Complexity: an Energetics Agenda

Complexity: An Energetics Agenda

Energy as the Motor of Evolution

1. INTRODUCTION rom galaxies to snowflakes, from stars and planets to life itself, modern science is weaving an intricate pattern describing all of natural history—a sweepingly F inclusive view of the order and structure of every known class of object in our richly endowed Universe. Cosmic evolution—the heart of this broad scenario—is the study of the sum total of the many varied developmental and generational changes in the assembly and composition of radiation, matter, and life throughout all space and across all time. These are the physical, biological, and cultural changes that have produced, in turn and among other systems, our Galaxy, our Sun, our Earth, and ourselves. The result is a grand evolutionary synthesis bridging a wide variety of scientific specialties—physics, astronomy, geology, chemistry, biology, and anthro- pology, among others—a genuine narrative of epic proportions extending from the beginning of time to the present, from big bang to humankind. ERIC J. CHAISSON Our appreciation for evolution now extends well beyond the subject of biology; the concept of evolution, generally considered, has become a powerful unifying factor in all of science. Yet questions remain: How valid are the interdisciplinary continuities among Nature’s many historical epochs, and how realistic is our quest Eric J. Chaisson is at the Wright Center, for unification among them? Can we reconcile the observed constructiveness of Tufts University, 4 Colby Street, cosmic evolution with the inherent destructiveness of thermodynamics? Is there a Medford, Massachusetts; e-mail: single driver of fundamental change writ large? We especially want to know more [email protected]; Website: about the origins of the richly ordered and diverse structures spanning both the www.tufts.edu/as/wright_center. terrestrial and extraterrestrial Universe, notably those systems often characterized by the intuitive term “complexity”—a state of intricacy, complication, variety, or in- volvement, as in the interconnected parts of a system. Particularly intriguing is the rise of complexity over the course of time, indeed dramatically so within the past half-billion years since the end of the pre-Cambrian on Earth. Resembling a kind of neo-Platonism, perhaps some underlying principle, a unifying law, or an ongoing process does create, order, and maintain all structures in the Universe, enabling us to study everything on uniform, common ground—“on the same page,” so to speak. Recent research, guided by notions of beauty and symmetry and bolstered by vast new databases, suggests affirmative answers to some of these queries: Islands of ordered complexity—namely, open systems such as galaxies, stars, planets, and life forms—are more than balanced by great seas of increasing disorder elsewhere in the environments beyond those systems. All can be shown to be in quantitative agreement with the principles of thermodynamics, especially nonequilibrium thermodynamics. Furthermore, flows of energy engendered largely by the expanding cosmos

14 COMPLEXITY © 2004 Wiley Periodicals, Inc., Vol. 9, No. 3 do seem to be as universal a process in FIGURE 1 the origin of structured systems as any- thing yet found in Nature. The optimi- zation of such energy ﬂows might well act as the motor of evolution broadly conceived, thereby orchestrating much of physical, biological, and cultural evolution [1].

