Homeostasis, from the Greek words for "same" and "steady," refers to any process that living things use to actively maintain fairly stable conditions necessary for survival. It is also synonymous with robustness and adaptability. Homeostasis is required on several levels: From stability of planetary systems to the balance and interdependence of ecological biochemical cycles, to homeostasis in life, and homeostasis on a molecular, intracellular level. The attempt to explain the origin of the stability of the solar system is one of the oldest problems in astronomy, dating back to Isaac Newton ( His God of the gaps explanation to explain the phenomena is famous ). The Earth-Moon System is also part and essential to maintain the stability, besides stabilizing the Earths tilt. The unusually circular orbit of the earth is also remarkable. The greater the eccentricity, the higher the temperature fluctuation on a planet’s surface. If the Earth’s oceans were alternately boiling and freezing solid, it is hard to imagine life as we know it having emerged and been sustained.
Lynn Margulis wrote in a science paper in 1973: The Earth's atmosphere is regulated by life on the surface so that the probability of growth of the entire biosphere is maximized. Acidity, gas composition including oxygen level and ambient temperature are enormously important determinants for the distribution of life. We recognize that the earth's atmosphere deviates greatly from that of the other terrestrial planets in particular with respect to acidity, composition, redox potential and temperature history as predicted from solar luminosity. We explore the concept that these anomalies are evidence for a complex planet-wide homeostasis.
The terrestrial nitrogen and carbon cycles are tighly coupled. Nitrogen availability plays an important role in controlling the productivity, structure and spatiotemporal dynamics of terrestrial ecosystems: perturbations in the nitrogen cycle will have repercussions in the carbon cycle, and vice versa. All five element cycles, the carbon, oxygen, nitrogen, water and phosphorus cycles are thus clearly interdependent and any change in one cycle will in the long term have a profound influence on the operation of the other four.
Guillermo Gonzalez wrote in: The privileged planet:
As it happens, our atmosphere strikes a nearly perfect balance, transmitting most of the radiation that is useful for life while blocking most of the lethal energy. In our universe, potassium-40 is probably the most dangerous light radioactive isotope, yet the one most essential to life. Its abundance must be balanced on a razor’s edge. It must be high enough to help drive plate tectonics but low enough not to irradiate life.
And Charles Langmuir from Harvard University in Cambridge, wrote: "Hydrothermal vents at ocean ridges are an essential part of the chemical balance of seawater. They support ecosystems not found anywhere at the surface and are thought to perhaps have been the sites of the early formation and evolution of life," he said.
We observe also homeostasis in all lifeforms, on an organism, down to the molecular level. The human body maintains steady levels of temperature and other vital conditions such as the water, salt, sugar, protein, fat, calcium, and oxygen contents of the blood. The human body uses a number of processes to control its temperature, keeping it close to an average value or norm of 98.6 degrees Fahrenheit. In order to function normally, unicellular and multicellular organisms must maintain a relatively constant internal environment under considerably varying conditions in the external environment. Homeostasis and mechanisms of its regulation are better known in mammals, especially in humans. These organisms maintain the level of many physicochemical parameters in their fluids constant. Among the most important homeostatic parameters they regulate are body temperature, pH, levels of electrolytes, hormones, growth factors, secreted proteins, etc.
One of the most remarkable facts is on an intracellular level, where molecules join to form autocatalytic metabolisms, we will have to find a source of molecular order, a source of the fundamental internal homeostasis that buffers cells against perturbations, a compromise that would allow the networks of the first living cells to undergo slight fluctuations without collapsing. No cell can function efficiently without a balanced electrolyte system. And, remarkably, at least fifteen life-essential elements and building blocks must also be finely regulated and have the right balanced levels, even trace elements. Below, I describe sodium, nitrogen, oxygen, hydrogen, potassium, chlorine, Calcium, Molybdenum, Iron, Sulfur, magnesium, phosphate, Zinc, Selenium and Copper.
Remarkable is Calcium. The ability of cells to maintain a large gradient of calcium across their outer membrane is universal. All biological cells have a low cytosolic (liquid found inside Cells ) calcium concentration, can and must keep this even when the free calcium outside is up to 20,000 times higher concentrated! The first forms of life required an effective Ca2+ homeostatic system, which maintained intracellular Ca2+ at comfortably low concentrations—somewhere around 100 nanomolar, this being ∼10,000–20,000 times lower than that in the extracellular milieu.
The question, of course, arises: How can the origin of homeostasis in nature be best explained? It should be self-evident that homeostasis is hard to create, making the emergence of stable networks vastly unlikely. Homeostasis is a major problem to be explained by secular science proposals where teleology has to be excluded, but it contrasts with the fact that the concept of function is central in physiology. The teleological, namely, the ‘goal-directed’ dimension of function, and its subsequent explanatory relevance, is a philosophical problem. Some attempts have been made:
In the early 1960s, Ernest Nagel and Carl Hempel showed that self-regulated systems are teleological.
