Three Big Bangs Page 7
The first big bang continues as an expanding universe, a 13 billion years’ explosion. The second big bang has continued expanding life on Earth, a 3 billion years’ explosion. Fast or slow is a matter of scale, both in space and on Earth, and at astronomical and atomic levels. If evolution takes centuries, the molecular biology that supports it can be fast. A single Escherichia coli cell, dividing every hour, synthesizes per second 4,000 molecules of lipid, almost 1,000 protein molecules, each containing about 300 amino acids, and 4 molecules of RNA. Some processes are autocatalytic; they feed back on themselves and escalate the speed of the process. Positive feedback can escalate and drive exponential changes. Allosteric enzymes can amplify metabolic processes a million times over the spontaneous rate without such enzymes. Other enzymes can accelerate reactions up to 1020 times the uncatalyzed reaction (Ringe and Petsko 2008). A developing fetal brain can generate over a quarter million neurons in the minute during which the reader is here pausing to consider whether the life processes are fast or slow (Cowan 1979). DNA replication, if it were expanded to the everyday world, would take place at the speed of a jet plane.
Yes, the evolution of life, like the evolution of stars, takes time, lots of it, especially to reach higher forms of life. In this evolution natural systems were often sustained in the past for long periods, even while they gradually modified. Some of the feedback loops, the feed loops, will seek equilibrium states, suppressing dynamic change. Negative feedback can stabilize processes, just as positive feedback can amplify them. Natural selection means continual changes, but it fails without order, without enough stability in ecosystems to make the mutations selected for dependably good for the time being.
There is genetic information, but such information must be appropriate to and operative in a stimulating environment, with feedback and feed-forward loops. Spring weather wakes up seeds, dry weather forces deeper roots, and both sorts of weather turn on genes to sprout, to root. Both weathers select for plants with the capacity to do this well. There is variation, more or less contingent, but without relative stability in environments, sustained patterns of evolutionary change cannot occur.
A rabbit with a lucky genetic mutation that enables it to run a little faster has no survival advantage to be selected for, unless there are foxes reliably present to remove the slower rabbits. Ecosystems have to be more or less integrated (in their food pyramids, for example), relatively stable (with more or less dependable food supplies, grass growing again each spring for the rabbits), and with persistent patterns (the hydrologic cycle watering the grass), or nothing can be an adapted fit, nor can adaptations evolve.
Evolution cannot occur in a world too chaotic to provide reliable life support, nor in one too stable to provide challenging new opportunities. The system is, from the short-term perspective, in equilibrium, even when from a longer-range perspective it is exploding. The explosion feeds off the equilibrium. Relatively simple individuals can form complex associations, ecologies, and this ecological complexity provides new niches that challenge the development of new forms of life. This is called “bootstrapping in ecosystems,” feed-forward loops that generate new niches that reinforce each other and open up new opportunities for species specialization (Perry et al. 1989). Cumulatively over the millennia, owing to the genetic capacity to acquire, store, and transmit new information, complexity can increase. There are advantages in specialized cells or organs, the efficiencies of the division of labor, and this couples more complex and more diverse forms of life.
A diverse environment is heterogeneous, and species are favored that are multiadaptable, not just well adapted to one homogeneous environment. Such adaptability requires complexity, capacities to search out better environments and migrate to them, and, once there, capacities to invade successfully, to prey on or resist predation by, or to find and share resources with, the different kinds of organisms that can live in both wet and dry, cold and hot, grassland and forested environments. Becoming more complex sometimes helps in dealing with the challenges and opportunities offered by diversity. Complexity helps in tracking changing environments.
Reptiles can survive in a broader spectrum of humidity conditions than can amphibians, mammals in a broader spectrum of temperature than can reptiles. Once there was no smelling, swimming, hiding, defending a territory, gambling, making mistakes, or outsmarting a competitor. Once there were no eggs hatching, no mothers nursing young. Once there was no instinct, no conditioned learning. Once there was no pleasure, no pain. But all these capacities got discovered by the genes. Once there was no metameric segmentation, as in worms; once there was no pentameric segmentation, as in starfish. But all these phenomena appeared, gradually, but eventually elaborating and escalating the diversity and complexity of life.
On geological scales, biologists can speak of explosions, as with the “Cambrian explosion,” when complex multicellular life vastly elaborated (Valentine 2004:chapter 14; Knoll 2003:6). Since the Cambrian, Motoo Kimura estimates that higher organisms have accumulated genetic information continually to the present at the average rate of 0.29 bit per generation (Kimura 1961). Interestingly, mass extinctions, though initially setbacks, can trigger subsequent explosions. There is typically a fast rebound, but these are not simply replacements. Respeciation takes off in new directions. Douglas H. Erwin says: “The end-Permian mass extinction triggered an explosion in marine diversity and a reorganization of marine communities…. Similar changes occurred on land” (Erwin 1993:261). In the Cenozoic era, Philip W. Signor documents for both plants and animals “a spectacular diversification over the past 100 million years” (Signor 1990:529). The challenges of survival and relentless pressures for better-adapted fit have driven escalating skills from the Cambrian to the present.
