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The Driving Force for Life s Emergence: Kinetic and Thermodynamic Considerations

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J. theor. Biol. (2003) 220, doi: /jtbi , available online at on The Driving Force for Life s Emergence: Kinetic and Thermodynamic Considerations Addy Pross
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J. theor. Biol. (2003) 220, doi: /jtbi , available online at on The Driving Force for Life s Emergence: Kinetic and Thermodynamic Considerations Addy Pross n Department of Chemistry, Ben-Gurion University of the Negev, P.O. Box 643, Beer Sheva 84105, Israel (Received on 26 December 2001, Accepted in revised form on 23 September 2002) The principles that govern the emergence of life from non-life remain a subject of intense debate. The evolutionaryparadigm built up over the last 50 years, that argues that the evolutionarydriving force is the Second Law of Thermodynamics, continues to be promoted bysome, while severelycriticized byothers. If the thermodynamic drive toward everincreasing entropyis not what drives the evolutionaryprocess, then what does? In this paper, we analyse this long-standing question by building on Eigen s replication first model for life s emergence, and propose an alternative theoretical framework for understanding life s evolutionarydriving force. Its essence is that life is a kinetic phenomenon that derives from the kinetic consequences of autocatalysis operating on specific biopolymeric systems, and this is demonstrablytrue at all stages of life s evolution F from primal to advanced life forms. Life s unique characteristics F its complexity, energy-gathering metabolic systems, teleonomic character, as well as its abundance and diversity, derive directly from the proposition that from a chemical perspective the replication reaction is an extreme expression of kinetic control, one in which thermodynamic requirements have evolved to play a supporting, rather than a directing, role. The analysis leads us to propose a new sub-division within chemistry F replicative chemistry. A striking consequence of this kinetic approach is that Darwin s principle of natural selection: that living things replicate, and therefore evolve, maybe phrased more generally: that certain replicating things can evolve, and may therefore become living. This more general formulation appears to provide a simple conceptual link between animate and inanimate matter. r 2003 Elsevier Science Ltd. All rights reserved. Introduction The nature of the driving force that led to the emergence of animate matter remains a subject of continuing debate and uncertainty. What physico-chemical principles led to the emergence of biological complexity, to the formation of increasinglycomplex far-from-equilibrium systems exhibiting purposeful structure and behavior? And given that it is the Second Law of n Tel.: ; fax: address: (A. Pross). Thermodynamics that governs the direction that all spontaneous processes must necessarilyfollow, how do these principles relate to the Second Law? One reason for much of the confusion that has enveloped this fundamental issue derives from the fault line that continues to separate biology and physics. The autonomy of biology approach to science, invoked some 200 years ago bykant (1952) with his natural purpose concept, and reinforced bymodern biologists such as Mayr (1988), had the unintended effect of impeding attempts to provide a physical /03/$35.00 r 2003 Elsevier Science Ltd. All rights reserved. 394 A. PROSS understanding of the evolutionaryprocess that led to biological complexity. But the magnitude of the problem of bridging between biologyand physics cannot be overstated: there is considerable inherent difficulty in applying physical principles, generallydeveloped for model systems of limited complexity, to biological systems that are characterized bycomplexityof overwhelming proportion. Dawkins (1986) opening line in The Blind Watchmaker says it succinctly: We animals are the most complicated things in the known universe. Nonetheless, despite the obvious difficulties, the need to place biological realityand its emergence within a context of physical law needs to be addressed. In discussing biologyand thermodynamics, we need to distinguish between two quite different issues: (a) the relationship between the operation of living systems and the Second Law, a problem that was resolved a centuryago, and (b), the process bywhich that complexityemerged, an issue which remains highlycontentious. The first issue F the functional operation of living systems F poses no thermodynamic dilemma because the complex, far-from-equilibrium nature of living systems is maintained through the continual utilization of energy, be it solar or chemical. But how did far-from-equilibrium systems emerge in the first place? A significant advance in our abilityto understand the emergence of non-equilibrium complexitytook place some 50 years ago when Schro dinger (1944), Bertalanffy(1952), and then Prigogine (1978), laid down the foundations