I have written in a blog about a scientific definition of life “What is Life, Really?” which broadens the biological or as I would like to call it the “textbook definition ” of life but considering life as a process would require that definition to be broaden much further by considering thermodynamics or energy flow and life is more of a process than a thing it that life reproduces, evolves, and makes a living generally and that definition would be broaden to include energy flow but of a specific kind of energy flow which is what life as a process.
Of course, we still consider what is it about life, that separates from nonlife and indeed we would include a definition or label for life and I think that this more than just a philosophical problem but there will come a time in the near future when sooner or later, we may actually have a tangible sample of a possible extraterrestrial life form and if it that ever happens, we would need to tell the difference between life and non life.
Which leads to the question: Is there a simple way to tell the difference assuming that there is such a difference? Consider that biology is the science that studies life and the word “biology” means “the study of life”. However the science of biology is divided into separate fields each with its own discipline of study, there is biochemistry or the study of the complex chemical processes that occur in organisms, genetics which studies how variations in living organisms appear and how these variations are transmitted to future offspring, ecology or the study of organisms and their environment, microbiology or the study of microbes or small one celled organisms and so on all within the science of biology so it seems that these fields of biology have given us multiple meanings of life, and of course it is life as we it know but there is no reason to think that what these fields of biology have found does not necessarily apply to terrestrial life but whatever broad definition of life that applies to our biosphere is also applicable to extraterrestrial biospheres so a definition of life would include life as a dynamic process and independent of what life is composed of. For one thing we know that life on earth shares a universal chemistry and that is a chemistry of carbon and water. A definition of life needs to take into account the fact that life is a non equilibrium thermodynamic process of a very specific kind and must be independent of any chemistry, whether that chemistry is carbon based, silicon based, arsenic based, or whatever. A definition like that would be universal to all life forms wherever there are found in the cosmos and does not take into account the kinds of chemistry whether it is carbon based or not.
This leads to the question which is at the heart of this blog: If there is a definition for life as a universal cosmic phenomenon then can this definition be applicable to biological systems small or large throughout the cosmos and can also this definition be summarized in something like an acronym? In an article in Science magazine, author Daniel Koshland Jr. (2002) has provided one such acronym that captures the core properties of life which is a really broadening of the “textbook” definition of life but includes processes and emphasis of processes and he given the acronym “PICERAS” (2002) and that stands for Program, Improvisation, Compartementalization, Energy, Regeneration, Adaptation, and Seclusion. We can look at each one of these definitions separately and give a broad definition which is within the scientific purview of biology, a biology that is universal.
Everything in our universe has a function and life is no different. Life is composed of a few chemical elements notably carbon, hydrogen, oxygen, and nitrogen and a few other elements such as phosphorous, sulfur,potassium, and iron and with carbon, hydrogen, oxygen, and nitrogen many complex polymers such as DNA is a universal molecule that carries biological information. Life can be considered something of a dynamic information processing systems and starting with DNA, there is a genetic code which is also universal in all terrestrial life forms and without DNA then it is likely conceivable that there would be no life for being a physical system it would be no different than an organic crystal such as quartz which is composed of silicon and oxygen and these two atoms are arranged in a regular and repeating way and a crystal like quartz can only respond to mechanical forces such as gravity and pressure but life has a higher degree of sophistication that is much complicated than a quartz crystal. Life because of a program requires information on how to synthesize molecules, which kind of molecules are used in energy conversions, how the organism will grow, how it will respond to environmental stresses, and how all this information can be passed on to the next generation.
Of all the polymers that occur in nature, DNA is the only polymer that carries all the necessary and vital information for all life forms from bacteria to humans and ever since the origin of life, there may been many molecules similar to DNA that could store and replicate information but it is possible that these polymers failed simply because of an inability to accurately replicate and store information but only DNA has the ability to store and replicate because of its structure as a double helix which consists of nearly stable covalent bonds or phosphodiester bonds where the outside of the molecule consists of phosphate and sugar bonded together while the inside of the molecule has four nitrogenous compounds called bases, each base bonded to a sugar but two bases that are linked by weak hydrogen bonds and in the inside of a DNA molecule or gene, a unit of heritable biological information, has a variable number of bases but there are still four bases nonetheless. It is the varying sequences of bases that carries the most information whereas in our quartz crystal, the information content in that crystal is low and simple since if you knew the pattern of atoms of oxygen bonded to silicon, that pattern is the same throughout the entire crystal. Not so for a gene, since the sequence of bases varies and it is this variation that carries a lot of information necessary for the organism to survive in whatever habitat it will be in.