2. ARROW OF TIME Figure 1(a) shows an archetypal illustra- tion of cosmic evolution, the “arrow of time.” Regardless of its shape or orien- (a) This symbolic “arrow of time” highlights salient features of cosmic history, from its fiery origins tation, such an arrow represents an in- some 14 billion years ago (at left) to the here and now of the present (at right). Labeled diagonally tellectual guide to the sequence of across the top are the major evolutionary phases that have produced, in turn, increasing amounts of events that have changed systems from order and complexity among all material systems: particulate, galactic, stellar, planetary, chemical, biological, and cultural evolution. Cosmic evolution encompasses all of these phases. Time is assumed simplicity to complexity, from inorganic to flow linearly and irreversibly, unfolding at a steady pace, much as other central tenets are assumed, to organic, from chaos in the early Uni- such as the fixed character of physical law or the mathematical notion that 2 ϩ 2 ϭ 4 everywhere. verse to order more recently. That se- (b) Sketched here qualitatively is the rise of order, form, and structure typifying the evolution of quence, as determined by a large body localized material systems throughout the history of the Universe. This family of curves connotes the of post-Renaissance data, accords well widespread, innate feeling that complexity of ordered structures has generally increased over the course of time. Whether this rise of complexity has been linear, exponential, or hyperbolic (as drawn with the idea that a thread of change here), current research aims to specify this curve, to characterize it quantitatively. links the evolution of primal energy into elementary particles, the transforma- tion of those particles into atoms, in ditions more conducive to stellar and turn of those atoms into galaxies and Our appreciation for evolution planetary formation; now, at least on stars, and of stars into heavy elements, now extends well beyond the Earth, cultural evolution dominates. the evolution of those elements into the subject of biology; the concept of Change in surrounding environments molecular building blocks of life, of evolution, generally considered, usually precedes change in organized those molecules into life itself, and of has become a powerful unifying systems, and the resulting system intelligent life into the cultured and factor in all of science. changes have generally been toward technological society that we now greater amounts of order and complex- share. Despite the compartmentaliza- ity. tion of today’s academic science, evolu- Time’s arrow implies no anthropo- Figure 1(b) sketches the widespread tion knows no disciplinary boundaries. centrism; it is not pointing at us. The impression that material assemblages As such, the most familiar kind of arrow merely provides a mental road- have become more organized and com- evolution—biological evolution, or map symbolically mapping the rise of plex, especially in relatively recent neo-Darwinism—is just one, albeit im- increasingly complex structures, from times. This family of curves refers to portant, subset of a much broader evo- spiral galaxies to rocky planets to repro- islands of complexity comprising sys- lutionary scheme encompassing more ductive beings. Nor does the arrow tems per se—whether giant stars, buzz- than mere life on Earth. In short, what mean to imply that “lower,” primitive ing bees, or urban centers—not their Darwinian change does for plants and life forms biologically changed directly vastly, indeed increasingly, disorga- animals, cosmic evolution aspires to do into “higher,” advanced organisms, any nized surroundings. A central task of for all things. And if Darwinism created more than galaxies physically change complexity science aims to—or ought an intellectual revolution by helping to into stars or stars into planets. Rather, to—explain this temporal rise of orga- free us from the notion that humans with time, the environmental condi- nization. That is the objective of this basically differ from other life forms on tions suitable for spawning primitive paper. our planet, then cosmic evolution ex- life eventually changed into those favor- tends that revolution by treating matter ing the emergence of more complex 3. NONEQUILIBRIUM on Earth and in our bodies no differ- species; likewise, in the earlier Uni- THERMODYNAMICS ently from that in the stars and galaxies verse, environments ripe for galactic Cosmic evolution, as understood today, beyond. formation eventually gave way to con- is governed largely by the laws of phys-