In his book: THE TINKERER’S ACCOMPLICE, How Design Emerges from Life Itself J . SCOTT. TURNER, writes at page 12 :
Although I touch upon ID obliquely from time to time, I do so not because I endorse it, but because it is mostly unavoidable. ID theory is essentially warmed-over natural theology, but there is, at its core, a serious point that deserves serious attention. ID theory would like us to believe that some overarching intelligence guides the evolutionary process: to say the least, that is unlikely. Nevertheless, how design arises remains a very real problem in biology. My thesis is quite simple: organisms are designed not so much because natural selection of particular genes has made them that way, but because agents of homeostasis build them that way. These agents’ modus operandi is to construct environments upon which the precarious and dynamic stability that is homeostasis can be imposed, and design is the result.
The logical fallacy of the author is evident. Wiki describes homeostasis as:
the stable condition of an organism and of its internal environment; or as the maintenance or regulation of the stable condition, or its equilibrium; or simply as the balance of bodily functions. 15 Homeostasis, from the Greek words for "same" and "steady," refers to any process that living things use to actively maintain fairly stable conditions necessary for survival. 16 As such, homeostasis is not an agent, but a state of affairs, resulting from a mechanism to be defined, either natural or an agent of volition implementing the state of affairs.
If life evolved, purposeless and unguided, why is there so much purpose and guidance within it?
December 6, 2017
Review of . J. Scott Turner’s Purpose and Desire What Makes Something “Alive” and Why Modern Darwinism Has Failed to Explain It, at MercatorNet:
The basic problem, he contends, is that current biology requires us to view life forms as machines. Yet a key characteristic of life forms is the intention of remaining alive and purposeful activity toward that end. For Turner, homeostasis (the way a life form balances itself within an environment and all of its cells balance themselves within it in order to stay alive) is central to understanding life, but largely ignored. It’s not hard to see why it is ignored. If life evolved, purposeless and unguided, why is there so much purpose and guidance within it?
The Emergence of Environmental Homeostasis in Complex Ecosystems
May 16, 2013
Explanations for this range from anthropic principles in which the Earth was essentially lucky, to homeostatic Gaia in which the abiotic and biotic components of the Earth system self-organize into homeostatic states that are robust to a wide range of external perturbations. Here we present results from a conceptual model that demonstrates the emergence of homeostasis as a consequence of the feedback loop operating between life and its environment.
The Gaya theory states that nature is essentially in balance, namely that ecological systems are usually in a stable equilibrium or homeostasis, According to Wiki, the theory was superseded by catastrophe theory and chaos theory. Research into the plausibility of Gaian homeostasis has continued in the context of natural selection; ecology and evolution; biogeochemical regulation; and complex adaptive systems. 17
To me, none of the " natural mechanisms only " proposals make sense or are compelling. In the same sense, as the question is: Did inanimated matter have the urge or drive to become alive and self-replicate, it can be asked: Why did matter have the urge to evolve into a state of affairs of homeostasis, and provide robustness to living systems? Matter has no goals, no purpose, therefore, naturalism has way to explain, why nature created living systems with the capacity and purpose of robustness, adaptability, homeostasis, recognition of what is, what should be, and self-correction to return to a homeostatic state of affairs and keep living complex systems alive.
Homeostasis in Life
Robustness in living organisms is homeostasis
In order to survive and reproduce, living organisms must be robust, tolerate injuries and undergo repair. Robustness in living organisms compares to robustness in human inventions, such as buildings and machines, which have to withstand occasional damage to avoid critical dysfunctions. However, the nature of robustness is fundamentally different in biology and in engineering. In living organisms, robustness is provided by homeostatic mechanisms, whereas in buildings and machines, it is provided by the redundancy of key elements. In this short essay, I discuss the nature of robustness in living organisms, and argue that redundancy, while important in engineering, is rare in biology. 43
The human body maintains steady levels of temperature and other vital conditions such as the water, salt, sugar, protein, fat, calcium and oxygen contents of the blood. 16 The human body uses a number of processes to control its temperature, keeping it close to an average value or norm of 98.6 degrees Fahrenheit. One of the most obvious physical responses to overheating is sweating, which cools the body by making more moisture on the skin available for evaporation. On the other hand, the body reduces heat-loss in cold surroundings by sweating less and reducing blood circulation to the skin. Thus, any change that either raises or lowers the normal temperature automatically triggers a counteracting, opposite or negative feedback.
Homeostasis as the Mechanism of Evolution
2015 Sep 15
Seen from a cellular-molecular perspective, homeostasis is the mechanistic fundament of biology, beginning with the protocell. 22
- Glucose homeostasis is regulated by the autonomic nervous system that controls the secretion of insulin and glucagons by the pancreas, as well as the metabolic state of liver muscles and fat tissue.