Because of it structure, DNA not only stores information in the variable sequences of its bases but it is the only molecule capable of replication and it can only do this inside the environment of that basic fundamental unit of life, the cell ( see my blog, “That Fundamental Unit of Life, The Cell”) while carrying all the information which is passed on to the next generation. The function of DNA is present in the structure and so the gene, as a region of DNA that codes for a protein molecule because of the genetic code where three bases or codons specify either a single amino acid or building block of protein or several amino acids and using terms from information technology, the sequence of bases and hence the genetic code is like the software or information while the molecular structure can be viewed as the hardware. Life in this sense, has mastered information technology for about 3 billion years whereas for Homo sapiens computer languages and hardware have been with us for about 60 years.
In the first definition in the acronym PICERAS, program is crucial for earth life as well as extraterrestrial life for without it, then any system of atoms would be a complete disarray, since there would be no information to organize these atoms into molecules, and molecules into various functioning structures that could interact with one another, providing a complete functioning organization of the highest integrity that is life. This would apply to extraterrestrial life since any extraterrestrial organism would , like earth life, would have to carry out metabolism, have an organization of some sort that would it from inanimate matter and reproduce and vary. It would be inconceivable even to me that a system that is biological can go without a program at all.
No organism exists independently of its environment but is a part of its environment. Our earth is about 4 billions years old and during its history, continents have shifted, resulting either in cooling or warming of seas, mountains have eroded producing a lot of sediment near oceans, magnetic fields have reversed polarity, and occasional meteorites have collided on the earth during it 4 billion year history. Since the origin of life, life had to deal with these changes in the environment. Many did not survive such natural cataclysms but some however did and so as life evolved into various species that could survive in each habitat, it could never do so for long because whatever is present in the environment never lasts forever but sooner or later changes and these changes could mean the difference between life or survival such as coping with these changes or death or the inability to deal with such changes.
The fossil record or that record within the earth’s crust which has fossils, the remains of once living organisms, which have been carefully studied reveals that there is a history of life beginning with simple to complex organisms more or less and this proves that species evolve into many distinct lineages. The reason for their extinction is simply because they could not tolerate the change in their environment and so died but life overall has coped with changes as witnessed by all species of organisms alive today. Their ancestors had to have survived the gradual changes and how was that possible?
The answer lies in the program, the first letter of the acronym, and as I have said it is the program that allows survival and it does this because not only does the program, in the form of DNA, is passed on to the next generation almost intact but because of thermal energies, radiation, and chemicals, the information encoded in the DNA will be altered and such natural alterations are called mutations. A mutation can result in a slightly different message as in the original and if the message is defective then chances are, the organisms may end up with a defective gene that does no good for the organism and the organism dies. Most mutations are harmful but a few can be beneficial such as the formation of a new protein molecule which was not present in the parents and if that protein molecule has a function that benefits the organisms in terms of survival then it will likely get passed on to its offspring and over time, will end up surviving in its environment. The environment acts to determine which organism in a population will live or die and such filtering mechanism is called natural selection, a term and mechanism defined by Darwin and confirmed by countless observations and experiment.
It is through natural selection along with mutations together in the case of sexually reproducing organisms, meiosis , that every population of organisms will have heritable variations because of the program in each organism and a variation will either help or harm the organisms, the helped ones will prosper while the harmed ones will perish and through improvisation of the program will determine the future outcome of the species through environmental changes.
An organism is either made out of one cells or many cells and cells are the building blocks of life. I have argued in in a previous essay “That Fundamental Unit of Life, Cells” that cells have all the basic functions associated with life but what is about cells which are complex biological systems as opposed to those systems that are simpler and non-biological? All cells have a boundary which defines the inside of the cell from the outside and this boundary is made of a compound called phospholipid. What is phospholipid? It is a molecule that consists of chains of hydrocarbon molecules or molecules of carbon and hydrogen but each hydrocarbon is bonded to a phosphate or phosphorous bounded to oxygen atoms. Both phosphorous and oxygen are electronegative atoms are atoms with a strong negative charges while the hydrocarbon molecule is an electrically neutral molecule. How does the molecular structure play a role in the boundary of the cell?