for nonequilibrium thermodynamics and the characterization of living systems as dissipative structures. This base has since served as the central element in the development of what has been termed the new evolutionaryparadigm (Wicken, 1985, 1989, 1998; Wiley& Brooks, 1982; Schneider and Kay, 1994; Swenson, 1997). According to this paradigm, the emergence of ordered, far-from-equilibrium, energydissipating systems poses no thermodynamic mystery, since it is a physically allowed response of an equilibrium system reacting to some perturbing potential. However, over the period of time that these ideas have been proposed, persistent dissenting voices have also sounded (see, for example, Weber et al., 1988; Elitzur, 1994; Peacocke, 1989; Thaxton et al., 1984, pp ; Corning & Kline, 1998a). One of the responses of the scientific communityto this scientific deadlock has been to widen the arena of discourse. Thus, in parallel, the debate has expanded so as to incorporate concepts from information theory(ku ppers, 1990), systems analysis (Corning & Kline, 1998a; Conrad, 1997), mathematics and computer science (Kauffman, 1993, 2000), even engineering (Corning & Kline, 1998b) F sciences whose epistemological frameworks might provide additional tools for addressing the highlycomplex nature of living systems. However as the abovecited literature makes clear, the issue remains controversial and far from resolved. In this paper, we address the problem of emergence from a chemical perspective, building on Eigen s (1971, 1992) kinetic approach initiated some 30 years ago, rather than on the physical perspective that characterizes much recent work. That recent work accepts the highly complex nature of living systems and the nonlinear dynamics associated with that physical complexityas a given (e.g. Kauffman, 1993, 2000; Conrad, 1997; Corning & Kline, 1998b), indeed the veryelement that requires explanation, and therefore tackles the problem from that point of view. But as we will discuss in more detail below, byexamining the evolutionary process at its earliest stages, we believe that much of the difficultyassociated with this inherent complexitycan be avoided. Secondly, following Eigen, we believe a chemical approach to be instructive simplybecause living systems are first and foremost chemical systems, whose description and governing principles should be explicable in chemical terms. It is the discipline of chemistrythat bridges between biologyand physics, and, with the benefit of hindsight, it is of less surprise that attempts to merge between the disciplines of biologyand physics have run into the difficulties theyhave. Thermodynamics, though formallyan integral part of physical science, does have its chemical aspects and orientation. Our analysis suggests that the role played by an element we term the kinetic imperative, though part of the evolutionary debate since Darwin, has not been adequately recognized. We will attempt to demonstrate that THE DRIVING FORCE FOR LIFE S EMERGENCE 395 life s unusual characteristics derive from dynamic rather than thermodynamic considerations, and that a broader perspective on the Darwinian principle mayresult when emergence and evolution are viewed through such a kinetic perspective. Discussion The Second Law of Thermodynamics, a fundamental tenet of physics and chemistry, requires that in an isolated system all transformations proceed irreversiblytoward a state of equilibrium, that state being defined as one of maximum entropy. In living systems however, we see a thermodynamic pattern that is striking and unusual. Though of course living systems fullyobeythe Second Law, all living entities are far-from-equilibrium systems that constantly consume energyin order to maintain the farfrom-equilibrium state so essential for life. For example, non-equilibrium ion concentration gradients, both within the cell and between the cell and its environment, are maintained bythe action of ion pumps that consume considerable metabolic energyin order to pump ions against the concentration gradient. Keyphysiological functions, such as nerve cell transmission, cruciallydepend on the maintenance of such non-equilibrium concentration gradients (Bolsover et al., 1997). Of course, as alreadynoted, living systems do not violate the Second Law since living systems exist in a situation of material and energy exchange with their environment. So just as a refrigerator, bythe consumption of energy, can transfer heat from a cold region to a hotter one, against the natural thermodynamic direction, so living systems can create order from disorder, and maintain themselves in a far-from-equilibrium state, through the constant utilization of energy F from food in the case of animals, or solar energyin the case of plants. But appreciating that living systems are functional thermodynamic entities that do not violate the laws of thermodynamics does not in itself resolve the dilemma. Indeed the starting point for the half centurylong debate began with the realization that the function of living beings are fullyconsistent with the laws