- In all the cases studied as of yet, normal levels of homeostatic parameters in vertebrates are maintained by hormones according to signals that ultimately originate in the brain.
- Empirical evidence shows that genome (including its duplication and gene expression) is itself regulated rather than the regulator of homeostasis.
- The study of the central mechanisms of control of animal functions is one of the main objects of animal physiology, and it is textbook knowledge that the maintenance of homeostasis and behavior in metazoans are functions of the nervous system.
- Epigenetic information is continually transmitted to target tissues and organs to induce them to produce new components and cells to compensate for relevant losses, thus maintaining homeostasis or restoring normalcy to the unavoidably degraded biological structures. Epigenetic information is computed in a structure, such as a nerve cell or neural circuit, and probably in the cytoskeleton of all groups of living organisms, including unicellulars. It is transmitted in the form of commands to particular structures, via neurohormonal algorithms (signal cascades) or directly by the information-generating structures, to produce a particular phenotypic result.
- Two neural mechanisms for the maintenance of bone homeostasis are operational in vertebrates: a central and a local mechanism.
- The neural control of body weight is also observed in lower animals such as planarians, where the adult animal size and tissue homeostasis are regulated by the neurally secreted, insulin-like neuropeptide Ilp-1.
- The liver is considered to be the metabolic power station of mammalians, where cholesterol homeostasis relies on an intricate network of cellular processes whose deregulations can lead to several life-threatening pathologies, such as familial and age-related hypercholesterolemia.
The Maintenance of Constant Internal Environment Is Prerequisite of Living Systems
In order to function normally, unicellular and multicellular organisms must maintain a relatively constant internal environment under considerably varying conditions in the external environment. Homeostasis and mechanisms of its regulation are better known in mammals, especially in humans. These organisms maintain the level of many physicochemical parameters in their fluids constant. 18 Among the most important homeostatic parameters they regulate are body temperature, pH, levels of electrolytes, hormones, growth factors, secreted proteins, etc.
Living organisms are open systems exchanging matter, energy, and information with their environment, whose dynamic equilibrium is maintained by a closed-loop control system. The state of dynamic equilibrium of multiple variables in living systems is a function of a built-in control system, which determines the normal levels of variables, or their set points. While any change in the level of the riverbed may change the shape and size of the lake, changes in the environmental factors do not affect the size, shape, or nature of the system; the built-in regulator maintains the system variables within the normal ranges, so long as the environmental changes do not override the adaptive capacity of the organism. In an oversimplifed form, the closed-loop control system in living organisms looks as shown in Figure below:
A diagrammatic representation of a closed-loop control system.
In order to sustain their vital functions, living systems need to create steady states (within certain limits) of their chemical and physical parameters, and they must maintain that state even under hostile environmental conditions. As shown in the preceding section, even without the influence of harmful environmental agents, the unavoidable degradation of structure determined by the thermodynamic forces of degradation will occur. In addition, the exceptional complexity of cells and multicellular organisms and the high improbability of their structure require an organism’s internal environment be within ranges that allow normal functioning of the cell or organismic machinery in multicellulars.
While Davies does not include homeostasis as being part of what characterizes life, I see it as life-essential feature, and as such, would include it.
One of the key properties of life is Regulation, including homeostasis.
The essential characteristic of living cells is homeostasis, the ability to maintain a steady and more-or-less constant chemical balance in a changing environment. Homeostasis is the machinery of chemical controls and feedback cycles that make sure that each molecular species in a cell is produced in the right proportion, not too much and not too little. Without homeostasis, there can be no ordered metabolism and no quasi-stationary equilibrium deserving the name of life. The question Why is life so complicated? means, in this context, Given that a population of molecules is able to maintain itself in homeostatic equilibrium at a steady level of metabolism, how many different molecular species must the population contain? From the fact that bacteria have generally refused to shrink below a certain level of complexity, we may deduce that this level is in some sense an irreducible minimum. I am conjecturing that the minimum population size required for homeostasis would be ten or twenty thousand monomer units. And more important, I am suggesting that the most promising road to an understanding of the origin of life would be to do experiments like the Spiegelman and Eigen experiments but this time concerned with homeostasis rather than with replication. 14
Free-living organisms, which are more directly exposed to environmental fluctuations, must often survive even harsher folding stresses. These stresses not only disrupt the folding of newly synthesized proteins but can also cause misfolding of already folded proteins. Bacteria have therefore sophisticated stress responses that can react to such threats to proteostasis through the induction of chaperones and proteases.