Phospholipid molecules form the boundary called cell membranes and one unique property of these molecules is that when placed in water, these molecules will form a bilayer where the phosphate groups will be in contact with the water molecules and that is because a water molecule is a polar molecule like the phosphate group and because a water molecule has one oxygen atom bonded to two hydrogen atoms, the oxygen atom will take up the electrons into its structure making the oxygen negative while both of the hydrogens are positive. With water molecles the phospholipid molecules will spontaneously assume the bilayer configuration with an environment of water on the outside of the boundary and also a watery interior in the inside.
The inside of a cell is not just water although water makes up most of the interior of the cell. In addition there are protein molecules and amino acids, sugars, vitamins, nucleotides for DNA and RNA synthesis and inorganic ions and the concentrations of these molecules and ions are different in the inside but few molecules are also present on the outside. Outside the boundary of the cell membrane, there are water molecules, oxygen, and carbon dioxide but concentration of organic molecules and inorganic ions inside, which is quite a difference.
The organic molecules inside are organized into polymers such as DNA for information storage, proteins, which carry out much of the activity inside, and also enzymes those subset of protein molecules that do much of the work of synthesizing molecules as well as breaking them down all of which takes place in the cytoplasm, the water based fluid inside the cell.
Of course the phospholipid molecules are impermeable to most molecules but some molecules need to be imported inside as well as exported and this done by transport proteins and these are imbedded into the cell membrane. These kind of molecules can import vital molecules inside as well export other molecules such as carbon dioxide outside.
The concentration of molecules and ions is vital for the entire functioning of the cell made possible by the membrane that if the membrane were to disappear, then all the molecules would disperse in all directions and vital functions would be lost. There has been any life form discovered where it did not have a boundary such as the cell membrane. Unicellular organisms such as bacteria and amoebae have cell membranes and so are all the cells of multicellular organisms. A boundary then is as important as a program and the ability to evolve.
To live is to do something and to do something is the ability to do useful work which is the simple definition of energy. Synthesizing information polymers like DNA, synthesizing protein molecules such as enzymes which can either catabolize or break down molecules into their monomers such as enzymes that break down protein molecules into amino acids, enzymes for breaking down carbohydrates into glucose, and there are enzymes that catalyze the formation of momomers into polymers such amino acids into proteins, glucose into carbohydrates, and nucleotides into DNA and RNA and this is the opposite of catabolism, anabolism or building up. Catabolism and anabolism are part of the process of metabolism and with metabolism organisms big or small can build shells, make nests, cause independent cells to aggregrate towards one another in response to stress, to fly, to swim, to spin a cocoon in order to metamorphose into a butterfly, to run, to scream, and to fall in love, in short to do all these things that are part of living.
All of this requires energy and through energy, life as we know it can live and evolve. There is never any life form that could not perform the basic functions of life without using energy and it is energy that powers the biosphere and all of it’s life forms.
Energy prevents life from reaching thermal equilibrium or death and although living things of course do die but to avoid death, life must find a way to use energy in a usable form in order to avoid death. Life forms off all kinds do this whether it is absorbing glucose, nitrogen, and oxygen in a nutrient media where bacteria are thriving to eating pizza in a restaurant, life takes in nutrients such as proteins, fats, and carbohydrates along with oxygen and water and giving off wastes such as sweat, carbon dioxide, and body heat.
Of course we also know from experience that things tend towards decay and there is even a term to call it, which is entropy or the amount of running down or for a more precise definition, entropy is the decrease of usable or free energy which is energy that can do work along with the increase of disorder or randomness so as randomness increases, free energy decreases until equilibrium results and nothing new results.