of thermodynamics. Living systems still appear highly improbable and extremelysurprising, as the refrigerator analogymakes clear. A refrigerator exists because it has been designed and built to function in a manner that counters the natural thermodynamic direction whereby heat flows from hot to cold. The reasons for its existence and function are inseparable. But how does one explain the emergence of a natural system that from a thermodynamic viewpoint seems to mimic refrigerator behavior? What general principle can explain the emergence of a highly complex system that taps into some external energysource F be it solar or chemical, in order to maintain its far-from-equilibrium state? Indeed, several of the noted physicists of the early 20th centuryargued that the laws of physics were inadequate for explaining biological phenomena. Schro dinger (1944) exemplifies this view when he observed in his book, What is Life? that we must be readyfor the fact that living matter works in a waythat cannot be reduced to the usual physical laws. There are other distinct characteristics of living systems that need to be addressed in the context of explaining life s emergence. Monod (1972), Dobzhansky et al. (1977), Ku ppers (1990) and others, have pointed out that life s direction is governed byits teleonomic character F that undeniable sense of purpose associated with the behavior and organization of living beings. As Dobzhansky(Dobzhanskyet al., 1977, p. 95) put it: Purposefulness, or teleology, does not exist in nonliving nature. It is universal in the living world. It would make no sense to talk of the purpose or adaption of stars, mountains, or the laws of physics. Adaptedness of living beings is too obvious to be overlooked. Thus, life s unique teleonomic character, expressed byjacob as the dream of everycell to become two cells (quoted in Monod, 1972, p. 20), seems intimatelylinked to life s unusual thermodynamic behavior. Indeed it is evident that the evolutionaryprocess has equipped living entities with the abilityto exploit the rules of thermodynamics to maximum advantage so as to enable them to pursue their teleonomic goals as efficientlyas possible. Monod considered the veryexistence of this teleonomic character as a flagrant epistemological contradiction, and 396 A. PROSS went so far as to state: In fact the central problem of biologylies with this verycontradiction, which if it is onlyapparent, must be resolved; or else proven to be utterlyinsoluble, if that should turn out to be the case. Dobzhanskyexpressed the same sentiment: The origin of organic adaptedness, or internal teleology, is a fundamental, if not the most fundamental problem of biology. Given the modern view that all living beings are physico-chemical systems that are merely following the laws of physics and chemistry, this teleonomic character must have some physico-chemical rationale, whose essence needs to be identified. THE NON-EQUILIBRIUM THERMODYNAMIC APPROACH In the past half century, new insights into the problem of emergence of biological complexity were obtained byextending the thermodynamic domain from equilibrium systems to non-equilibrium ones (Bertalanffy, 1952; Prigogine, 1978; Peacocke, 1989; Babloyantz, 1986; for more recent treatments, see: Weber et al., 1988, Schneider & Kay, 1994; Swenson, 1997). When a system at equilibrium is perturbed by some external force, small random fluctuations are induced that can amplifydramaticallyand lead to spontaneous self-organization and order F so-called dissipative structures. According to this view, living beings are not unique in their highlyorganized non-equilibrium state. Living beings simplyrepresent one particular class of organized complex systems far-from-equilibrium, that are able to maintain their highly ordered structure bythe constant transfer of energyand matter between the system and its environment. Thus, according to this approach the emergence and existence of complex nonequilibrium biological systems poses no thermodynamic dilemma F life s unusual thermodynamic character is explained byits characterization as a dissipative structure. While the non-equilibrium thermodynamic approach does remove some of the mystery regarding the veryexistence of biological complexityin its far-from-equilibrium state, there is continuing and persistent opposition to its central claim (see, for example, Peacocke, 1989; Weber et al., 1988; Elitzur, 1994; Corning & Kline, 1998a; Collier, 1988; Thaxton et al., 1984). Firstly, the categorization of life as a dissipative structure appears too general. Biological systems are clearly different to dissipative structures, such as heated liquids and whirlpools, with which theyare compared. Dissipative structures tend to be relativelytransient, generallyform in response to some immediate perturbation, and are characterized by order. In contrast, the existence and function of each and every biological system on the planet today, be it single or multi-cell, is characterized by organization (rather than order), and is directlylinked to events that took place almost