Two pairs of sandals made from recycled automobile tires. I bought the large pair at a street market in Delhi, India, and the smaller pair at the recycling market of Nairobi, Kenya.
Tire strips cut for sandals at the recycling market of Nairobi.
This little prologue may strike you as interesting enough, but dubiously related to the intended subject of this essay—evolution and the history of life. Yet the theme of recycling for purposes almost comically different from original intent not only occupies a central place in the history of life but also completes an argument that I left half-finished in the previous essay. The last piece examined Darwin’s justification for predictable progress in the history of life and argued that recent discoveries about the frequency, rapidity, and extent of mass extinction suggest a much more quirky and uncertain path. I pointed out that Darwin did not locate the source of progress in the basic mechanics of natural selection itself—for he recognized natural selection as a theory of local adaptation only, not a statement about general advance. He justified progress with another argument about nature embodied in his favorite metaphor of the wedge. Nature is chock-full of species (like a surface covered with wedges) all struggling for a bit of limited space. New species usually win an address by driving out others in overt competition (a process that Darwin often described in his notebooks as “wedging”). This constant battle and conquest provides a rationale for progress, since victors, on average, may secure their success by general superiority in design.
Sandal salesmen at a street market in Delhi.
I argued that mass extinction prevents wedging from establishing long-term patterns in the history of life. Progress by competition may occur in normal times, but episodes of mass extinction undo, disrupt, and redirect this process so frequently that wedging cannot put a dominant stamp upon the overall course of life. I do not believe that mass extinctions work with absolute randomness, treating each species as a coin to be flipped or a die to be rolled. Survivors probably prevail for reasons, but here’s the rub (and the role for the wheel of fortune): The rules for survival change in these extraordinary episodes, and features that help species to prevail through catastrophes need not be the sources of success in normal times. Getting through a mass extinction may require a stroke of fortune in the following special sense: A feature evolved for one function in the wedging of normal times must, by good luck and for different reasons, provide a crucial benefit under the different rules of episodic catastrophe. (The diatoms of Essay 21, the only group of planktonic microorganisms to sail through the Cretaceous debacle, had evolved a life cycle with a dormant stage in order to weather predictable seasonal fluctuations of light and nutrients in normal times. If an impacting comet entrained a dust cloud that darkened the earth for several months, the survival of diatoms might be linked to a capacity for dormancy evolved for quite different reasons. Mammals may have prevailed, in large part, by virtue of small body size. But we can hardly label limited size as a preparation for long-term success or even as an active adaptation at all. Mammals may have remained small for primarily negative reasons—because dinosaurs dominated the ecospace of large creatures, and mammals could not displace them by wedging in normal times.) Thus, mass extinction sets a quirky and interesting course for life by opening opportunities to new groups and by basing success upon fortuitous side consequences of features evolved for other reasons. Who cares how well a tire works when the rules change and we run out of gas; continued existence now depends upon a fortuitous capacity for conversion to some other, utterly unanticipated role—retreading for sandals, for example.
But this argument—as far as I went in Essay 21—grants the wheel of fortune only half a loaf. For any champion of the wedge will swiftly reply: Mass extinction is a negative force. It makes nothing and can only pick and choose among creatures fashioned by natural selection. Sure, mass extinction can disrupt a trend, wipe out a complex group, or send life down an unpredicted channel—but evolution is about making, not about differential removal. The creative force in evolution, the motor of construction, must still reside in the processes of normal times, building creatures that will one day pass for review before the sieve of mass extinction. And the controlling process of normal times is wedging by competition.
I could rebut this argument in several ways. I might argue, for example, that sorting by differential death in mass extinction mimics, on a grander scale, the ordinary mode of operation for natural selection in normal times. Natural selection is a sieve, not a sculptor. Many are called; few are chosen. Natural selection is powered by differential birth and death; the few survivors accumulate and intensify their favorable features bit by bit on the relentless sieve of each generation. What is mass extinction but a grander sieve, sorting out species rather than organisms. If the sorting of organisms in normal times yields change that we label as positive, why not grant the same status to differential death in episodic catastrophes? Mass extinction need not be viewed as a negative force opposed to progress in normal times.
But I would choose an opposite tack to provide the other half-loaf to the wheel of fortune. Rather than promoting the wheel of mass extinction by its formal similarity to processes at smaller scale, I would proceed in the reverse direction. I will argue, in short, that the fundamental principle of quirky and unpredictable success by fortuitous side consequences pervades all scales. This vital principle of the wheel does not “click in” only at global catastrophe, leaving most of time to the wedge of ordered progress. The tires-to-sandals principle works at all scales and times, interacting with the wedge to permit odd and unpredictable initiatives at any moment—to make nature as inventive as the cleverest person who ever pondered the potential of a junkyard in Nairobi.