Here is an analogy that I would like to use in order to help you understand why entropy is a quantity that measures the amount of disorder. Imagine that you have a deck of cards and you exert energy in arranging these cards in a particular sequence such the hearts, arranged from 1 to 9, then the clubs from 1 to 9, and up to the King of hearts, Queen of hearts down to the last card which are the joker card. After you arrange them in that order then throw the entire deck up in the air and watch them fall down and they fall on the floor, take each card randomly and form a deck. What are the chances that if you keep throwing them in the air and randomly pick each card and form a deck will the original sequence be present? In others words, would the deck that you form have the same exact sequence as before? The answer is of course is no, and that is because of all the random throwing and picking, it is not likely ever to be the same as before because there is so much randomness occurring that you more likely end up with a different sequence of cards everytime you throw the deck.
This randomness in the distribution of cards in the deck represents random shuffling and its get more and more random and this is what is meant by entropy being random or rather the measure of randomness, getting larger and larger. Every physical system such as a cube of ice where all the molecules are arranged in a regular order of a specific pattern but as that cube of ice is allowed to melt, the molecules will have form more room to move and their positions, velocities, and momenta will be more spread and also the water will evaporate into a gas which is way more spread out as the space between molecules gets larger and larger so the overall entropy increases.
The science of thermodynamics is that field of physics that studies energy change and there are two laws of thermodynamics. The first law states that energy can change into many different forms but the total amount is the same. An example is the biosphere which depends on solar energy, which is really a form of electromagnetic energy and through the process of photosynthesis, that form of energy is converted into chemical energy which for photosynthesis is water and carbon dioxide and through sunlight, which is composed of light of various frequencies from violet to red and hence different energies, the water and carbon dioxide are converted into carbohydrates which the plant cells use for its own use in metabolism while plants serve as food for herbivorous organisms that are adapted to draw nutrients from eating plants whether leaves or fruits. Through metabolism, the chemical energy is converted into energy of movement of these animals small, medium, or large, using the energy to move, find food, and to find mates, while being food for carnivorous animals that actively hunt them as well as parasitic organisms which are adapted to live on the surface of either carnivorous animals or herbivorous animals and even inside them.
Through what are called trophic layers, energy is passed from one layer, which is photosynthetic organisms up to carnivores, and energy is really being transferred while each organism up gives off wastes such as heat so useful energy is converted into another form of energy heat, which a form of energy that is random and disordered and here we come to the second law of thermodynamics and that is law of increasing entropy.
The second law states that for any process that converts energy into other forms, entropy inevitably increases and whatever free energy is present, will decrease until equilibrium is reached. Once entropy reaches a maximum, there is no more energy to do useful work and things come to a complete halt.
Physical systems move towards entropy but life is a physical system that evolves, reproduces, and metabolize, and life through its evolution tends to get more and more complex and in the 4.5 billion year history of earth, life from its origin to the present has been on this planet for about 3 billion years. According to the second law, life, as a physical system, and hence the biosphere should be in equilibrium but clearly it is not.
How is that possible? Is life violating the second law? The answer is no and we could clearly see that organisms if deprived of food, space, and mates will die and their molecules will escape their dead bodies, and like the molecules of the melting ice cube, will spread out farther but how do organisms avoid death?
The second law of thermodynamics was originally formed within the study of closed systems or systems where there is no flow of matter in or out but only energy and through the study of these systems was the conclusion reached that entropy increases in closed as well as in isolated systems but life forms are not isolated and they are not even closed systems, they are open systems and open systems take in energy and matter , organize themselves into more intricate forms with a degree of function, and this is done because of flows of matter and energy.
In biology, metabolism is that complex process of building up and breaking down but life does it in a very precise and specific way, but it does this nonetheless thanks to flows of matter whether it is in the form of carbon dioxide and water along with inorganic ions and under the presence of sunlight, and in the process of photosynthesis, a form of anabolism, carbohydrates are produced and plant cells use carbohydrates to form various kinds of biomolecules such as amino acids, nucleotides, and vitamins, while wastes such as oxygen gas are released, Organisms that use oxygen as an energy source combine it with glucose, amino acids, and fatty acids, the building blocks of fats to release chemical energy which is converted into various biological activities while heat is released slowly along with water and carbon dioxide, and this is through a process called respiration. Both photosynthesis and respiration are like mirror images of one another and form the major cycles of life that defines the biosphere and this happens because earth intercepts a small amount of visible light which powers photosynthesis while powering respiration while oxygen, carbon dioxide, and heat are released but these processes continue as long as sunlight flows in and heat flows, out, carbon dioxide is taken in while oxygen flows out, and oxygen and food goes in and heat and carbon dioxide flows out so flows of energy and matter goes in each organism and wastes goes out and the cycle repeats.