Physiology of hemodynamic homeostasis
Homeostasis of hemodynamics refers to the regulation of the blood circulation to meet the demands of the different organ and tissue systems. This homeostasis involves an intimate interaction between peripheral metabolic needs, vascular adaptations to meet these needs and cardiac adaptation to provide the driving force to circulate the blood. 24
Hypothalamic circuits regulating appetite and energy homeostasis: pathways to obesity
2017 Jun 1
Obesity results from the dysregulation of energy metabolism (Crowley et al., 2002). The central nervous system (CNS) plays a key role in sensing and controlling the energy status of the organism (Myers and Olson, 2012), and the hypothalamus in particular has emerged as an integrating, superordinate master regulator of whole-body energy homeostasis. 25
Air Breathing: Oxygen Homeostasis
Metazoan species ( Metazoan animals are multicellular, mitochondrial eukaryotes ) maintain oxygen homeostasis through the activity of hypoxia-inducible factors, which are transcriptional activators that regulate the expression of hundreds of genes to match O2 supply and demand. A constant supply of O2 must be delivered to all cells of metazoan organisms for their continued survival. O2 is utilized as the final electron acceptor in the process of oxidative phosphorylation. The function of the respiratory chain is optimized for physiological PO2 levels and sustained deviations from normoxia cause increased production of reactive oxygen species (ROS)* by the electron transport chain (ETC). ROS cause oxidation of lipids, proteins, and nucleic acids that can result in cellular dysfunction or death. Thus, homeostatic mechanisms exist to tightly regulate O2 levels within cells and tissues. 3
All eukaryotic organisms must maintain oxygen homeostasis. A number of defense and regulatory mechanisms have been developed to protect the cell from low as well as high oxygen levels. 20
Homeostasis and Body Fluid Regulation
Powerful multiple homeostatic mechanisms maintain sodium concentration and osmolarity of the ECF of most bony fishes and amphibians, and birds and mammals, to about one-third that of the sea. Such mechanisms operate to avoid either desiccation or overhydration 26 Responses to sodium loss or gain provide an example of how different types of mechanisms contribute to body fluid homeostasis. Homeostatic mechanisms in general may operate in a reaction mode, usually involving negative feedback, or through a combination of reaction with prediction or anticipation
What Critical Role Does Water Play in Homeostasis?
April 24, 2017
Water serves a wide range of functions: it's a nutrient, a building material, a regulator of body temperature, a participant in carbohydrate and protein metabolism, a lubricant and a shock absorber. Water balance, or homeostasis, with respect to the internal environment is essential for survival. 27
Vitamin D, calcium homeostasis and aging
18 October 2016
Vitamin D is the principal factor that maintains calcium homeostasis. Increasing evidence indicates that the reason for disturbed calcium balance with age is inadequate vitamin D levels in the elderly. 28
Regulation of metabolism by long, non-coding RNAs
25 March 2014
Our understanding of genomic regulation was revolutionized by the discovery that the genome is pervasively transcribed, giving rise to thousands of mostly uncharacterized non-coding ribonucleic acids (ncRNAs). Long, ncRNAs (lncRNAs) have thus emerged as a novel class of functional RNAs that impinge on gene regulation by a broad spectrum of mechanisms such as the recruitment of epigenetic modifier proteins, control of mRNA decay and DNA sequestration of transcription factors. We review those lncRNAs that are implicated in differentiation and homeostasis of metabolic tissues and present novel concepts on how lncRNAs might act on energy and glucose homeostasis. Finally, the control of circadian rhythm by lncRNAs is an emerging principles of lncRNA-mediated gene regulation. 29
Homeostasis on a molecular level
If we are to believe that life began when molecules spontaneously joined to form autocatalytic metabolisms, we will have to find a source of molecular order, a source of the fundamental internal homeostasis that buffers cells against perturbations, a compromise that would allow the protocell networks to undergo slight fluctuations without collapsing. 45
One of the cutting-edge developments in cell biology and genetics is the realization that there are networks of molecules that are regulated by other molecules. Some molecules stimulate growth while others repress it. The dynamic interplay between signals, hormones, repressors and other processes somehow leads to “homeostasis” – a dynamic balance that is responsive to the environment and able to adapt to changing needs. 44
No cell can function efficiently without a balanced electrolyte system. Ions of sodium, potassium and chlorine are the key electrolytes having a range of critical functions in the well-being of cells and therefore animals. 32
Chlorine is a fascinating element that is found in all living tissue. It is essential for the function of cleansing the body of debris. It is also exchanged in the stomach to produce hydrochloric acid, a very necessary acid for protein digestion. Chlorine is a member of a group of elements called the halogens. Others in this group are fluoride, iodine, and bromine. The body maintains a delicate balance between all these elements. 33
Molybdenum and iron mutually impact their homeostasis in cucumber (Cucumis sativus) plants.