I will go further and make a statement that may seem paradoxical. The wedge of competition has been, ever since Darwin, the canonical argument for progress in normal times. I claim that the wheel of quirky and unpredictable functional shift (the tires-to-sandals principle) is the major source of what we call progress at all scales. Advance in complexity, improvement in design, may be mediated by the wedge up to a certain and limited point, but long-term success requires feinting and lateral motion, with each zig permitting another increment of advance, and progress crucially dependent upon the availability of new channels. Evolution is an obstacle course not a freeway; the correct analogue for long-term success is a distant punt receiver evading legions of would-be tacklers in an oddly zigzagged path toward a goal, not a horse thundering down the flat.
Let us take the broadest possible look at just three prerequisites for continuity in the sequence that, in our parochialism, we view as the crucial and representative case of evolutionary advance—the origin of human consciousness. At each true turning point, each leap in the capacity for substantial advance, we meet the wheel spinning its way toward a new use for old features (sandals from tires); the wedge can only promote, for a limited time and extent, what the wheel makes possible.
1. Origin of the genetic flexibility for major advance in complexity. For evolutionists, perhaps the most intriguing and unexpected discovery of molecular genetics emerged during the 1960s when study after study proved that, in multicellular organisms, only a small percentage of the total genetic material consists of functional genes in single copies. Most of the genetic material may be “junk” with respect to information needed to build and maintain a working body. Moreover, many genes exist in multiple copies for obscure reasons unrelated to the necessary functions of bodies.
Nonetheless, it soon became clear to evolutionists that redundancy of multiple copies might be the crucial prerequisite for evolution of complexity. Suppose that the original unicellular ancestor of all complex creatures had only one copy of each gene and that each gene coded for a vital function. (This claim is not mere conjecture, but represents the actual state of the simplest creatures alive today and the best models for ultimate multicellular ancestors.) These creatures work very well; they may be honed to an optimum by n
atural selection, shorn of all unneeded fat and flab. But now, a conundrum for evolution: These creatures may excite our admiration for efficiency, but how can they change in the crucial sense of adding new capacities in complexity? Each gene codes for something vital; it can only alter by improvement in its own channel. This kind of genetic system offers no flexibility, no play, no capacity for adding something truly new.
This conundrum led evolutionists to grasp the fundamental role of multiple copies in permitting the evolution of complexity. If genes exist in several copies, but only one supplies the body’s functional need, then the other copies are free to experiment, vary, and add capacity through occasional good fortune.
Well and good so far, but we now face a logical puzzle: Unless we thoroughly misunderstand the fundamental nature of causality, multiple copies cannot arise “for” their potential use in permitting complexity millions of years in the future. Multiple copies are the key to complexity, but they must have evolved for other reasons—the tires-to-sandals principle of quirky functional shift.
This case is particularly interesting because the initial reason for duplication (harvesting of rubber for tires) may not, itself, have much to do with natural selection as traditionally conceived in terms of organisms struggling for reproductive success. Duplication may arise by selection at the lower level of genes, a process invisible in the larger world of the wedge. Genes also play the game of natural selection in their own realm, and those that develop the capacity to duplicate and move (transposons, or jumping genes) secure advantages at this lower level, just as organisms win in Darwin’s world by leaving more surviving offspring. In fact, gene duplication may be abetted by producing no effect upon bodies, for invisibility at Darwin’s level of the wedge provides hiding space and assures that no negative pressures of natural selection shall impede the accumulation of “unneeded” extra copies. Yet these “redundant” duplicate genes may house the latent source of later complexity.
2. The evolution of complex cells. Many biologists would place nature’s fundamental distinction not between plants and animals, or even between unicellular and multicellular organisms, but at a division within unicellular creatures. The structurally simple prokaryotes, bacteria and cyanophytes, have no organelles within their cells—no nucleus or chromosomes, no mitochondria. The complex eukaryotes have evolved the array of internal structures that grace (or disfigure according to your view or status) nearly every high school biology final with its inevitable question: Label all the parts of the cell and state their functions.
This increment of complexity from prokaryote to eukaryote is deemed fundamental, in part because we view eukaryote organization as an absolute prerequisite to the later evolution of multicellular organisms. (To cite just one standard argument: Darwinian evolution of complexity requires copious variation to fuel natural selection; most variation arises from the mixture, via sexual reproduction, of two differing genetic systems in each offspring; sexual reproduction requires a mechanism for exact division of genetic material so that 50 percent of each parent reconstitutes the needed 100 percent in offspring; meiosis by reduction division of paired chromosomes is the biological invention that secured equal separation; prokaryotes, lacking chromosomes and other organelles, cannot produce an exact genetic halving.)
We now encounter the same conundrum faced in the last example: We can see why multicellular life required the evolution of organelles, but eukaryotic cells arose at least 800 million years before the origin of multicellular animals—so progress to multicellular complexity cannot be the reason why organelles evolved.
A favored theory for the origin of some organelles (the mitochondrion and chloroplast but not, alas, the nucleus, for which no good theory now exists) invokes the process of symbiosis. Mitochondria and chloroplasts look uncannily like entire prokaryotic organisms (they have their own DNA and are the same size as many bacteria). Almost surely, they began as symbionts within cells of other species and later became more highly integrated to form the eukaryotic cell (so that each cell in our body has the evolutionary status of a former colony). Now, one can argue that the wedge drove the ancestors of mitochondria to a life of symbiosis. These bacteria, gaining protection or whatever, did not enter the primordial eukaryote in order to provide an opportunity for multicellular complexity a billion years down the road. Symbiosis occurred for immediate Darwinian reasons; then the wheel turned and the rubber made for symbiosis put the first footprints on the path of multicellular complexity.