Overall the entropy of the biosphere becomes negative or more organized, more ordered , and with free energy while the total entropy of the space surrounding the whole biosphere increases. In order to have negative entropy which defines the biosphere and hence life, there has to be disorder increasing somewhere else and by focusing on the biosphere as an opens system along with fluxes, we can see that life does not violate the second law but it is because of the second law that life is possible.
At the most fundamental level of life, that is recognizable as life which is the single cell, it is a complex network of biochemical reactions, all under the influence of the second law as well as the first law of thermodynamics. Each reaction is catalyzed by enzymes in various steps, whether it is a breakdown of a large molecule or synthesis up into another large molecules. Some reactions proceed quickly, some slowly but in this process, sooner or later, a molecule may not present where it needs to be and it could get lost into the environment.
If this were to happen and if it did, then the cell will slowly succumb to the second law and every molecule will spread out much more farther and the cell will cease being a cell, or in other words it would be dead.
Because its take energy to keep organisms from reaching death, some of that energy is allocated into processes that replaced any damaged parts and the damage that occurs is kept at a minimum and this is what is meant by regeneration in that acronym.
Sometimes we may not think that it is a important prerequisite of life but when you think about it, it is essential and consider this analogy.
For those who own a car or truck, we know that sooner or later it will break down (thanks to the second law) and in order to keep a car or truck running, if it does break down you just take it to your nearest repair shop and have expert mechanics replace any damage or defective parts. Likewise for a computer or laptop, if a hardware component is damaged or corrupted by a virus, then you take it to your nearest computer store and have trained technicians replace the damage part or infected software program ( the second law applies not just to complex mechanical systems such as automobiles but to information processing devices as well).
For any artificial systems that does something which uses energy such as cars, you have replace anything that is damaged but what about biological systems such as yourself?
It turns out that organisms have the ability to repair any damage without anything external. An example is if you were by accident that you cut your finger, and in response to the cut, a complex series of biochemical reactions will occur such as forming blood clots to prevent blood loss, certain chemicals to cause cells to divide in the tissue layers of the skin, and antibodies and white blood cells to prevent infections. Not a single one of these components come from the outside but from within and that is one characteristic of life, the process of regeneration.
Life has evolved that ability in an universe dominated by the second law, whether it is wound healing or the entire regeneration of a new organism from a previous organism which is reproduction.
Regeneration is necessary for replacing molecules that are loss to propagating species into the future while in the latter case, the information for doing so is also propagated into the offspring such as how the organisms will grow, feed, behave, and reproduce. No man made structure(just as yet) has every had that ability to regenerate without requiring human agents to do so. Even with regeneration however, no single organism can truly be immortal, although the only organisms that are immortal in a sense are bacteria but a bacterium will divide into more bacteria but this fact is true much so for multicellular organisms like ourselves and as we grow from infant to adulthood, the energy for regeneration gradually decreases until the biochemical systems for regeneration lose their ability to counteract the constant losses and this adds up to the process of aging.
The only way that life has evolved immortality was to reproduce. It has done this ever since the origin of life and bacteria are in a way immortal and it was because of cellular reproduction but in the evolution of multicellular organisms, various cells specialized to do various tasks such kidney cells for excreting urine, muscle cells for motor movement and so on while a specialized set of cells were formed for reproduction and these are the gametes or the sperm and egg which carries half the genetic instructions and these kinds of cells are formed through meiosis which acts not only to halve the genetic instructions of the organisms but to rearrange the genes into new combinations in either sperm or egg so through fertilization, a new organisms with a new complement of genes from both parents result along with the vital information for growth, metabolism, and reproduction. In that sense, the only multicellular organisms that are immortal are the germ cells which can continue from one generation to the next or go extinct.
