Principles of self-generation and self-maintenance
An der Heiden U., Roth G. & Schwegler H. (1985) Principles of self-generation and self-maintenance. Acta Biotheoretica 34: 125–138. Available at http://cepa.info/4148
Table of Contents
4. Self-generating systems
5. Self-maintaining systems
6. Implications of self-maintenance
Living systems are characterized as self-generating and self-maintaining systems. This type of characterization allows integration of a wide variety of detailed knowledge in biology.The paper clarifies general notions such as processes, systems, and interactions. Basic properties of self-generating systems, i.e. systems which produce their own parts and hence themselves, are discussed and exemplified. This makes possible a clear distinction between living beings and ordinary machines. Stronger conditions are summarized under the concept of self-maintenance as an almost unique character of living systems. Finally, we discuss the far-reaching consequences that the principles of self-generation and self-maintenance have for the organization, structure, function, and evolution of single- and multi-cellular organisms.
Are there properties common to all living beings that distinguish them from non-living beings? In view of the enormous richness of forms and structures developed in the realm of the living on the one hand, and of the increasing success in reducing many processes in living systems to physico-chemical relationships on the other, both an unambiguous delimitation of the living from the non-living and a common characterization of all living beings appears to be a futile endeavour from the very beginning.
Nevertheless, each of us cannot deny the feeling, that in general there is no problem in deciding whether or not a given thing is a living being. Is a justification of this feeling possible by specifying some criteria with scientific scrutiny, thus allowing a clear distinction? Or are living systems only gradually distinguished from non-living systems by their degree of complexity? In the latter case we can never fail by investigating living systems just like others. However, if the first case holds, then it may happen that, even if all our scientific investigations are correct, negligence of these criteria may exclude a comprehensive, profound insight into the special nature of living beings as well as the development of concepts unifying the mass of details in so many fields of biology. In fact we all suffer from the abundance of specific knowledge accumulating in many special areas.
In our opinion there are principles of organization shared by all living beings which are realized neither in natural non-living systems nor in artificial systems up to now created by man. Such principles are stated and derived here. For a more detailed discussion the reader is referred to .
We start with the observation that the physico-chemical world has a structure consisting of spatially and temporally related events. Any spatio-temporal domain which in some way is separated from its (spatiotemporal) environment is called a process. A process can be characterized by a connected four-dimensional spatio-temporal domain B and certain physico-chemical entities or quantities V1, V2, …, Vn occurring in this domain. These quantities are comprised into a vector V = (VI, V2, …, Vn). E.g. VI might be an electric field, V2 a magnetic field distributed in the domain B. Or VI might be a distribution of mass across B. Some of the components of V could represent concentrations of certain types of molecules existing in B. (For simplicity we do not consider here problems of description connected with quantummechanic and relativistic phenomena. However, these may be accounted for without changing the theory essentially.) Note that connectedness of B implies connectedness of the time domain T of the process. For each point of time t from T the (three-dimensional) shape or Gestalt Bt of the process is just the restriction of B to that value of t. It is not necessary that Bt is a connected set in three- dimensional space.
The separation or delimitation of a process P = (B, V) from its environment consists of two parts: spatial and temporal delimitation The two-dimensional spatial delimitation at a moment t is given by a steep (eventually discontinuous) spatial gradient of at least one of the intensities V1, V2, …, Vn. In this way the spatial boundary dBt at time t of the process is defined not numerically precisely but identifiably. E.g., it may be given by a steep gradient of the concentration of some type of molecules.
Temporally a process starts in one of three forms: either from several others by fusion or by division of a previous process or by the building- up of a new steep spatial gradient separating the new process from its environment. Correspondingly a process ends by dividing into several others or by fusion with some others or by smoothing of the steep gradient defining its spatial boundary. In this way also the temporal boundaries of a process, its beginning and its ending, are identifiably defined.
In a broad sense the environment of a process is everything outside its domain B. However, due to the principle of nearest action only those conditions of the environment that actually meet the boundary of the process can have some effect on the process. These conditions decompose into the initial conditions given by the state of the world in the neighbourhood of the place where the process starts, and into the boundary conditions. For each time t of the process the latter are given by the limit values of the intensities external to the process observed at the spatial boundary dBt.