3. The basic features of human consciousness. The wheel and the wedge then interact for more than half a billion years to the separation of our lineage from the ancestry of chimpanzees some six to eight million years ago. The wedge produces some forward motion (and more blind alleys of overspecialization), but the wheel inaugurates each domain of change—the limbs on an odd group of fishes, by an unusual arrangement of fin bones, can bear the body’s weight on land (see Essay 4); mammals get a chance after 100 million years in the backwaters because dinosaurs succumb in a mass extinction.
Australopithecus now begins the process that textbooks used to call, before we reformed our language to include all people, the “ascent of man.” Doesn’t the wedge finally prevail? Isn’t the unreversed trend of increasing brain size, from Australopithecus to Homo habilis to Homo erectus to us, driven by ordinary natural selection working on the advantages of superior cognition? Let me take the most conservative argument of the wedge (not my actual view) and reply: Yes, fine; I agree. The human brain got large because natural selection directly favored some traits of cognition that gave bigger-brained people advantages in competition.
Does such an admission imply that the foundations of human cognition, the universal traits that we define as “humanity” or “human nature,” were built directly by the wedge? Of course not—and no argument is more important for our understanding of human nature, yet less widely appreciated, than this. Yes, the brain got big by natural selection. But as a result of larger size, and the neural density and connectivity thus imparted, human brains could perform an immense range of functions quite unrelated to the original reasons for increase in bulk. The brain did not get big so that we could read or write or do arithmetic or chart the seasons—yet human culture, as we know it, depends upon skills of this kind. If you label me as a hopelessly parochial academic for citing only the skills of an intellectual elite, I reply that the fortuitous side consequences of large brains include the defining activities of all people. What about language, the most widely cited common denominator and distinguishing factor of humanity? And I don’t mean using sound or gesture for communication, as many complex animals do. I refer to the unique syntax and underlying universal grammar of all languages. I can’t prove that language was not the selected basis of increasing brain size, but the universals of language are so different from anything else in nature, and so quirky in their structure, that origin as a side consequence of the brain’s enhanced capacity, rather than as simple advance in continuity from ancestral grunts and gestures, seems indicated. (I lay no claim to originality for this argument about language. The reasoning follows directly as an evolutionary reading for Noam Chomsky’s theory of universal grammar.)
To cite another example, consider Freud’s argument on the origin of religion—or at least of a belief in some form of persistence after death as a common feature of this institution. Freud held that all religions maintain some belief in personal persistence after death—whether in heaven, by reincarnation, in a universal soul, or merely by continuity of tradition. This belief marks the common basis of religion because our large brains “forced” us to learn and acknowledge the fact of personal mortality (a concept not clearly grasped by any other animal). Now you cannot argue that our brains became large so that we would appreciate the fact of our death; knowledge of mortality is an inevitable (and largely unfortunate) side consequence of mental power evolved for other reasons. Yet this unwanted knowledge forms the basis of an institution often regarded as the
most fundamental consequence of human nature.
If such features as language and the basis of religion are side consequences of the wheel, not direct gifts of the wedge, then is human nature a predictable product of organic improvement, honed in the fires of competition, or a set of oddly cobbled side consequences rooted in an unparalleled neural complexity built for other reasons? We are a bit of both—though more, I suspect, quirks of the wheel than boons of the wedge—and in this mixture lies our hope and our destiny.
If I have upset your equanimity by attributing the genuine complexity of human cognition to fortuity piled upon fortuity (with a little yardage for predictability after each spin of the wheel), then I must apologize for one further disturbance in conclusion. We talk about the “march from monad to man” (old-style language again) as though evolution followed continuous pathways of progress along unbroken lineages. Nothing could be further from reality. I do not deny that, through time, the most “advanced” organism has tended to increase in complexity. But the sequence from protozoan to jellyfish to trilobite to nautiloid to armored fish to dinosaur to monkey to human is no lineage at all, but a chronological set of termini on unrelated evolutionary trunks. Moreover, life shows no trend to complexity in the usual sense—only an asymmetrical expansion of diversity around a starting point constrained to be simple. Let me explain that last cryptic remark: For reasons of organic chemistry and the physics of self-organizing systems, life arose at or very near the lower limit of preservable size and complexity in the fossil record. Since diversity, measured as number of species, has increased through time, extreme values in the distribution of complexity can move in only one direction. No species can become simpler than the starting point, for life arose at the lower limit of preservable complexity. The only open direction is up, but very few species take this route. Increasing complexity is not a purposeful trend of an unbroken lineage but only the upper limit of an expanding distribution as overall diversity increases. We focus on this upper tail and call its expansion a trend because we crave some evolutionary rationale for our perception of ourselves as a predictable culmination.