The Structure of Evolutionary Theory -Stephen Jay Gould’s Critique of Progress

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The Structure of Evolutionary Theory

Stephen Jay Gould’s Critique of Progress

Richard York and Brett Clark

A question of central importance in the interpretation of patterns of evolution is whether history had to turn out the way it did. From before Charles Darwin’s time up to the present it has been commonly assumed that history, both human history and the history of life in general, unfolded in a somewhat deterministic manner, that the present was inevitable, either ordained in Heaven or, in the scientific view, mechanically produced by deterministic natural laws. This view contrasts with that of the historian: that the quirks, chance events, and particularities of each moment make history, and that the world could have been other than it is.

The renowned paleontologist and evolutionary theorist Stephen Jay Gould struggled throughout his career to come to terms with the nature of history and to understand the interplay of general laws and historical particulars, the respective importance of necessity and contingency.1 He developed a sophisticated and nuanced position that recognized both the importance of general laws and the role of contingency, arguing that, although natural laws limit the pathways that can be taken, the particular pathway—one of the many available—that is actually taken depends on numerous contingent events. Thus the world could not have been just any way, but many worlds are possible, of which we live in just one.

The consideration of history opens up important philosophical as well as political questions, as Gould clearly recognized. It raises questions that have been central to modernist thinking: Is history directional? Is there a continual march to a future that will be better than the past? In short, is there progress in human and natural history? The answers to these questions hold implications not only for understanding the nature of evolution but for our political world as well. If contingency played little part in how history turned out, if the present was inevitable, then it makes little sense to challenge the status quo. However, if contingency dominates history, the future is open, and the world can be another way, as radicals of all varieties have long believed. Of course, as Gould well recognized, our personal or political preferences should not be imposed on nature—the laws of nature and the patterns of evolution are independent of what we make of them. However, our biases influence how we interpret what we observe in nature. Therefore, it is important to be aware of our biases, both our personal ones and the ones preval‎ent in society at large.

Questions about the nature of history go to the heart of assumptions buried in Western culture, and Gould was a major critic of the biases that assume a progressive nature to history and the inevitability of the present. These biases can be seen in the common view in evolutionary theory that more recently emerging species are superior to their predecessors since surviving species have won out in the struggle for existence. Given human arrogance and the preval‎ence of progressivist ideology, it is commonly presumed that the emergence of Homo sapiens is the inevitable apex of evolutionary processes. Counter to this view, Gould argued that, although natural selection led to some degree of “progress” on short timescales in the limited sense that it dialectically adapted creatures to their environments, over longer scales of time there was no deterministic direction to the history of life. The fundamental importance of contingency in history was perhaps the most centrally important feature of Gould’s thinking.

Gould was clearly interested in the invariant natural laws, such as those in physics and chemistry, which underlie biological phenomena and constrain all relationships within the world, including human society.2 But he argued that much of the order observed within the biological world is due to historically emergent structures, such as the evolved developmental pathways of organisms embedded in the genetic code. As a result, biology (as well as other historical sciences, such as geology) attempts not only to understand the general forces that shape natural phenomena, but also to explain how and why history developed as it did. (The social sciences share a similar orientation in their own specified context, seeking to comprehend forces that influence social phenomena.) Gould saw the importance of assessing the available pathways to a specific end in order to develop a proper explanation. Here, the particularities of biological systems and their history need to be understood in their own terms.

The Wedge of Progress and the Paradox of the First Tier

As a consummate student of Darwin, Gould often returned to Darwin’s writings to work out difficult issues in evolutionary theory. In trying to understand the patterns of evolutionary history and the question of whether there was a progressive trend in that history, Gould reflected on Darwin’s thinking on the issue:

No question troubled [Darwin] more than the common assumption, so crucial to Victorian Britain at the height of industrial and imperial success, that progress must mark the pathways of evolutionary change. Darwin clearly understood that the basic mechanics of natural selection implied no statement about progress, for the theory only speaks of local adaptation to changing environments….To resolve this troubling discordance between the mechanics of his basic theory and his fundamental impression of pattern in life’s history, Darwin called up [an] ecological principle encompassed by the metaphor of the wedge.3