© 2004 Wiley Periodicals, Inc. COMPLEXITY 15 ics, particularly those of thermodynam- provided we reason in broad, interdis- FIGURE 2 ics. Note the adjective “largely,” for this ciplinary terms. is not an exercise in traditional reduc- Figure 2 schematically diagrams, tionism; no attempt is made here to along the lines developed by Prigogine “reduce” biology to physics, rather to [4], the emergence of structure in the broaden physics to include biology. For presence of energy flow. Physicists are of all the known principles of Nature, familiar with the type of curves at the thermodynamics has perhaps the most top; biologists more comfortable with to say about the concept of change—yet those at bottom. Upon crossing certain change dictated by a combination of energy thresholds that depend on a sys- chance and necessity, of randomness tem’s status, bifurcations can occur, and determinism. Even so, the cosmic- fostering the emergence of whole new evolutionary narrative is much too rich hierarchies of novel structures that dis- and diverse to be explained merely by play surprising amounts of coherent be- equilibrium thermodynamics. All struc- havior. Such dissipative structures ex- tures, whether galaxies, stars, planets, port some of their entropy (or expel or life forms, are demonstrably open, some of their energy) into the external nonequilibrated systems, with flows of environment with which they interact. energy in and out a central feature. And Accordingly, order is created and often it is this energy, the so-called available, maintained by routine absorption of or “free” energy, that goes to work and substances rich in energy, followed by (a) The extent to which open systems depart helps build structures. discharge of substances low in energy. from equilibrium is drawn here as a function By utilizing energy order can be This process, often misnamed, is not of both time and energy. The time axis achieved, or at least the environmental makes clear that this is an historical, evo- conditions made conducive for the po- lutionary process, whereas the parallel energy axis denotes the free energy flowing tential rise of order within open systems In short, what Darwinian change through an open system as a vital part of its ripe for growth. Whether it’s electricity does for plants and animals, being. At certain critical energies, labeled cosmic evolution aspires to do powering a laser, sunlight shining on a here Ec, the system can spontaneously plant, or food consumed by humans, for all things. change, or bifurcate, into new, nonequilib- energy flows do play a key role in the rium, dynamic steady states. Statistical fluctuations—that’s chance—affect which creation, ordering, maintenance, and really self-ordering; it is ordering in the fork the system selects—that’s necessity— fate of complex systems—all in quanti- upon bifurcation (vertical arrows), namely presence of energy. “Self-organization” tative accord with thermodynamics’ which spatial structure is achieved. The reflects an essential tension between celebrated second law. None of Na- process of change, as always, is an inter- energy inflow and dissipative outflow; ture’s ordered structures, not even life, play of randomness and determinism; these systems do not function magically therefore the end result is inherently unpre- is a violation (nor even a circumven- by themselves. dictable, as with virtually all of evolution. (b) tion) of this law. For both ordered sys- Events in evolutionary biology mimic those The onset of order from a state orig- tems as well as their surrounding envi- of the physical diagram in (a), although the ronments, we find good agreement with inally having none is relatively straight- results are richer in structural detail, system function, and energy flow. In phases modern, nonequilibrium thermody- forward. Fluctuations—random devia- tions from some average, equilibrium marked A, species survive and persist until namics. No new science is needed [1]. the environment changes (vertical arrows), value of, for example, density, temper- Championed decades ago by Berta- after which further evolution occurs—along lanffy [2] and later espoused by ature, or pressure—inevitably yet sto- phase B toward renewed survival or phase Schroedinger [3], the need for energy chastically appear in any system having C toward extinction. The upward rising graphs (drawn solid in both a and b) imply has come to be recognized as an essen- many degrees of freedom. Normally, as no progress, but they do suggest a general in equilibrium thermodynamics, such tial feature, not only for biological sys- trend toward increasing complexity. tems such as plants and animals that instabilities regress in time and disap- process foodstuffs in the course of liv- pear; they just come and go by chance, ing, but also for physical systems such the statistical fluctuations diffusing as laxed. Nor can open systems near equi- as stars and galaxies that convert grav- quickly as they arise. Even in isolated librium evolve spontaneously to new itation potential into heat and light, in- systems, such internal fluctuations can and interesting structures. But should deed for social systems, too, such as generate local, microscopic reductions those fluctuations become too great for cities that economize the inward flow of in entropy, but the second law ensures an open system to damp, that system food and fuel and the outward flow of that they will always balance out. will then depart far from equilibrium products and wastes. The analysis is Minute temperature fluctuations, for and be forced to regroup. Such reorga- much the same for any open system, instance, are said to be thermally re- nization generates a “dynamic steady