Molybdenum (Mo) and iron (Fe) are essential micronutrients required for crucial enzyme activities in plant metabolism. 30
Regulation of Potassium Homeostasis
Potassium is the most abundant cation in the intracellular fluid, and maintaining the proper distribution of potassium across the cell membrane is critical for normal cell function. Potassium plays a key role in maintaining cell function. Almost all cells possess an Na+-K+-ATPase, which pumps Na+ ( Sodium ) out of the cell and K+ ( Potassium ) into the cell and leads to a K+ gradient across the cell membrane (K+in>K+out) that is partially responsible for maintaining the potential difference across the membrane. This potential difference is critical to the function of cells, particularly in excitable tissues, such as nerve and muscle. The body has developed numerous mechanisms for defense of serum K+. These mechanisms serve to maintain a proper distribution of K+ within the body as well as regulate the total body K+ content. 11
Potassium is an essential mineral micronutrient and is the main intracellular ion for all types of cells. It is important in maintaining fluid and electrolyte balance in the bodies of humans and animals. Potassium is necessary for the function of all living cells and is thus present in all plant and animal tissues. 34
The Calcium gradient :
The ability of cells to maintain a large gradient of calcium across their outer membrane is universal. All biological cells have a low cytosolic (liquid found inside Cells ) calcium concentration, can and must keep this even when the free calcium outside is up to 20,000 times higher concentrated! The first forms of life required an effective Ca2+ homeostatic system, which maintained intracellular Ca2+ at comfortably low concentrations—somewhere around 100 nanomolar, this being ∼10,000–20,000 times lower than that in the extracellular milieu. Damage the ability of the plasma membrane to maintain this gradient and calcium will flood into the cell, precipitating calcium phosphate, damaging the ATP-generating machinery, and kill the cell. 12
An information hypothesis for the evolution of homeostasis
Homeostasis also evolves to minimize noise in physiological channels. Fluctuations in physiological factors constitute inescapable noise that corrupts the transfer of information through physiological systems. 21
Local and systemic regulation of sulfur homeostasis in roots of Arabidopsis thaliana.
Nutrients are limiting for plant growth and vigour. Hence, nutrient uptake and homeostasis must be adjusted to the needs of the plant according to developmental stages and environmental conditions. 35
CELLULAR MAGNESIUM HOMEOSTASIS
2011 May 27
Magnesium is the second most abundant cellular cation after potassium. High concentrations of total and free magnesium ion (Mg2+) have been measured within mammalian cells through a variety of techniques. These concentrations are essential to regulate numerous cellular functions and enzymes, including ion channels, metabolic cycles, and signaling pathways as attested by the large number of observations gathered in the last twenty years. 36
Regulation of cellular Mg2+ homeostasis
The cartoon summarizes the principal mechanisms controlling cellular Mg2+ homeostasis, compartmentation, and transport in and out of mammalian cell, as well as the main cellular functions regulated by changes in Mg2+ content within different compartments. For mere practical purpose, the entry mechanisms have been assigned to the apical domain of the cell. Abbreviation used in the figure: ER = endoplasmic reticulum; G6Pase = glucose 6 phosphatase; Mito = mitochondria; β-AR = β-adrenergic receptor; AC = adenylyl cyclase; NMx = Na+/Mg2+ exchanger; Me2+ = divalent cations; VP-R = vasopressin receptor; PMA = Phorbol-Myristate Acetate; PKC = Protein Kinase C.
Iron Homeostasis in Health and Disease
2016 Jan 20
Iron is required for the survival of most organisms, including bacteria, plants, and humans. Its homeostasis in mammals must be fine-tuned to avoid iron deficiency with a reduced oxygen transport and diminished activity of Fe-dependent enzymes, and also iron excess that may catalyze the formation of highly reactive hydroxyl radicals, oxidative stress, and programmed cell death. 37
Eukaryotic Phosphate Homeostasis: The Inositol Pyrophosphate Perspective
November 19, 2016
Phosphate, as a cellular energy currency, essentially drives most biochemical reactions defining living organisms, and thus its homeostasis must be tightly regulated. Investigation into the role of inositol pyrophosphates (PP-IPs) has provided a novel perspective on the regulation of phosphate homeostasis. Recent data suggest that metabolic and signaling interplay between PP-IPs, ATP, and inorganic polyphosphate (polyP) influences and is influenced by cellular phosphate homeostasis. 38
Phosphatonins and the Regulation of Phosphate Homeostasis
17 March 2007
Inorganic phosphate (Pi) is required for energy metabolism, nucleic acid synthesis, bone mineralization, and cell signaling. The activity of cell-surface sodium-phosphate (Na+-Pi) cotransporters mediates the uptake of Pi from the extracellular environment. Na+-Pi cotransporters and organ-specific Pi absorptive processes are regulated by peptide and sterol hormones, such as parathyroid hormone (PTH) and 1α,25-dihydroxyvitamin D (1α,25(OH)2D3), which interact in a coordinated fashion to regulate Pi homeostasis. 39
Intracellular zinc homeostasis and zinc signaling
14 August 2008
Zinc (Zn) is an essential heavy metal that is incorporated into a number of human Zn metalloproteins. Zn plays important roles in nucleic acid metabolism, cell replication, and tissue repair and growth. Zn deficiency is associated with a range of pathological conditions, including impaired immunity, retarded growth, brain development disorders and delayed wound healing. Zn contributes to intracellular metal homeostasis and/or signal transduction in tumor and immune cells. 40
Selenium homeostasis and antioxidant selenoproteins in brain: implications for disorders in the central nervous system.