Explaining this wedge metaphor, Gould highlighted a passage from Darwin’s unpublished long manuscript, the shortened version of which became the Origin of Species, that gives a clear presentation of the worldview that Darwin had developed regarding the struggle for existence:

Nature may be compared to a surface covered with ten thousand sharp wedges, many of the same shape, and many of different shapes representing different species, all packed closely together and all driven in by incessant blows: the blows being far severer at one time than at another; sometimes a wedge of one form and sometimes another being struck; the one driven deeply in forcing out others; with the jar and shock often transmitted very far to other wedges in many lines of direction.4

Gould notes, “The Origin of Species contains several passages about progress in the history of life, and all are validated, not by the bare bones mechanism of natural selection, but by the second principle of the wedge, the vision of a full world ruled by overt competition among organisms.”5 He goes on to quote a passage from the Origin of Species in which Darwin demonstrates his commitment to the idea of progress (although Darwin struggled with this view, also arguing against inappropriate progressivism):

The more recent forms must, on my theory, be higher than the more ancient; for each new species is formed by having had some advantage in the struggle for life over other and preceding forms….I do not doubt that this process of improvement has affected in a marked and sensible manner the organization of the more recent and victorious forms of life, in comparison with the ancient and beaten forms.6

In considering the extent to which a progressive trend could emerge in the history of life, Gould recognized that different processes occur at different temporal scales. He argued that it is useful to think of evolutionary processes unfolding across three tiers of time. The first tier is “normal” ecological time: the day-to-day struggle of organisms to survive and reproduce. This is the tier of time in which natural selection in the traditional Darwinian sense operates, and this is the tier of time that has been the focus of the modern synthesis in biology—the neo-Darwinian theory based on the merger of Darwinian natural selection and Mendelian genetics that began to take shape in the early 1930s and solidified by the 1950s. Supporters of the modern synthesis have typically assumed that all long-term changes in organic evolution can be extrapolated from processes happening in this first tier.

Gould, however, argues that processes also occur on two other tiers of time, which disrupt any trajectory that may emerge from processes occurring on the first tier. The second tier is “normal” geological time between episodes of mass extinction. This is where Niles Eldredge and Gould’s theory of punctuated equilibrium addresses the deployment of species and origin of trends over long stretches of time—more specifically, the history of most species is best characterized as long periods of relative stasis (the equilibrium), where there is only minor, non-directional change in organismal structure, punctuated by brief periods of rapid change where new species emerge from old in a geological “moment,” which may last thousands or tens of thousands of years. Thus, the second tier is dominated by processes independent of the first tier, such as those stemming from the characteristics of lineages that lead to different rates of speciation.

The third tier of time is dominated by mass extinction events that are due to processes not reducible to the first or second tiers, such as the asteroid impact that led to the Cretaceous extinction. Gould notes that these extinction events appear to operate with their own rules, and are thus independent of processes occurring in the other two tiers. At the same time, events, such as those causing mass extinctions, influence evolutionary processes and history. He conceives of mass extinctions as “a separate process, and a cardinal shaping force for patterns of life’s history” that disrupts the “accumulated achievements at lower tiers.”7 Emphasizing this decisive power, as well as its potential, he explains, “Mass extinctions are not unswervingly destructive in the history of life. They are a source of creation as well, especially if the…view of external triggering is correct….Mass extinction may be the primary and indispensable seed of major changes and shifts in life’s history. Destruction and creation are locked in a dialectic of interaction.”8

Gould succinctly summarizes these three tiers:

The first tier includes evolutionary events of the ecological moment. The second encompasses the evolutionary trends within lineages and clades that occur during millions of years in “normal” geological time between events of mass extinction….[Mass extinction represents] a third distinct tier with rules and principles of its own.9