16 COMPLEXITY © 2004 Wiley Periodicals, Inc. state,” provided the amplified fluctua- myriad ordered systems evident in Na- simplest of all structures; the energy tions are continuously driven and stabi- ture. density of matter began to dominate. lized by the flow of energy from the This represents a change of first magni- surroundings—namely, provided the tude—perhaps the greatest change of 4. STANDARD COSMOLOGY energy flow rate exceeds the thermal all time—for it was as though an earlier, The origin of Nature’s many varied relaxation rate. Global, coherent cycling blinding fog had lifted; cosmic unifor- structures is closely synonymous with is often the result, because under these mity was punctured, its symmetry bro- the origin of free energy. Time marches conditions the spontaneous creation of ken, its equilibrium spent. The Universe on, equilibrium fails, and free energy macroscopic structures dissipates en- thereafter became transparent, as pho- flows because of cosmic expansion ergy more rapidly than the ensuing, and tons no longer scattered aimlessly and damaging, heat can damp the gradients [5,6], all of it typified by the run of den- destructively. The bright Radiation Era and destroy those structures. Further- sity and temperature shown in Figure gradually transformed into the darker more, because each successive reorder- 3(a). Here, the essence of change is Matter Era; it occurred about 300,000 ing causes more complexity than the plotted on the largest scale—the truly years after the big bang, which is when preceding one, such systems become big picture, or “standard model,” of the the free energy began to flow. even more susceptible to fluctuations. whole Universe—so these curves per- More technically, although thermal Complexity itself consequently creates tain to nothing in particular, just every- and chemical (but not gravitational) en- the conditions for greater instability, thing in general. They track the main tropy must have been maximized in the which in turn provides an opportunity trends, minus the devilish details, of early Universe, hence complexity in the for greater reordering. The resulting modern cosmology: the cooling and form of any structures then nonexistent, phenomenon—termed “order through thinning of radiation and matter, largely the start of the Matter Era saw the en- fluctuations”—is a distinctly evolution- based on observations of distant reced- vironmental conditions become more ary one, complete with feedback loops ing galaxies and of the microwave back- favorable for the potential growth of or- that can drive a system farther from ground radiation—all this change fun- der, taken here as a “lack of disorder.” equilibrium. But only in the presence of At issue was timing: As density (␳) de- energy, for otherwise Nature abhors a creased, the equilibrium reaction rates “Self-organization” reflects an gradient (and not just a vacuum). And (ϰ␳) fell below the cosmic expansion essential tension between energy 1/2 as the energy consumption and result- rate (ϰ ␳ ) and nonequilibrium states inflow and dissipative outflow; ing complexity accelerate, so does the became possible. Thus we have a para- these systems do not function evolutionary process—all of which put doxical yet significant result that, in an magically by themselves. us into the realm of true thermodynam- expanding Universe, both the disorder ics, the older, traditional subject of that (i.e., net entropy) and the order (maxi- name more properly labeled “thermo- damentally driven by expansion of the mum possible entropy minus actual en- statics.” Universe. tropy at any given time) can increase Numerous such systems come to Radiation completely ruled the early simultaneously, the former globally and mind, and not only in the physical Universe. Life was then nonexistent and the latter locally [1]. world of convection cells, river eddies, matter itself only a submicroscopic pre- atmospheric storms, and even artifi- cipitate suspended in a glowing fireball 5. THE RISE OF COMPLEXITY cially made devices such as refrigerators of intense light, X-rays, and gamma Recall the task we set out to address: to and lasers among a whole host of sim- rays. Structure of any sort had yet to quantify the rise of complex systems, ilar examples of systems that experience emerge; the energy density of radiation ideally for all such systems in the same coherent order when amply fed with was too great. If single protons captured way, lest special effects prescribe some sufficient energy. Biological systems, single electrons to make hydrogen at- systems, not least perhaps life. But how too, obey the rules of nonequilibrium oms, radiation was then so fierce as to shall we characterize complexity, a slip- thermodynamics, for we and our living destroy those atoms immediately. Pre- pery term for many researchers? In bi- relatives are demonstrable examples of vailing conditions during the first few ology alone, much as their inability to dynamic steady states that emerge and tens of millennia after the origin of time reach consensus on a definition of life, flourish via energetically rich neo-Dar- were uniform, symmetrical, equili- biologists cannot agree on a complexity winism. As are Lamarckian-type cul- brated, and boring. We call it the Radi- metric. Some count nonjunk genome tural systems of more recent times, for ation Era. size [7], others employ morphology and energy again is the principal broker in Eventually and inevitably, as de- flexibility of behavior [8], whereas still the making of today’s bricks and chips. picted by Figure 3(b), the primacy of others chart numbers of cell types in The upshot is that life and its social radiation gave way to matter. As the organisms [9] or appeal to cellular spe- inventions differ not in kind, but merely expanding Universe naturally cooled cialization [10]. Each of these attributes in degree—specifically, degree of com- and thinned, charged particles assem- of life is useful in qualitative ways, but plexity and energy use—among the bled into neutral atoms, among the they evade quantification and apply