2013 Aug 15
The essential trace element selenium, as selenocysteine, is incorporated into antioxidant selenoproteins such as glutathione peroxidases (GPx), thioredoxin reductases (TrxR) and selenoprotein P (Sepp1). a low selenium status was associated with faster decline in cognitive functions and poor performance in tests assessing coordination and motor speed. 41
Copper-Regulatory Domain Involved in Gene Expression
Copper ion homeostasis in yeast is maintained through regulated expression of genes involved in copper ion uptake, Cu(I) sequestration, and defense against reactaive oxygen intermediates. 42
Homeostasis and information transfer.
(a) Schematic of a typical negative feedback loop underlying physiological homeostasis. Physiological sensors monitor
the level of some factor, and compare it to a reference signal or set point (labeled ‘Ref.’). Those sensors then produce signals (nerve impulses, endocrine molecules, cell
surface proteins, etc.) whose levels are inversely proportional to the deviation from the reference, and which direct other physiological entities (i.e., cells, tissues, or
organs) to do work that counteracts the deviation.
(b) Shannon’s scheme for information transfer. Transmitters encode and send out signals along a channel; those
signals are received and decoded at the other end. Noise can corrupt the signal, either in transit or during the transmitting or receiving steps. This schematic is simplified
from the one that Shannon showed, which described the process as having separate devices for the source and transmitter of information and, similarly, separate
devices for the receiver and the destination. For our purposes, that distinction is less relevant. (c) The variables regulated by homeostasis provide quiet physiological
backgrounds for the transmission of any kind of information within organisms. (d) The parts of a feedback cycle are coupled communication systems. In the first system,
the sensor (indicated by blue squares) communicates, via some kind of physiological signal, with the tissues or organs doing physiological work (yellow squares). In the
second system, the working component communicates (blue squares) with the sensor via the regulated variable (yellow squares). Homeostasis of one variable creates a
quiet background for the communication that occurs within other homeostatic feedback loops; that is, multiple homeostatic systems likely are self-reinforcing.
Abbreviation: CNS, central nervous system.
Air Breathing: Oxygen Homeostasis and the Transitions from Water to Land and Sky
Balance and interdependence of ecological biochemical cycles
The most important biogeochemical cycles are the carbon cycle, nitrogen cycle, oxygen cycle, phosphorus cycle, and the water cycle. The biogeochemical cycles always have a state of equilibrium. The state of equilibrium occurs when there is a balance in the cycling of the elements between compartments. Ecologists may also be interested in the sulfur cycle, the nutrient cycle, and the hydrogen cycle; however, ecologists are more interested in studying the carbon, nitrogen, oxygen, phosphorus, and water cycles. 3 Odum (1959) describes what are called more or less "perfect" cycles: biogeochemical cycles that involve equilibrium states. That is, there exists in nature a balance in the cycling of the element between various compartments, with the element or material moving into abiotic compartments about as fast as it moves into biotic compartments. Certain ecosystems may experience "shortages", but overall a balance exists on a global scale. 13
Homeostasis, complexity, and the problem of biological design 4
The harmonious melding of structure and function—biological design—is a striking feature of complex living systems such as tissues, organs, organisms, or superorganismal assemblages like social insect colonies or ecosystems. How designed systems come into being remains a central problem in evolutionary biology: adaptation, for example, cannot be fully explained without understanding it.
Biological Modulation of the Earth's Atmosphere
November 6, 1973 , LYNN MARGULIS
The Earth's atmosphere is regulated by life on the surface so that the probability of growth of the entire biosphere is maximized. Acidity, gas composition including oxygen level, and ambient temperature are enormously important determinants for the distribution of life. We recognize that the earth's atmosphere deviates greatly from that of the other terrestrial planets in particular with respect to acidity, composition, redox potential and temperature history as predicted from solar luminosity. These deviations from predicted steady state conditions have apparently persisted over millions of years. We explore the concept that these anomalies are evidence for a complex planet-wide homeostasis that is the product of natural selection. Possible homeostatic mechanisms that may be further investigated by both theoretical and experimental methods are suggested. 19
THE PRIVILEGED PLANET, HOW OUR PLACE IN THE COSMOS IS DESIGNED FOR DISCOVERY, Guillermo Gonzalez and Jay W. Richards, page 66
As it happens, our atmosphere strikes a nearly perfect balance, transmitting most of the radiation that is useful for life while blocking most of the lethal energy. Water vapor in the atmosphere is likewise accommodating,
3 a fact that even the fifteenth edition of the staid Encyclopaedia Britannica picks up on: “Considering the importance of visible sunlight for all aspects of terrestrial life, one cannot help being awed by the dramatically
narrow window in the atmospheric absorption . . . and in the absorption spectrum of water.” The oceans transmit an even narrower window of the spectrum, mainly the blues and greens, while halting the other wavelengths near the surface, where they nourish the marine life that figures prominently in Earth’s biosphere
Earth’s water results in skies that, on average, are about 68 percent cloudy. Clouds help balance the global energy by contributing to the global albedo, that fraction of sunlight reflected back into space. Earth currently
reflects about 30 percent of the sunlight that strikes it. Four major types of reflecting surfaces contribute to the global albedo: land, ice, oceans, and clouds, each with its own particular reflective properties.