Among paleontologists, there is continuing debate about how radical or rapid such transitions were: the relationship, for example, between the novel Cambrian fauna and the Ediacaran (pre-Cambrian) animals they replaced; whether at that time the plants evolved as rapidly as the animals (Vickers-Rich and Komarower 2007). In addition to the famous Burgess Shale in Canada, two more recently discovered Cambrian sites, Chengjiang in China and the Sirius Passet formation in Greenland, show a rich and diverse fauna, supporting the claim that the Cambrian radiation was truly explosive. Always, on geological scales, fast or slow is a matter of perspective; still, two dozen paleontologists analyze what they term “major evolutionary radiations” across evolutionary history. “Short-lived, highly ‘creative’ evolutionary phases of rapid diversity increase and morphological divergence characterize the evolutionary history of many groups” (Taylor and Larwood 1990:vi). There are repeated spurts of evolutionary novelty and resulting adaptive radiation.
Paleontologists also recognize critical thresholds, revolutions in the evolutions, so to speak. John Maynard Smith, the dean of theoretical biologists, says: “Heredity is about the transmission, not of matter or energy, but of information…. The concept of information is central both to genetics and evolution theory” (Maynard Smith 1995:28). He and his colleague, Eörs Szathmáry, analyze “the major transitions in evolution” with the resulting complexity, asking “how and why this complexity has increased in the course of evolution.” “Our thesis is that the increase has depended on a small number of major transitions in the way in which genetic information is transmitted between generations” (Maynard Smith and Szathmáry 1995:3).
Maynard Smith finds that each of these innovative breakthrough levels is surprising, not scientifically predictable on the basis of the biological precedents, but with dramatic results. What makes the critical difference in evolutionary history is increase in the information possibility space, which is not something inherent in the precursor chemical and physical materials, nor in the incrementally developing evolutionary system. The vital genesis of emergent information channels punctuates evolutionary history. This happens when the genetic code is discovered, when DNA becomes concentrated in the nucleus, with the appearance of meiotic sex,
transforming capacities for information transfer. Another threshold is crossed with multicellular life, with specialized capacities for organ hierarchies (livers, kidneys, legs), elaborating large networks for distributed discrete functions. Yet another transformation comes with the generation of neurons and brains, with their capacities for acquired learning, for language. Each transformation breaks former constraints and escalates evolutionary possibilities.
Genes speciate new kinds in response to environmental challenge, as much as they reproduce existing kinds to the maximum possible extent. Maynard Smith pinpoints the limitless potential novelty in genetics: “There are today, in the living world, only two systems capable of unlimited heredity, that is, of transmitting an indefinitely large number of different messages: these are the genetic system based on nucleic acids, and human language” (Maynard Smith 2000:215). That couples, interestingly, what we are here calling the second and the third big bangs. “Descent with modification” results in ever-ongoing “ascent with modification.” As Francisco Ayala puts it, there is “increase in the ability to gather and process information about the environment” (Ayala 1974:344). Genes are set to generate biodiversity without end.
A genetic sequence has a potential for being an ancestor in an indefinitely long line of descendant genotype/phenotype reincarnations. Genes act directed toward a future, under construction, producing a functional unit that does not yet exist. In physics and chemistry as such, there can be only sources, never resources. In biology, the novel resourcefulness lies in the epistemic content conserved, developed, and thrown forward to make biological resources out of the physicochemical sources. A crucial line, a singularity, is crossed when abiotic formations get transformed into loci of information. The factors come to include actors that exploit their environment.
A genome conserves a form of life, but a genome is equally a search program. Genes are as dynamic as they are stable units. Mutation, crossing over, drift, allelic variation, cutting and splicing, insertions, deletions—all this disrupts conserving the inherited genes; but such processes also make genes information generators. There is “facilitated variation” (Kirschner and Gerhart 2005). Genetic sets may be tested for their “evolvability,” their flexibility in encounters with shifting environments (Kirschner and Gerhart 1998). Genes generate trial-and-error solutions, some of which yield novel information discovered. Genes interweave possibilities and explore new possibility space. A result of this “highly sophisticated information processing” (recalling Shapiro 2002:10; 2005) is open-ended, explosive radiation of biodiversity. Genes in living systems explore a combinatorially immense space of possibilities through the evolutionary process.
The construction of complexity begins with the biologically simple, one-celled organisms; the story is from the bottom up. After certain thresholds are reached, parts can be differentiated. There is division of labor, a principal result of multicellularity. Now evolutionary development compounds the growth of biodiversity and biocomplexity. Often these relations are nonlinear; causes and effects are not proportionally related. Organic systems build by reiteration and modification of modules (mutations enabling survival in colder weather), though there may also be novel sorts of modules (enabling in mammals nursing mothers, unknown in reptiles).