© 2004 Wiley Periodicals, Inc. COMPLEXITY 17 FIGURE 3 events in the early Universe [Figure 3(b)]. Moreover, it is the rate at which free energy transits a complex system of given mass that seems most important. Hence, free energy rate density, symbol- ⌽ ized by m, is an operational term whose meaning, measurement, and units (erg sϪ1 gϪ1) are clearly understood. Figure 4 plots a sampling of many findings, where free energy rate densities are graphed as horizontal histograms for various systems’ evolutionary ⌽ ϳ 3 ages. As expected, plants ( m 10 erg sϪ1 gϪ1) are more complex than stars (ϳ1) or planets (ϳ102); animals (ϳ104) (a) The essence of big-bang cosmology: A plot of the density (␳) of matter and the temperature (T)of ϳ 5 radiation, from near the origin of the Universe to the present. The thicker width of the density curve and their brains ( 10 ) more complex represents the range of uncertainty in total mass density, whose value depends on the (as yet yet; and society collectively (ϳ106) unresolved issue of) “dark matter.” By contrast, the cosmic background temperature is very accurately among the most complex of all known measured today, and its thin curve can be accurately extrapolated back into the early Universe. The ordered systems. That is, although the startling possibility, recently discovered, that universal expansion might be accelerating should not much affect these curves to date. (b) The temporal behavior of both matter energy density (␳c2) and total energy flowing through a star or radiation energy density (aT4) illustrates perhaps the greatest change in history, where a is the planet is hugely larger than that radiation constant and c is the speed of light. Where the two curves intersect, neutral atoms began through our human body or brain, the to form. By some 300,000 years, the Universe had changed greatly as thermal equilibrium and particle specific rate (per unit mass) is much symmetry had broken, and the Radiation Era transformed into the Matter Era. A uniform, featureless larger for the latter. The modeled flow state characterizing the early Universe thus naturally changed into one in which order and complexity were thereafter possible. of normalized energy for a wide range of open systems, be they alive or not, closely resembles the intuitive rise in complexity implied by Figure 1(b). only to biological systems. Here, the Complexity (as treated here energeti- goal is to push the envelope beyond Machines, too, and not just cally for localized structures) has in fact mere words, indeed beyond biology. computer chips, but also ordinary risen throughout the course of natural Putting aside as unhelpful the idea of motors and engines that impel history, and at a rate at least exponen- information content (of the Shannon- our fast-paced, 21st-century tial in more recent times [1, 17]. Weaver type, which is admittedly useful society, can be likewise cast in This is not to say, by any means, that yet controversial in some contexts) and evolutionary terms…. galaxies per se evolved into stars, or of negative entropy (or “negentropy,” stars into planets, or planets into life. which Schroedinger [3] first adopted Rather, this analysis suggests that galax- and then quickly abandoned), I prefer is surely more complex than any inani- ies gave rise to environments suited to to embrace the quantity with greatest mate object. Thus, absolute energies are utilize flows of energy for the birth and appeal to physical intuition—energy. not as telling as relative values, which Given that energy is the most universal maturation of stars, that some stars depend on a system’s size, composi- currency known in the natural sciences, spawned environments energetically tion, and efficiency. Nor are maximum energy has a central role to play in any conducive to the formation and main- energy principles or minimum entropy attempt to unify physical, biological, tenance of planets and that at least one states likely relevant; rather, organiza- and cultural evolution. Not that energy planet fostered an energy-rich environ- tional complexity is mostly governed by has been overlooked in previous studies ment ripe for the origin and evolution of of Nature’s many varied structures. the optimum use of energy—not too lit- life. Cosmic evolution, to repeat, incor- Physicists [11, 12], biologists [13, 14], tle as to starve a system, yet not too porates both developmental and gener- ecologists [15, 16], to cite but a few, much as to destroy it. ational change, spanning physical, bio- have championed the cause of energy’s To characterize complexity objec- logical, and cultural systems, across a organizational abilities. Even so, the tively—that is, to normalize all such or- wide and continuous hierarchy of com- quantity of choice cannot be just energy dered systems on that same page—I plexity from big bang to humankind. alone, for a star clearly has more energy adopt a kind of energy density, much And in an expanding, nonequilibrated than an amoeba, a galaxy much more like the competing energy densities of Universe, energy is a natural underlying than a single cell. Yet any living system radiation and matter that dictated driver for the rise of that complexity.