Examples of processes are: stones, waters, the earth, cells, organs, organisms, molecules, machines, chemical reaction systems.
Any process may be considered as a system with components V1, V2, …, Vn. However, more generally we imagine systems whose components are processes. To this end we define two processes P1, P2 to be disjoint if their domains Bl, B2 have in common only boundary points. Two disjoint processes P1, P2 interact if they come into contact at some time, i.e. if there is at least one time t at which they have parts of their boundary in common (not necessarily fusing). A system S is a union of mutually disjoint processes P1, P2, … satisfying the condition that any two of these processes are connected via a chain of interactions within this set of processes. P1, P2, … are called the constituent subprocesses or the constituent parts of the system. Subprocesses of constituent subprocesses are examples of nonconstituent subprocesses.
Machines are examples of systems. However, in order to define a system properly it is necessary to specify which parts of the system form the constituent parts. Normally these will not be the molecules, but some macroscopic objects (which are also processes in our sense). E.g., in a mechanical clock these may be the cog-wheels, springs, and casing. Considering the molecules of the clock to be its constituent subprocesses would define another system.
There is another problem in defining a system. No one would call a set of cog-wheels, springs, and casing a clock when these things are lying disconnected on the table. Generally also a certain set of relations between the constituent parts have additionally to be specified in order to define a system completely. The relations defining a system are called its constituent relations (there are generally many more relations in a system, e.g. relations between non-constituent subprocesses are non- constituent relations). Hence a system is defined by a set of constituent subprocesses and a (possibly empty) set of constituent relations between these subprocesses. Therefore the existence of a system is not only bound to the existence of its subprocesses but also to the satisfaction of its constituent relations. Only in this way it is possible that constituent parts of a system may exist before or after the system exists, namely before or after the constituent relations are satisfied.
With machines it is generally the case that its constituent parts exist already before the machine exists. Building a machine not only requires building its parts but also constituting certain relations between these parts.
4. Self-generating systems
An essential point of this paper is to show that the constituent parts of a system do not necessarily exist prior to the system. This observation leads to the concept of self-generating systems which produce their constituent parts by and within themselves, and moreover, constitute the relationships between these parts by themselves.
In an absolute sense a system cannot generate itself. If it has an origin, this origin must exist before and outside of it. Each originating system has a history determining its initial condition. However, the notion of self-generation attains a realistic meaning by the following definition: A system is called self-generating if there is some time of its existence after which it contains only constituent subprocesses originating within the system. “Originating within the system” means that the initial conditions of the corresponding subprocess are constituted at least partially by other constituent subprocesses belonging to the system.
Examples of non self-generating systems are machines composed of parts. These parts do not display the property of participating in the generation of the other parts of the machine. Whereas in non-self-generated systems the parts exist independently of the system, in self-generating systems the system is a condition for the existence of its constituent parts, at least after some time.
Examples of self-generating systems are chemical reactions in the course of which all reactants are newly formed. Most remarkable in this context is the well-known Belousov-Zhabotinsky reaction, where in a cyclic fashion new substances (which may be viewed as the constituent parts) are permanently synthetized. Other examples are populations consisting of individuals as subprocesses connected by a network of descent. When we look at multicellular organisms there are many ways to decompose them
into constituent parts or processes. One way would be to consider the organs as constituent parts, a second one the cells, a third one biological macromolecules, finally atoms and elementary particles. In a strict sense another system is defined each time (despite the fact that in all cases
the same “whole” is given). If the lowest level, the atoms and elementary particles, is chosen, then the system is not self-generating, since in this case the constituent parts exist before and independent of the system. However, if the organism is conceived to be composed of its organs then clearly these constituent subprocesses are generated within the system, and in this respect the organism is a self-generating system. The same result holds if the cells are defined to be the constituent subprocesses.
At this point it becomes clear that to say an organism is nothing else than a complicated machine may be misleading. Organisms are self-generating systems whereas machines are not (at least those existing up to now). It may even be the case that there are rather simple self-generating systems and on the other hand rather complicated machines which are not self- generating (e.g., sophisticated computers).
5. Self-maintaining systems
With respect to life the fundamental question arises how its process was able to persist without interruption for a period of several billion years under a tremendous number of destructive environmental influences.