These biological achievements are quite impressive, wonderful. But now the generative context of order and contingency no longer seems fine-tuned. Rather it is often groping, exploratory, with false starts and setbacks, ragged and wasteful. Indeed, at the same time that astrophysics and nuclear physics have been describing a universe “fine-tuned” for life, evolutionary and molecular biology seem to be discovering that the history of life is a random walk with much struggle and chance, driven by selfish genes. The process is prolific, but no longer fine-tuned. To the contrary, evolutionary history can seem tinkering and makeshift at the same time that, within structural constraints and mutations available, it optimizes adapted fit. Mutations are random, unrelated to the needs of the organism; most are worthless or harmful. Mutations may have causes (a cosmic ray, a chemical mutagen absorbed), but these involve the intersection of unrelated causal lines, statistical probabilities, or even (with a mutation caused by radioactive decay) quantum indeterminism—the latter a sort of causal gap.
Even if, more comprehensively seen, the genetic searching is highly sophisticated information processing, it involves trial and error, in which mutations play a part. When immense possibility spaces open up in the combinatorial explosion, fine-tuning no longer seems the relevant, the desirable, or even the possible route of creativity. The heavens may be clockwork, but the evolutionary epic is adventure. Creativity is more open-ended, more opportunistic in an ambiguous and challenging world. On Earth there is wild nature, wilderness, and there is something almost logically contradictory about fine-tuning the wild. Fine-tuning may be required to produce the elements of life, but the struggle for adapted fit is at another level of creativity. Recalling Andrew Knoll, “life’s long history abounds in… narrative verve” (Knoll 2003:xi). That struggle, ongoing in wild nature, can produce some quite well-adapted fits, impressively competent at both phenotypic and genotypic levels. If you wish to phrase it this way, the evolutionary epic has been a struggle for ever better “tuning” of life in its environments. The results can be “fine-tuned,” even if the process cannot.
Biodiversity and Biocomplexity
There are two dimensions to the explosions involved in the second big bang: biodiversity and biocomplexity. Life started in the seas, so we should first look at the marine record. J. W. Valentine, after a long survey of evolutionary history, concludes for marine environments that both complexity and diversity increase through time. First, with regard to diversity:
A major Phanerozoic trend among the invertebrate biota of the world’s shelf and epicontinental seas has been towards more and more numerous units at all levels of the ecological hierarchy. This has been achieved partly by the progressive partitioning of ecospace into smaller functional regions, and partly by the invasion of previously unoccupied biospace. At the same time, the expansion and contraction of available environments has controlled strong but secondary trends of diversity…. The biosphere has become a splitter’s paradise. (Valentine 1969:706)
There are ups and downs in numbers of families and species, due to the contingencies of drift; nevertheless, biologically, the trend is upward.
Complexity also increases:
A sort of moving picture of the biological world with its selective processes that favor increasing fitness and that lead to “biological improvement” is projected upon an environmental background that itself fluctuates…. The resulting ecological images expand and contract, but, when measured at some standardized configuration, have a gradually rising average complexity and exhibit a gradually expanding ecospace. (Valentine 1973:471)
Environments fluctuate, as Valentine recognizes. We worried with the primordial explosion of matter-energy that explosions can be messy, and found that there is a resulting mixture of both order and disorder. The explosion of life on Earth is cybernetic, but now there are crooked lines, ups and downs, twists and turns, as one might expect when billions of species evolve over billions of years. The history of life is something of a thicket. Just how explosive which parts of marine invertebrate evolution were remains under analysis, with some recent studies agreeing that the rise was sometimes steep but also less steep than previously thought in more recent times (Alroy et al. 2008). Others still find “dramatic evolutionary radiations” (Stanley 2007:11). Much of the debate turns on the extent of biases in the fossil record, sampling errors, regional collecting, local versus global trends, ice ages.
A common interpretation of the periods of stasis in the seas is that Earth’s tectonic plates were configured to fuse the landmasses, resulting in a saturation of kinds of species that had at that point evolved on the continental shelves. Since marine life is primarily on the continental shelves, it may be especially susceptible to c
ontingencies in continental drift. Meanwhile, there seems little doubt that in the seas there are both fluctuations and gradually rising diversity, which can sometimes be exponential.
M. J. Benton summarizes the global picture in three graphs (figs. 2.1–3, Benton 1995), which convey several impressions. One is that the number of families starts low and ends high, alike for all organisms on land and in sea. Another is that life’s expansion speeds up in later geological time, although the diversifying processes of early life may have fossilized poorly or be lost in the ancient fossil record. In marine life there is a flat part of the curve from Ordovician to Permian times. But one has also to remember that in that period life moved from sea to land, which demanded considerable novel complexity in speciation, since the terrestrial environment is more demanding. By species count, most of the world’s plants and animals live on land, despite the much larger sea environment. Life in the sea does retain more basic animal forms (thirty-five phyla) than life on land (ten phyla), but the overall species count on land is some ten times higher than in the sea.
Figure 2.1 Diversification of life through time for all organisms (Benton 1995)
Figure 2.2 Diversification of life through time for continental organisms (Benton 1995)
Figure 2.3 Diversification of life through time for marine organisms (Benton 1995)