Gould indicates that the logic of Darwin’s argument, particularly with regard to the power of natural selection and the wedge metaphor, did suggest that there could be some form of “progress” in ecological time, but only in the limited sense of species becoming more adapted to their local environments, which, over the course of time, are subject to change. However, despite long-standing assumptions to the contrary, no clear directional trajectory can be found in the history of life indicating some form of overall progress. Gould refers to this failure to find a progressive signal in life’s history as the paradox of the first tier:

The dilemma of the modern synthesis for paleobiology lay in its claim that all theory could be extrapolated from the first tier, thus converting macroevolution from a source of theory to a simple phenomenology….But if the tiers of life create pattern by emergent rules not predictable from processes and activities at lower tiers, then paleobiology adds its special insights without contradicting principles for lower tiers.10

Gould concludes, “A theory of mass extinction would largely resolve the paradox of the first tier. If anything like progress accumulates during normal times (and punctuated equilibrium casts doubt even upon this proposition), the vector of advance may be derailed often and profoundly enough to undo any long term directionality.”11

The belief in progress is at the heart of the modern Western worldview, so it is not surprising that it finds its way into theories of natural history. Darwin developed his theory during what was perhaps the height of progressivist thinking, the Victorian era, and, ever since, his theory has been widely interpreted as suggesting that organisms improve over the course of evolutionary history.12 Gould worked to counter the long-established view, dating back to before Darwin, that evolution was directional and progressive. As discussed above, Darwin himself, although much more sophisticated and nuanced in his thinking than many of his subsequent acolytes, argued that there should be some degree of progress in evolution, at least with regard to adaptation to the local environment. But he was skeptical of such claims in regard to historical progress, as indicated in his correspondence with U.S. paleontologist Alpheus Hyatt, who contended that there was necessary progress. Darwin, in some degree of contradiction with the passage quoted above from the Origin, in a letter dated December 4, 1872, wrote, “After long reflection I cannot avoid the conclusion that no inherent tendency to progressive development exists.”13 Nonetheless, the progressive view became central to the modern synthesis. Gould points out:

Progress is not merely a deep cultural bias of Western thought…it is also…the explicit expectation of all deterministic theories of evolutionary mechanism that have ever achieved any popularity, from Darwinian selection to Lamarckism to orthogenesis. I do not, of course, mean progress as an unreversed, unilinear march up the chain of being; Darwin did away with this silly notion forever. But even Darwinism anticipates that an imperfect, irregular, but general ascent should emerge from all the backing and forthing inherent in a theory based on a principle of local adaption to changing circumstances.14

The popular view of “survival of the fittest” indicates that the unfit are the ones weeded out over time. Is not extinction, after all, a mark of failure?15 Are not dinosaurs, for example, of an intrinsically lower grade than mammals, as evidenced by the persistence of the latter to the present?

Running counter to much prevailing thought, one of Gould’s central themes is that of historical contingency—events (such as those characterized by the third tier of time), often occurring effectively by chance and that are not predictable beforehand (although they may be rendered sensible in hindsight), may change the course of history, foreclosing some options and opening others. The trilobites—marine arthropods that flourished before their disappearance in the greatest of all mass extinctions, which ended the Permian period approximately 250 million years ago—surely did not vanish due to inherent inferiority. After all, they had thrived for 300 million years, longer than mammals have been around, and over one thousand times longer than Homo sapiens has trod upon the earth. But their existence blinked out likely due to bad luck in an unpredictable, and still unexplained, global shake-up that took with it over 90 percent of all species then extant.16 Likewise, had an asteroid not collided with the earth 65 million years ago, at the close of the Cretaceous period, dinosaurs almost surely would have persisted in their dominance over the inconsequential rat-like ancestors of mammals, and our lineage would have taken a different path. There is every reason to suspect that if this contingent event had not occurred, dinosaurs would have survived to the present, and humans would never have evolved. Dinosaurs and mammals, after all, appear in the fossil record at approximately the same time, coexisting for over 100 million years, with dinosaurs arguably being more successful than mammals. Gould stresses, “Dinosaur should be a term of praise, not of opprobrium. They reigned for 100 million years and died through no fault of their own; Homo sapiens is nowhere near a million years old and has limited prospects, entirely self-imposed, for extended geological longevity.”17