The notion of self-generating systems opens the view on systems that show the property of outlasting their own parts or subprocesses. Normally a system perishes together with its parts. This is the fate of all machines having existed so far, if their parts are not exchanged. Likewise it happens with all non-self-generating composite systems. Sooner or later they disintegrate because of influences of the environment or internal fluctuations. But also most self-generating systems cannot persist forever. E.g. chemical reactions generally terminate after several reaction steps in a state of local thermodynamic equilibrium. But if there remains a steep gradient separating the system from its environment the system must disintegrate although this can take place very slowly, e.g. in the case of a solid system at low temperatures. Only in the exceptional case that conditions of coexisting phases are fulfilled precisely, a spatially limited system can persist in full thermodynamical equilibrium. But even in this exceptional case, maintenance is quite different from our concept of self-maintenance. Self-maintaining systems, as we define them here, are not in a state of thermodynamic equilibrium, and nevertheless persist in principle forever.
One could imagine a machine persisting for ever in the way that each of its parts is repaired again and again or replaced by new parts. But obviously such a machine needs other systems that perform the repairs and replacements. Thus the problem of maintenance and persistence is deferred to these other systems. Essentially, a self-maintaining system is a system that can do these operations by and within itself.
A system of machines that repair each other in a way such that they altogether persist would be a self-maintaining system. However, that the machines, which in this case are the constituent subprocesses, persist, would be too strong a postulate in our context. We only require that the system as a totality persists, whereas the constituent subprocesses may come and go. I.e. we imagine a system of machines which persists indefinitely despite the fact that the individual machines do not necessarily persist within the system. Of course when individual machines permanent ly stop existing or are brought out of the system, then maintenance of the system requires that new machines are built permanently.
Moreover, we do not postulate that repair of a machine or construction of a new machine always leads to the same type of machines. We allow series of repairs that may transform the machine drastically. Of course in normal language one would not speak of “repair” in this case. If we would only allow repair and replacement in the sense of exact restitution of a previous state then evolution of a self-maintaining system would be impossible.
We must also abandon certain teleological aspects in the word repair. Moreover, the term machine has to be replaced by constituent subprocess. Destruction of the machine (making necessary repair or replacement) then simply means ending of a subprocess. Repair and replacement simply mean generation of new subprocesses. Thus in a self-maintaining system there is permanent generation and disappearance of constituent subprocesses. Self-maintenance means self-generation in permanence.
The principles of self-maintenance so far discussed are summarized as follows:
(i) a self-maintaining system exists permanently in an environment the fluctuations of which suffice to disrupt the gradient of its boundary locally
(ii) the constituent subprocesses are not in thermodynamic equilibrium with each other
(iii) all constituent subprocesses exist only for a finite time within the system, whereas the system virtually exists forever.
Virtually in condition (iii) means that permanent existence may be prohibited by one of the following factors: 1. there may occur fluctuations in the environment that are strong enough to interrupt the generation of subprocesses underlying self-maintenance, 2. condition (ii) implies that all constituent subprocesses and hence the total system will end if there is not sufficient supply of energy and matter from the environment, hence existence forever presupposes permanent availability of energy-rich material in the environment.
The definition of self-maintenance we want to propose here is not yet complete. There are chemical reactions, like those of the Belousov-Zhabotinsky type, which under constant supply of matter and energy do not equilibrate, but periodically or aperiodically change the concentrations of their reactants indefinitely. This behaviour makes them appear fascinatingly “lively”: the components come and go but the reaction as a totality persists. However, these reactions are unable to maintain the gradient of their boundary. They always take place in reaction vessels, the walls of which supply the boundary of the reactions (at least large parts of it). Without the supporting walls of the vessel the reaction would disintegrate after a short while. The vessel and hence also its walls forming part of the boundary of the reaction exist before the reaction starts. Quite contrarily the surface (boundary) of a living being does not exist before and independently of the living being. In this case the boundary of the system comes into existence just with the system itself. We take this property of a “self-determined boundary” as another important feature of self-maintaining systems and include it as part (iv) of its definition:
(iv) a self-maintaining system has a self-determined boundary, i.e. the boundary does not exist before and independently of the system.