In our universe, potassium-40 is probably the most dangerous light radioactive isotope, yet the one most essential to life. Its abundance must be balanced on a razor’s edge. It must be high enough to help drive plate tectonics but low enough not to irradiate life.
Water is still being added to the hydrosphere by volcanism, but some water molecules in the atmosphere are continually being destroyed by ultraviolet radiation from the Sun. These two processes balance each other.
Charles Langmuir, an Earth and planetary scientist at Harvard University in Cambridge, Massachusetts, and study co-author, says that hydrothermal vents are central to the function of the Earth system and the life that is part of it.
"Hydrothermal vents at ocean ridges are an essential part of the chemical balance of seawater. They support ecosystems not found anywhere at the surface and are thought to perhaps have been the sites of the early formation and evolution of life," he said. 31
The incredible stability of the Earth and our Solar System
The stability of the solar system is one of the oldest problems in theoretical physics, dating back to Isaac Newton. After Newton discovered his famous laws of motion and gravity, he used these to determine the motion of a single planet around the Sun and showed that the planet followed an ellipse with the Sun at one focus. However, the actual solar system contains eight planets, six of which were known to Newton, and each planet exerts small, periodically varying, gravitational forces on all the others.
The puzzle posed by Newton is whether the net effect of these periodic forces on the planetary orbits averages to zero over long times, so that the planets continue to follow orbits similar to the ones they have today, or whether these small mutual interactions gradually degrade the regular arrangement of the orbits in the solar system, leading eventually to a collision between two planets, the ejection of a planet to interstellar space, or perhaps the incineration of a planet by the Sun. The interplanetary gravitational interactions are very small—the force on Earth from Jupiter, the largest planet, is only about ten parts per million of the force from the Sun 1
Most of the calculations agree that eight billion years from now, just before the Sun swallows the inner planets and incinerates the outer ones, all of the planets will still be in orbits very similar to their present ones. In this limited sense, the solar system is stable.
Jacques Laskar of the Bureau des Longitudes in Paris carried out the most extensive calculations for investigating the long-term stability of the solar system. He simulated the gravitational interactions among all the eight planets over a period of 25 billion years or for a period five times the age of the solar system. He found that the eccentricities and other elements of the orbits undergo chaotic excursions. This simulation makes it impossible to predict the locations of the planets after a hundred million years. Does Laskar’s result indicate that the Earth might eventually find itself in a highly elliptical orbit, taking it much closer to and farther from the Sun? In other words, will it mean the solar system could lose a planet? The immediate answer is perhaps, no. This is because of the fact that even the chaos has to operate within physical limits. As an example, meteorologists cannot predict the weather that is another chaotic system as far as a month in advance, but they can be quite confident that those conditions will fall within a certain range. This is because external constraints, for example, the Sun’s brightness and the length of the day, set certain limits on the overall system. Interestingly, Laskar observed that despite the influence of chaos on the exact locations of the planets, their orbits remain relatively stable over billions of years. This means that the long-term configuration is highly unpredictable, and the orbits remain well behaved for preventing collisions between the neighboring planets. In this case, an external constraint is imposed by the conservation of angular momentum in the system, which limits the excursions of orbital eccentricity for bodies of planetary mass. 5
Long-term integrations and stability of planetary orbits in our Solar system
21 October 2002
We present the results of very long-term numerical integrations of planetary orbital motions over 10^9 -yr time-spans including all nine planets. A quick inspection of our numerical data shows that the planetary motion, at least in our simple dynamical model, seems to be quite stable even over this very long time-span. A closer look at the lowest-frequency oscillations using a low-pass filter shows us the potentially diffusive character of terrestrial planetary motion, especially that of Mercury. The behavior of the eccentricity of Mercury in our integrations is qualitatively similar to the results from Jacques Laskar's secular perturbation theory (e.g. emax∼ 0.35 over ∼± 4 Gyr).