Gould pointed to Mark Twain’s sarcastic derision of the notion of progress and purpose in evolution as an insightful critique.18 Drawing upon the science of his time—such as Lord Kelvin’s estimate of Earth’s age—Twain pointed out that the world had existed for hundreds of millions of years (the best estimate today is around 4.5 billion years), but Homo sapiens had been present for well less than 1 percent of this time. Nonetheless, it was commonly assumed that all of history was directed toward preparing the world for the eventual rise of human beings. Twain mockingly explained, following this logic of argument, that if the present was simply meant to be, many important steps were necessary to set the stage for current needs. For instance, “Man would have to have the oyster,” so “preparation was made for the oyster.” But such a process is not easy, as all of the oyster’s ancestors would have to be made first, so a variety of invertebrates had to precede the oyster. Twain explained that many of the invertebrates

will die and become extinct, in the course of the nineteen million years covered by the experiment, but all is not lost, for the Amalekites will fetch the homestake; they will develop gradually into encrinites, and stalactites, and blatherskites, and one thing and another as the mighty ages creep on and the Archaean and the Cambrian Periods pile their lofty crags in the primordial seas, and at least the first grand stage in the preparation of the world for man stands completed, the oyster is done.

To drive the point home, he added:

An oyster has hardly any more reasoning power than a scientist has; and so it is reasonably certain that this one jumped to the conclusion that the nineteen million years was a preparation for him; but that would be just like an oyster, which is the most conceited animal there is, except man. And anyway, this one could not know, at that early date, that he was only an incident in a scheme, and that there was some more to the scheme, yet.19

After dealing with oysters, Twain, with wry wit, detailed how the preparation of the earth for humans required fish, and of course such a cuisine needed coal to fry it. So the history of the earth also involves millions of years where forests grew only to be buried within the earth to create fossil fuel, awaiting the eventual evolution of Homo sapiens. To represent symbolically the human haughtiness that conceived of the history of the earth in this light, Twain wrote: “If the Eiffel Tower [then the tallest human-built structure in the world] were now representing the world’s age, the skin of paint on the pinnacle-knob at its summit would represent man’s share of that age; and anybody would perceive that the skin was what the tower was built for.”20 This quotation summarizes Gould’s view of history and human arrogance well. It is no more reasonable to assume that humans were the necessary outcome of evolution than it is to assume that the Eiffel Tower was built to put paint on top of it.

Due to the dominance of contingency in natural and social history, the world of the present is only one of the many worlds that are possible; it does not represent a foreordained order. Or to use Gould’s powerful metaphor, if we were to “replay the tape of life,” a different history would unfold, almost surely without the appearance of humans or any creature especially similar to us; a history that would appear just as sensible and even as “inevitable” as the history that actually occurred.21 At each moment in history, we stand at a gate of worlds, and although it is not predictable where any path will ultimately lead, our actions nonetheless influence the path along which we travel, and the organization of the larger world will influence what constraints and possibilities we will confront as a new present emerges and eventually becomes history.

The Question of Convergence

One of the important issues in debates that Gould had with other scholars centered on the extent to which the evolutionary principle of convergence made the outcome of history inevitable in its broad features, if not in its particulars. Convergence refers to the fairly common phenomenon where two species from lineages that are not closely related will come to share common features, such as the morphological similarities between marine mammals, such as dolphins, and fishes, since these species face common selection pressures due to shared environmental constraints. Gould, like other evolutionary biologists, fully recognized the ubiquity and importance of convergence and its illustration of how natural selection shapes organisms to fit their environments. However, Gould also was leery of hyper-adaptationist assumptions that attributed all similarities across species to selection, noting that many examples of convergence may reflect common structural underpinnings, such as the Hox genes shared by many phyla. These genes control developmental processes, such as those related to body plans (the deep structural organization of the body of an organism), and presumably originated early in the history of multi-cellular animal life. In particular, although Gould accepted the importance of convergent evolution, he was highly skeptical of what might be called meta-convergence, the argument that the overall patterns of evolution and the structure of the modern biotic world necessarily had to be as they are.

