or are these just simulations of life? Or are these just simulations of life? Heavily dependent on personal values and prejudices Heavily dependent on personal values and prejudices

13. Middle Ground Some properties can be universally agreed upon Some properties can be universally agreed upon Response to stimuli, adaptation, self- organizationResponse to stimuli, adaptation, self- organization Not all criteria satisfied by all life forms 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 reproduceBut we can all agree that a mule or a worker ant is alive despite inability to reproduce Many non-living things satisfy some of our criteria 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.But we can all agree that fire is not alive, despite its ability to grow, adapt, maintain homeostasis, metabolize non-organic material, etc.

14. Intuitive For lack of a better terminology, we could say that we “just know” if most things are alive For lack of a better terminology, we could say that we “just know” if most things are alive Obviously there are grey areas Obviously there are grey areas Hence the study of “theoretical life”, “artificial life”, etc. Hence the study of “theoretical life”, “artificial life”, etc.

15. Study the Fringes Most biological life can be easily categorized and defined by reasonable criteria Most biological life can be easily categorized and defined by reasonable criteria Theoretical life studies the oddities, exceptions, and borderline cases Theoretical life studies the oddities, exceptions, and borderline cases These cases help to further our understanding and expand our thinking about what constitutes lifeThese cases help to further our understanding and expand our thinking about what constitutes life If we can assimilate these difficult cases into a unified definition of life, we can eliminate some of the confusion If we can assimilate these difficult cases into a unified definition of life, we can eliminate some of the confusion

16. Conclusion It seems nearly impossible at this point to come up with an all- encompassing definition of life that everyone can agree on It seems nearly impossible at this point to come up with an all- encompassing 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 Best to agree on the most common characteristics, which define most life as we currently know it Deal with the exceptions on a case-by- case basisDeal with the exceptions on a case-by- case basis