The Earth-Moon System and the Dynamical Stability of the Inner Solar System
Evidence from self-consistent solar system n-body simulations is presented to argue that the Earth-Moon system (EM) plays an important dynamical role in the inner solar system, stabilizing the orbits of Venus and Mercury by suppressing a strong secular resonance of period 8.1 Myr near Venus's heliocentric distance. The EM thus appears to play a kind of "gravitational keystone" role in the terrestrial precinct, for without it, the orbits of Venus and Mercury become immediately destabilized. 6
The Earth’s moon appears to play a vital role in moderating the climate by stabilizing the axial tilt. It is suggested that a chaotic tilt may be a deal-breaker in terms of habitability: A satellite the size of the Moon is not only helpful but also produces stability. 9
Unusually circular orbit of the earth
The unique arrangement of large and small planetary bodies in the solar system may be required to ensure the stability of the system. In addition, it is readily apparent from the cycle of ice ages that the earth is at the edge of the life zone for our star. Although the earth has one of the most stable orbits among all the planets discovered to date, its periodic oscillations, including changes in orbital eccentricity, axial tilt, and a periodic elongation of Earth's orbit, results in a near freeze over (Kerr, R. 1999. Why the Ice Ages Don't Keep Time. Science 285: 503-505, and Rial, J.A. 1999. Pacemaking the Ice Ages by Frequency Modulation of Earth's Orbital Eccentricity. Science 285: 564-568.). According to Dr. J. E. Chambers, simulations of planetary formation "yield Earth-like planets with large eccentricities (e ~ 0.15)," whereas the Earth has an e value of 0.03. He goes on to say, "Given that climate stability may depend appreciably on e, it could be no coincidence that we inhabit a planet with an unusually circular orbit." (Chambers, J. E. 1998. How Special is Earth's Orbit? American Astronomical Society, DPS meeting #30, #21.07) With this new information, it seems very unlikely that stable planetary systems, in which a small earth-like planet resides in the habitable zone, exist in any other galaxy in our universe. This does not even consider the other design parameters that are required for life to exist anywhere in the universe. 2
Orbit and Rotation
Stability is an important consideration in evaluating the effect of orbital and rotational characteristics on planetary habitability. Orbital eccentricity is the difference between a farthest and closest approach of a planet to its parent star divided by the sum of said distances. It is a ratio of the shape of the elliptical orbit. The greater the eccentricity, the higher the temperature fluctuation on a planet’s surface. Living organisms can only stand so much variation, particularly if the fluctuations overlap both the freezing and boiling point of the planet’s main biotic solvent (e.g., water on Earth). If the Earth’s oceans were alternately boiling and freezing solid, it is hard to imagine life as we know it having evolved. The orbit of the Earth is almost circular, with an eccentricity of less than 0.02; other planets in the solar system (except Mercury) have eccentricities that are similarly benign. Data gathered on the orbital eccentricities of extrasolar planets show that 90% have an orbital eccentricity larger than that obtained within the solar system and the average is fully 0.25 . It means that the majority of planets have highly eccentric
orbits and the average distance from their star is deemed to be within the HZ. The movement of a planet around its rotational axis must reveal certain criteria if life is to have the opportunity to evolve. The significant assumption is that the planet should have moderate seasons. If there is little or no axial tilt relative to the perpendicular of the ecliptic, seasons will not occur. If a planet is radically tilted, seasons will be extreme and make it more difficult for a biosphere to achieve homeostasis. 8
As a matter of fact, relative loneliness is ultimately what a life-bearing system demands. If the Sun were crowded among other systems, the chance of being fatally close to dangerous radiation sources would increase significantly. Again, close neighbors might disrupt the stability of different orbiting bodies such as Oort cloud and Kuiper belt objects, which can bring catastrophe if knocked into the inner solar system. A star as metal rich as the Sun would perhaps not have formed in the very outermost regions of the Milky Way given a decline in the relative abundance of metals and a general lack of star formation. Therefore, a suburban location such as that the solar system enjoys is preferable to a galaxy’s center or farthest reaches. 10
galaxy location (9) (p = 0.1)
if too close to dense galaxy cluster: galaxy would be gravitationally unstable, hence unsuitable for life
if too close to large galaxy(ies): same result
number of stars in the planetary system (10) (p = 0.2)
if more than one: tidal interactions would make the orbits of life-supportable planets too unstable for life
if fewer than one: no heat source would be available for life chemistry
distance from parent star (13) (p = 0.001)
if greater: planet would be too cool for a stable water cycle
if lesser: planet would be too warm for a stable water cycle
rate of decline in tectonic activity (26) (p = 0.1)
if slower: crust conditions would be too unstable for advanced life
if faster: crust nutrients would be inadequate for sustained land life
5. Solar Planetary Systems: Stardust to Terrestrial and Extraterrestrial, Chapter 8, page 217
8. Solar Planetary Systems: Stardust to Terrestrial and Extraterrestrial, Chapter 14, page 381
9. ibid. page 382
10. ibid page 388
14. Freeman Dyson, Origins of Life, page 73
18. Building the Most Complex Structure on Earth, An Epigenetic Narrative of Development and Evolution of Animals, page 13
45. At Home in the Universe: The Search for the Laws of Self-Organization , page 39
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