18 COMPLEXITY © 2004 Wiley Periodicals, Inc. 6. EVOLUTION, BROADLY CONSIDERED FIGURE 4 The word evolution need not be the sole province of biology, its usefulness of value only to life scientists. Charles Dar- win never used it as a noun, in fact only as a verb in the very last sentence of his 1859 classic, On the Origin of Species [18]. Nor need natural selection be the only cause of evolutionary change, past and present. Darwin surely embraced it, as we do today to describe much of biological change, but here too he cau- tioned us: “I am convinced that Natural Selection has been the main but not exclusive means of modification.” Actually, the term “selection” is itself a bit of a misnomer, for no known agent in Nature deliberately selects. Selection is not an active promoter of evolution as much as a passive pruning device to weed out the unfit. As such, selected objects are simply those that remain after all the poorly adapted or less fortu- nate ones have been removed from a population of such objects. A better term might be “nonrandom elimina- tion” [19], for what we really seek to explain are the adverse circumstances responsible for the deletion of some members of a group. Accordingly, selection can be broadly taken to mean pref- ⌽ The rise in free energy rate density, m, plotted as histograms starting at those times when various erential interaction of an object with its open structures emerged in Nature, has been rapid in the last few billion years, much as expected environment, an acknowledged factor from human intuition and Figure 1(b). The solid curve approximates the increase in normalized energy in the flow of resources into and out of flows best characterizing the order, form, and structure for a wide range of systems throughout the any open system, and not just life history of the Universe. The circled insets show greater detail of further measurements or calculations of the free energy rate density for stars and planets, plants and animals, and societies and machines, forms. All systems are selected by their each of them typifying physical, biological, and cultural evolution, respectively (Adapted from Chaisson ability to utilize energy; and this ener- [1, 17].). gy—the ability to do work—is a “force,” if there is any at all, in evolution. Liber- ally interpreted, selection does occur in stars. Stellar size, color, brightness, and evolve in the strict and limited biologi- the inanimate world, often providing a composition all change, while progress- cal sense. Yet close parallels are appar- ⌽ ϳ formative step in the production of or- ing from protostars at “birth” ( m 0.5 ent, including system selection, simpli- der. A handful of cases will suffice to erg sϪ1 gϪ1), to main-sequence stars at fied adaptation, and perhaps even a illustrate the increased use of energy mature “mid-life” (ϳ2), and to red gi- kind of reproduction among the stars, density among a spectrum of systems in ants near “death” (ϳ100). Those paren- all of it reminiscent of the following successive phases of cosmic evolution. thetical values are their respective en- Malthusian-inspired scenario. First, consider stars as an example of ergy rate densities, our newly devised Galactic clouds spawn clusters of physical evolution. Growing complexity complexity metric, plotted among other stars, only a few of which (the more serves as an indicator of stellar evolu- values in the circled inset at the bottom massive ones unlike the Sun) cause (via tion as stars’ interiors naturally foster of Figure 4. At least as regards energy supernovae) other, subsequent popula- steeper thermal and elemental gradi- flow, matter circulation, and structural tions of stars to emerge in turn, with ents during repeated cycles of nuclear development while undergoing change, each generation’s offspring showing fusion; more data are needed to de- stars have much in common with life. slight variations, especially among the scribe the higher-temperature, gas-dif- None of which is to claim that stars are heavy elements contained within. ferentiated, onion-like layers as heavier alive, a common misinterpretation of Waves of “sequential star formation” elements fuse near the cores of aging such an eclectic stance. Nor do stars propagate through many such clouds