The evolution of meiosis has speeded up biological evolution of multicellular organisms because of the ability to rearrange genes on chromosomes into different combinations, but at a price and that is the price of aging. Ultimately the organism that reproduces with meosis will after reproduction will die sooner or later.
The pillars of the definition are information, energy, a boundary, and regeneration and with all these pillars, then each organism will have to find a fit and with information on how to reproduce and metabolize, how will it survive in its own habitat? What are the features the organisms possese to make a living, whether its environment is the oceans, in a mountain range, in a hot desert, or in a forest? How will the organism able to withstand temperatures that are either hot or cold, how will it defend itself against dangerous microbes or hungry predators?
Every organism has features that will either allow it to survive if its environment is stable or able to cope with changes that do occur and that pillar is adaptation. Survival is universal attribute of all life form and it can do what it must or what it has in order to survive well up into reproduction.
As Darwin (1859) was the first to point out, nature selects those individuals with the characterstics that enable an organism in a population to survive and it was he who called it “natural selection” or the mechanism that results in the evolution of a population in a changing environment.
Natural selection favors those organisms with a novel way of surviving while culling those without such survival enhancing capabilities. These features are known as “phenotypes” and they can be anything ranging from metabolic networks in cells up to the complex instinctual behaviors of animals. Whatever happens to appear in a generation , however small, that allows an organism to survive, natural selection will favor it until the environment changes.
Natural selection, should really be called “nonrandom elimination” according to Mayr (2001) since a confusion when learning about the concept of natural selection is that something or someone is doing the selection. Originally natural selection was developed in analogy to human selection of heritable traits in animals and plants for breeding and if humans can create changes in generations of animals or plants, then nature gradually can act on every phenotype allowing either survival or death and since evolution by natural selection is a process that changes the populations making it different than the ancestral population. In a population , natural selection or rather nonrandom elimination can slowly transform a population to adapt to new changes in the environment and if in a population, the organisms all have the same phenotype in a given environment that is stable, then that phenotype will allow survival.
What happens if the environment changes? In every generation, no two individuals in a population are alike because of reproduction. If you have followed through my explanations of the pillars of life up to adaptation, as well as in my first blog “Populations are the Key to Evolution”, then you will realize that an individual is an unique blend of genetic traits or genotypes from each parent (assuming of course sexual reproduction) mainly as a consequences of observed facts that give support to Program, Improvisation, Compartementalization, Energy, and Regeneration (or Reproduction), then you can see that adaptation will slowly but inevitably result and it should but whatever changes are present after each generation, if the changes allow survival then natural selection will favor that change, however slight, or if it does not, then the offspring of the organism with that kind of change will become fewer or go extinct.
There is one characteristic of systems far from thermodynamic equilibrium that must be considered and that is feedback. Feedback is a kind of behavior that acts on systems in response to any changes and this only happens for open systems kept away from equilibrium as a result of fluxes.
Feedback is present in every level of life from unicellular organisms to organ systems in living organisms up to ecosystems. Environment and system are intimately coupled, they have to be since I have stressed repeatedly that life is an example of an open system that has been on this earth for 3.5 billions of year and it is because that it is an open system that life has survived up until now.
Two kinds of feedback are present, positive which allows for increase of something and negative which allows decrease of something. At the level of cells, biochemical reactions occur constantly in response to changes in environment, any biochemical reaction can adjust to any changes in the amount of oxygen present, the amounts and kinds of nutrients present and so forth. This is where feedbacks come in. A biochemical reaction that is needed for synthesize of molecules requires energy and all life that use oxygen or aerobic metabolism utilize energy from foodstuffs such as glucose and for energy to be available as free energy, there are step by step catalysis of the molecule but it is done in a careful and controlled way as far as the whole organism is concerned and indeed such careful control is favored by natural selection.
Consider the biochemical reaction
A to B to C to D to E
A the starting molecule or reactant is catalyzed into another different kind of molecule B, which converts to C, and so on but at a point along the sequence of reactions, there is a final product that results which has the ability to continue to further its own synthesis , call it Q and if these steps all lead to Q, then Q will continue to make more of itself. This is an example of positive feedback, and it is when the final product makes more and more of itself in this example and this is illustrated in the reaction
A to B to C to D to E….Q which continues to A which makes more Q
Of course there has to be a limit in the production of the same substance or the organism may not be able survive too much of that substance and this is where negative feedback comes.