four billion years ago. Thus the highlyintricate and organized complexityof biological systems, one based on heritable coded information, makes biological complexityunique and quite distinct from the relativelytransient and arbitrary order of typical non-biological dissipative structures. But even if we accept the theoretical basis for the thermodynamic paradigm, a serious difficultyremains. Modeling living systems on dissipative structures fails to provide insights into the nature of biological function, and, in particular, into the specific processes bywhich that function emerged. As Collier (1988, p ) has pointed out, there is no evidence that the laws of non-equilibrium thermodynamics applyto biological systems in a non-trivial fashion. Also it should be noted that the other striking life characteristic discussed earlier F teleonomy F finds no resolution within the dissipative structure approach. Corning & Kline (1998a), for example, have recentlypointed out that life s teleonomic character is not derivable from the Second Law nor from anyof its thermodynamic parameters. In the last two decades, attempts have been made to overcome the above-mentioned deficiencies of the non-equilibrium approach by focussing on features that characterize biological systems specifically F in particular information, and incorporating them into a thermodynamic description (Wiley& Brooks, 1982). However, the thermodynamic problem was greatly magnified bythe introduction of information into the physically grounded discourse. The concept of entropyhad now to accommodate, not just an THE DRIVING FORCE FOR LIFE S EMERGENCE 397 energetic and statistical aspect, but an informational one as well. As a result, attempts to merge these various concepts seem to have led to even greater confusion. Thus argument regarding the distinction between order and organization, the various definitions of entropy F some mutually contradictory, and the conflicting thermodynamic treatments of information, have all conspired to create a thermodynamic paradigm whose derivation and particulars remain uncertain. The problem maybe summarized as follows: attempts to applyrigorouslydefined physical parameters, derived from well-defined physical systems, to the broad sweep of highly complex biological systems, where the corresponding parameters are not readilyquantifiable, and in some cases are not even definable, remain theoreticallycontroversial. These sentiments are reflected in Corning and Kline s (1998a) detailed critique of the thermodynamic paradigm that concludes: Monolithic thermodynamic theories of evolution are fundamentally flawedy Interestingly, using insights from cosmology, Layzer (1988) has also concluded that the Second Law is not the driving force responsible for life s emergence, though his analysis does lead him to support the Wiley and Brooks view that entropyand information do indeed grow together. We would conclude therefore bysaying that the non-equilibrium thermodynamic framework, while able to resolve the apparent paradox inherent in the very existence of non-equilibrium biological systems, fails to provide fundamental insights into the evolutionaryprocess bywhich biological complexityemerged. In an attempt to provide some alternative description of the evolutionarydriving force, there is one additional characteristic of life beings that needs to be mentioned. We are referring to the diversityand widespread nature of living beings. As one looks over the planet, it is evident that life in one form or another has overwhelmed it F a fact that Darwin was alreadywell aware of over 150 years ago. We are not referring here to just plant, animal and marine life that maybe found on most parts of the planet, but in particular to microbial life. As pointed out bygould (1996), life is extraordinarilyabundant. A small sample of garden soil might contain billions of microbes belonging to thousands of different species, a square centimeter of our skin might house some 10 5 microbes, and it has been estimated that fully 10% of a human body s dried weight consists of bacteria, manyof which we cannot survive without. The growing awareness of the existence of a class of bacteria termed extremophiles that survive without difficultyin extreme environmental conditions, such as high salinity, high pressure, high and low temperatures, etc. are further evidence for life s extraordinaryadaptability. Indeed, in a memorable comment Gold (1992) has stated: Microbial life exists in all the locations where microbes can survive. Simply put, at least with respect to our planet, life is almost everywhere. Thus, the unusual thermodynamic characteristics of living beings mentioned earlier, manifest themselves within one of the most widespread set of chemical reactions on the face of the earth F the metabolic reactions of life. So given the increasinglywidespread view that the emergence of biological complexityis not a direct manifestation of the Second Law, what is the physical principle or principles that can be considered re
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