© 2004 Wiley Periodicals, Inc. COMPLEXITY 19 like slow-motion chain reactions over Onward across the bush of life (or the 21st-century society, can be likewise eons of time—shocks from the death of arrow of time)—cells, tissues, organs, or- cast in evolutionary terms—though old stars triggering the birth of new ganisms—we find much the same story here the mechanism is less Darwinian ones—neither any one type of star dis- unfolding. Cold-blooded reptiles (ϳ104) than Lamarckian, with its emphasis on ⌽ playing a dramatic increase in number have m values higher than globally av- accumulation of acquired traits. Either nor the process of regeneration ever be- eraged plants (ϳ103), warm-blooded way, energy remains the driver. Aircraft ing perfect. Those massive stars se- mammals typically more (ϳ5 ϫ 104); ex- engines, for example, display clear evo- lected by Nature to endure the fires amining animal life with finer scale, sed- lutionary trends as engineering im- ϳ ϫ 4 ⌽ needed to produce heavy elements are entary humans ( 2 10 ) have less m provement and customer selection over 4 in fact the very same stars that often than for laboring humans (ϳ6 ϫ 10 ), generations of products have made en- create new populations of stars, thereby which, in turn, have less than bicycling gines not only more powerful and effi- ϳ 5 both gradually and episodically enrich- humans ( 10 ), and so on [20]. Starting cient but also more intricate and com- ing the interstellar medium with greater with life’s precursor molecules (the realm plex, all the while utilizing enriched elemental complexity on timescales of chemical evolution) and all the way up flow of energy density, from the Wright measured in millions of millennia. As to human brains exemplifying the most ⌽ ϳ 6 engine of the early 1900s ( m 10 erg always, the necessary though perhaps complex clump of matter known (neuro- sϪ1 gϪ1), to the Boeing-747 jumbo jet of not sufficient conditions for the growth logical evolution), the same general trend the last few decades (ϳ107), to the F-117 of complexity depend on the environ- characterizes plants and animals as for stealth aircraft of the present (ϳ108). mental circumstances and on the avail- stars and planets: The greater the appar- Automobiles, from the pioneering mod- ability of energy flows in such (here, ent complexity of the system, the greater el-Ts (ϳ105) to today’s gas-guzzling, galactic) environments. On and on, the the flow of free energy density through gadget-rich SUVs (ϳ106), can be simi- cycles churn; build up, break down, that system—either to build it, or to larly analyzed [16], as can the vaunted change—a version of stellar “reproduc- maintain it, or both. silicon chips that accelerate the global tion” minus any genes, inheritance, or Finally, consider human society as an economy [1]. Remarkably, even fine- overt function, for these are clearly the example of cultural evolution. Here, the scale evolution (and complexification) value-added qualities of biological evo- cosmic-evolutionary narrative continues, of the typical American passenger car lution that go well beyond physical evo- with greater energy flows to account for can be traced over the past decades, lution. the rise of our decidedly complex, far- made clear by growing horsepower-to- Next, consider plants as an example from-equilibrium civilization—to the dis- weight ratios provided by the U.S. High- of biological evolution. Here, we can may of some anthropologists and econo- way Traffic Safety Administration: ⌽ ϭ trace the rise in complexity with evolu- mists, let alone sociologists, who often m 5.9 ϫ 105 ergs sϪ1 gϪ1 in 1978, 6.8 ϫ 105 tion among plant life (as for myriad cringe at the notion of thermodynamic in 1988, and 8.3 ϫ 105 in 1998. other life forms). And here natural se- principles being used to model their