All properties discussed so far are not only shared by living beings but also by populations of living beings which are connected phylogenetically. If the boundary of a population is defined as the union of the boundaries of all its individuals, then this boundary does not exist before the population, and hence it is self-determined. However, since we do not want to classify arbitrary populations as self-maintaining systems, we need another distinctive condition which does not exclude living beings. As a necessary condition we postulate that a self-maintaining system is spatially connected at each moment of its existence. By this constituent relation we mean that any two constituent subprocesses existing at a time t are in direct contact to each other or in indirect contact via other constituent subprocesses existing at the same time t.
Spatial connectedness also allows to define the unity and wholeness of a self-maintaining system. This is necessary, because otherwise both a cell within a multi-cellular organism and the total organism may turn out to be self-maintaining. To escape this difficulty we call only those systems self-maintaining which at each time of their existence satisfy the following maximality condition: A self-maintaining system is the maximal spatially connected unit satisfying all the other conditions of self-maintenance.
Summing up we arrive at the following characterization: A system is called self-maintaining if the following conditions are satisfied:
it exists permanently in an environment the fluctuations of which suffice to disrupt the gradient of its boundary locallythe constituent subprocesses are not in thermodynamic equilibrium with each otherall constituent subprocesses exist only for a finite time within the system, whereas the system virtually exists foreverit has a self-determined spatial boundary, i.e. the spatial boundary does not exist before and independently of, but is created with the systemat each time of its existence it is spatially connectedit is the maximal spatially connected unit obeying properties (i) to (v).
Remark: It follows from (iii) that self-maintaining systems are self- generating systems.
Conditions (i)–(vi) are abstract principles of self-maintenance. They do not tell which special mechanisms and arrangements of processes are able to guarantee these properties. But they allow one to decide whether a given system is self-maintaining without knowing the mechanisms underlying self-maintenance.
Indeed, it can be easily seen that living beings satisfy these conditions, hence they are self-maintaining. There is only a difficulty with condition (iii) which is in conflict with the fact that living beings generally have only a finite life-time. It can be assumed that organisms could persist in principle forever (in a suitable environment) if they had no genetically determined mechanisms defining upper limits of their life-time. Nevertheless they generally exist much longer than their constituent subprocesses. Such systems we call “real” self-maintaining systems, whereas the definition above characterizes “ideal” self-maintenance. The definition does not exclude the aspect that we consider a single line of cells following each other from one generation to the next as a single system. If it is true that all cells existing today derive from early cells existing several billion years ago then it is justified to say that cell lines virtually exist for ever.
Another advantage of defining self-maintenance in this abstract way is that only then it becomes clear that there may be completely different self-maintaining systems. There are in fact enormous differences between different species of living systems. But it may even be that somewhere else in the cosmos or in later times on earth self-maintaining systems exist which not even use the genetic code or the cell structure to maintain themselves.
Therefore self-maintenance is not used here to define “living.” However, since up to now living systems are the only self-maintaining systems on earth, this property may be used to characterize living beings. In this way it is possible to draw a border-line between biological and non- biological entities.
6. Implications of self-maintenance
The criteria of self-maintenance can be used in conjunction with known physical or chemical laws and in conjunction with knowledge of historical conditions on earth in order to derive a great number of conclusions on the structure and organization of living systems and their relationship to the environment. This procedure nearly automatically brings into perspective many details of the living world which otherwise appear to be combined arbitrarily and at random.
Without explicit deduction (for details see ) we only mention some points:
(a) Living systems as self-maintaining systems have to obey certain rules of stability. Since the individual subprocesses are in a sense unstable (iii) global and macroscopic properties of organisms have to exist which are attractors in terms of system theory. This is necessary because otherwise the always present external fluctuations (i) could drive the system into a state of collapse. The attractor properties of so-called “climax processes” have been emphasized by Schwegler .
(b) All equilibria observable in self-maintaining systems have adynamic nature being realized by a balance of production and destruction (this was recognized by many biologists, well known in this context is the concept of steady state or “Fliessgleichgewicht” ). It follows that the global structures must be self-regulated and self-organized. Examples of global properties are the number of subprocesses at a given time, the total energy content, the amount of in- and outflux of matter and energy, shape and anatomy of the organism.