At a certain level, if too much of Q is produced then another set of biochemical reactions occur to prevent too much of Q being synthesized. Negative feedback is as critical as positive feedback for the long term survival as natural selection would have favored the first simple forms that could control biochemical reactions with both negative and positive feedbacks. Those life forms that only used positive feedbacks were likely at a disadvantages since if there was nothing to control the synthesis of a vital substance, then too much of it would have end up killing the organism while if there was too much negative feedback that too would have been bad.
So much for biochemistry. What about behavior? Behavior is also part of the organism’s phenotype and it is common in every life form , even in microbes. As a dedicated and practicing amateur biologist, I have observed the movements of microbes like Paramecium that move effortlessly through water and when a paramecium encounters an obstacle , it will respond , shift direction and move away. I have even seen bacteria form clusters , from bacteria that swim singly but also can form large groups. These kinds of behaviors serve a purpose, and like biochemical reactions, behavior allows survival and this is evident in the ability of microbes to discriminate between food and poison. If there is food such as glucose, microbes will swim towards or if there are poisons, they will do the opposite.
Behavior then is a form of feedback, favoring individuals of populations to come together such as birds flocking together for mating, which is considered positive feedback or if a hungry predator is present then the birds will fly away.
In the evolution of life, as multicellular organisms become specialized in terms of the kinds of cell that perform specific function, the ability to respond to the inside as well as physiological changes occurring inside of each organism became more sophisticated and this was paralleled with the evolution of nervous and hormonal systems. A nervous system allows an organism to sense its environment, allow it distinguish between predators and mates, and move towards sources of food, while a hormonal system allows for reproduction and for changes in physiological states such as hunger or thirst and both systems were favored by natural selection and evolutionary mechanisms, those mechanisms that favored the evolution of nervous systems and hormonal systems in the evolution of animals, can also be seen as feedback mechanisms, the survival and thus speciation of populations are like the positive feedback while extinctions are negative feedbacks, ultimately as a change in heritable adaptations. Indeed there can never be adaptation if natural selection was not a process of the two kinds of feedback.
I have looked into the definitions of life but I’ll admit that I’ve never encountered a definition of life which included seclusion. What is seclusion and what does it have to do with this definition of life according to Koshland Jr (2002)? Seclusion would mean that every biochemical chain acts independently of one another and can proceed without any interference. Recall that in the cell, everything is bounded by a cell membrane and there are plenty of molecules all reacting with one another and in each biochemical reaction, they are catalyzed by enzymes. An enzyme has a molecular structure that can react with each molecule and each enzyme is very specific to the substrate or molecule that the enzyme has bounded to in order for the reaction to proceed. It is the specificity of enzymes that prevents any unwanted products from other reactions to interfere and this has been favored by natural selection. Of course some reactions have to be flexible to receive messages from outside the cells such as hormones interacting with specific cells which can change the internal biochemistry but in a narrow and specific way so with this property of seclusion, there is the balance of stability and flexibility which is a characteristic of life.
Biology in keeping up with the definition of life as a scientific objective should keep up with what biology would be intended to study , which is life and I believe that an acronym like PICERAS is a good label and would consider both terrestrial (the forms of life that we are familiar) and extraterrestrial (any forms of life that evolved independently from earth and from which we know nothing at all). If we do find evidence of extraterrestrial life, which is possible in the near future, an acronym like PICERAS which is part of biology, can be used to determine whether or not, an extraterrestrial life form satisfies the criteria for being a form of life. Does it have a genetic program? Are there compartmentalization for metabolisms and so on.
PICERAS will proves its universality if any life forms outside of earth satisfies all definitions of life, that we can reasonably come up or it may be life but it may challenge our assumption and so we would have to modify the objectives for defining life in ways that we cannot yet imagine. Either way it is part of the challenges as well as delights of the scientific method.
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Goodsell, D.S (2009) The Machinery of Life: New York, NY: Springer Verlag
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Martinez, A (2015, August 19). What is Life, Really?
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Mayer, E (2001) What Evolution Is New York, NY: Basic Books
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