sub- Humankind is now moving toward a lection—genuine neo-Darwinism—is jects. As nonetheless noted in Figure 4 time, possibly as soon as within a few clearly at work, making use of free en- (top circled inset), we can trace several generations, when we shall no longer be ergy rate densities well in excess of progressive stages for a variety of human- able to expect Earth to provide for us those for galaxies, stars, and planets. As related cultural advances among our naturally the environmental condi- shown in Figure 4 (middle circled inset), hominid ancestors: Quantitatively, that tions—especially per capita energy energy-flow diagnostics display an in- same energy rate density increases from flow—needed for survival. Rather, soci- crease in complexity among various hunter-gatherers of a million years ago ⌽ ϳ 4 ety itself will have to increasingly engi- plants that locally and temporarily de- ( m 10 ), to agriculturists of several crease entropy: Photosynthesis requires thousand years ago (ϳ105), to the early neer the very conditions of our own more normalized energy flow for pin- industrialists of some 200 years ago ecological existence. From the two, so- ⌽ ϳ ϫ 3 Ϫ1 Ϫ1 ϳ ϫ 5 ciety and the biosphere, will likely ewood ( m 3 10 erg s g ) than ( 5 10 ). The import of rising energy for simple hay or grass (ϳ5 ϫ 102), and, expenditure per capita has reached a cur- emerge a socially controlled bioculture. in turn, still more energy for more effi- rent high for today’s well-lit (18-terra- Here the components will become cient, highly cultivated corn (ϳ6 ϫ 103), watt) world in the energy-crazed United ideas, artifacts, technology, and hu- ⌽ ϳ ϫ 6 Ϫ1 Ϫ1 mans, among all other living organisms among a host of other more organized States with m 3 10 erg s g , thus woodstuffs. System functionality and empowering our technologically “sophis- on Earth—the epitome (thus far) of genetic inheritance are factors, above ticated” society well beyond the 2800 complexity known anywhere in Nature. and beyond mere system structure, kilocalories that each of us typically con- Indeed, we are perched at the dawn of a which enhance order among animate sumes daily. whole new cosmological era—the Life systems that are clearly living compared Machines, too, and not just com- Era—wherein sentient, manipulative, to inanimate systems that are clearly puter chips, but also ordinary motors energy-efficient beings truly become not. and engines that impel our fast-paced, the agents of change.

20 COMPLEXITY © 2004 Wiley Periodicals, Inc. 7. CONCLUSION have been especially alert for any pro- ilizations. One might prefer to em- cesses—developmental or generative, brace information content to describe This article has taken the liberty of us- gradual or punctual—in the universal and unify complex systems in the nat- ing the word “evolution” in an inten- environment that have allowed for, in- ural world, but until such time that tionally provocative way, to capture on- deed potentially driven, evolution from information can be satisfactorily tological, ecological, and phylogenetic time immemorial. quantified, energy will remain a useful change on all spatial and temporal More than any other single factor, descriptor of complexity. Energy, spe- scales by means surely including, but and quantitatively so, energy flow cifically humankind’s use of it wisely not restricted to, natural selection. I would seem to be a principal means and optimally, will likely guide our have sought, within the grand context whereby all of Nature’s diverse sys- fate along the future arrow of time, for of cosmic evolution, general trends tems naturally became increasingly we, too, are part of the cosmic-evolu- among Nature’s myriad changes during complex in an expanding Universe, in- tionary scenario, an epic-class story of an extremely long line of temporality, cluding not only galaxies, stars and rich natural history for the new from big bang to humankind. And I planets, but also lives, brains and civ- millennium.

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