(c) If a system indeed persists for a very long time compared with the duration of its constituent processes then it is not very likely that always completely new kinds of subprocesses arise. Rather it has to be expected that sequences of processes will run into (if they are not already in) circular arrangements of processes where subprocesses of the same types are generated again and again in a definite order. There are innumerable examples of circular arrangements of processes repeating themselves endlessly in living systems. We only point to the citric acid cycle and many other basic metabolic cycles which have not changed in many millions of years despite other drastic evolutionary changes. The cell cycle is another outstanding example of circular organization. Even the stasis of many species across thousands of generations can only be explained on the basis of predominant circular organization .
(d) The maintenance of a steep boundary gradient, damaged punctually by environmental influences (i), requires that the system operates far from thermodynamic equilibrium, i.e. self-maintaining systems are energetically open systems.
(e) All local parts of the system must be accessible both to the aquisition of energy-rich material and to the outward transport of energy- poor material. All voluminous living systems have very refined transport systems.
(f) There is another type of circularity in self-maintaining systems concerning the relationship between parts and whole: The whole determines the kinds and operations of the parts and vice versa (whereas in “normal systems, where the parts exist before and independently of the whole, the parts seem to play the major role). This is a special type of functional organization and makes living systems appearing sometimes mysterious, giving rise to theories like vitalism and some variants of holism. However, failure of such theories does not imply that biological entities can be well understood without any holistic principles or aspects. In the sense of this paper proper account of the parts – whole relationship with respect to morphology was given by Dullemeijer .
This remark is also important with respect to the problem of the relation between genome and organism, between genotype and phenotype. There is growing evidence that the organism does not simply receive “commands” from the genome but that also the organism regulates the genome. We cannot go into details of this problem here.
(g) It is to be expected that a theory of self-maintaining systems will shed new light on evolutionary processes. We suppose that the theory of self-maintaining systems if developed further allows a distinction between those properties of organisms which may undergo evolutionary changes and those which have to be invariant for the process of life to go on. It is very important to note that despite the enormous variety of structures and forms of life on earth there are some, though at first glance only formal, properties common to all creatures as self-maintaining systems. These properties are invariants of evolution .
In spite of the formal nature of these principles their universality is reflected in the universality of some material structures of living beings. It is very remarkable that all known forms of life are coupled to the cell structure. Under this aspect it is not so much astonishing that single cell organisms are those which have maintained themselves for the longest time in the history of life, ranging to some billion years. But also with respect to all other types of organisms it is most remarkable that the cellular structure “survived” all mutations throughout (the whole of) evolution.
There are many other examples such as the citric acid cycle or the machinery of protein and nucleic acid synthesis which were conserved across a billion years. They confirm us in our opinion that self- maintenance is characteristic for the organization of the living. The importance of the circular relationships between nucleic acids and proteins for evolutionary developments was elaborated in the theory of hypercycles , .
In this context we point to a certain explanatory antagonism. In one type of explanation constancy and stability of a species are based on a selective advantage this species has to nearby relatives. In this model a species is preserved because it represents a stable equilibrium in a competitive process. This type of explanation requires the existence and permanent production of less fit individuals in order to ensure that again and again the most fit ones are selected and conserved.
In the other type of explanations the specific properties of individuals or species remain unaltered because they are properties of processes which are stable by themselves due to their internal organization. In this case, for the existence of a long sequence of generations it is not necessary that other individuals or populations of individuals with differing properties exist.
It is very important to see that these two kinds of explanation, despite their alternative character, are by no means contradictory or mutually exclusive. We believe that they are complementary to each other and that both of them play an important role in evolution. Without some internal stabilization the competitive process and hence selection could not even take place. For any theory of evolution it is crucial to distinguish between those properties of organisms which can change without violating the principles of self-maintenance and those properties which must not or cannot change. This antagonistic twin relationship can be highlighted by the paradoxical formulation “Evolution would stop if the property of organisms to produce mutants were not a stable variant.”
We appreciate very much the incentive stimulation the work of H.R. Maturana and F. Varela on “autopoietic” systems had on our ideas presented here. A discussion of the connection between their and our concepts will be given elsewhere . In addition the relationship to other characterizations of living systems (e.g. those by von Bertalanffy and Schrödinger) will be considered there.
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