Gould presents a sophisticated view of evolutionary change, which includes the structural, historical legacies of organisms. He points out that organisms are not mere putty to be sculpted over the course of their phylogeny (evolutionary history) by external environmental forces, but rather their structural integrity constrains and channels the variations on which natural selection operates.22 In this, Gould challenges the notion that phenotypic variation is isotropic, equally likely in all directions. Although mutation produces genetic variation that is random relative to selective advantage, this does not mean that phenotypic variation is not more likely in some directions than in others. Gould notes that the structural nature of the development of an organism throughout its life course (ontogeny) limits the types of phenotypic variation that are possible, because changes at one stage of the developmental process have consequences for later stages. Therefore, many characteristics of an organism cannot simply be modified without having substantial ripple effects throughout the whole organism.

For example, enlarging the body of a deer leads to enlarging disproportionately the antlers due to allometric processes where antlers grow at a different rate than body size. The inherited patterns of development do not readily allow for all types of modification—it would be difficult (although not necessarily impossible) for natural selection to make a larger deer with smaller antlers since this would require a restructuring of the development process. Hence, the evolutionary process is a dialectical interaction between the internal (inherited structural constraints) and the external (environmental selection pressure), just as the ontogeny of individual organisms is a dialectical interaction between their genes and the environment. Such an understanding helps restore the organism to a concept in biology—“an integrated entity exerting constraint over its history,” situated in a specific environmental context.23

Gould emphasizes the importance of recognizing both the reality of structural constraint and that structures have historical origins.24 Here, he helps unite insights from both sides of the age-old debate between functionalist biologists, such as Darwin and Jean-Baptiste Lamarck, and formalist (structuralist) biologists, such as Étienne Geoffroy Saint-Hilaire and Johann Wolfgang von Goethe. The functionalists typically stressed that features of organisms existed for utilitarian reasons (they were adaptations to their environments), and formalists underscored the structural unity of type common across similar organisms. Formalists often denied the possibility of evolution because they believed that only superficial change was possible, not fundamental change of underlying structures. This division was undermined when Darwin and subsequent evolutionists recognized that structures had evolved, although after their emergence, these structures may indeed constrain the evolutionary pathways available to organisms. In this, Darwin fundamentally reoriented the functionalist-formalist debate by adding a new dimension to the functional (active adaptation) and formal (constraints of structure) dichotomy: history (contingencies of phylogeny).25

Gould made a case for what might be called contingent convergence, whereby one of the reasons convergence is observed is because distantly related lineages share common structural features that are resistant to change, but whose emergence is the result of historical contingencies. Up to the Cambrian period, for example, the world was relatively underoccupied by metazoans (multicellular animals), providing the space for a bounty of evolutionary experiments to emerge. Early in the evolutionary history of metazoans, before developmental pathways were set, mutations could more fundamentally alter the development of organisms and produce widespread diversification. New body plans could emerge more readily than after deep structures became firmly established. And, just as contingency played an important role throughout history, the particularities of these structures (such as Hox genes) were not inevitable. Although these structures, once established, might lead to considerable convergence due to the constraints they place on developmental pathways, their origin was contingent.

Recognizing the profound importance of contingency, and wary of cultural biases leading others to see history as a march of progress, Gould, in his book Full House, provided one of his most compelling critiques of the assumption that evolution inevitably proceeds in a progressive manner, moving from simple to complex.26 Gould argues there is no predisposition toward evolutionary progress; that is, there is no general trend in the history of life whereby organisms typically become more complex, intelligent, or otherwise “superior” to their ancestors. Rather, there has been simply an increase in the variability of the complexity of organisms since the origin of life—that is, the history of life has been one of diversification not “improvement” per se.27 Bacteria, the simplest form of life, have remained the most common form of life throughout history.28 Although the most complex organism at any particular time in the past was less complex than the most complex organism alive today, the modal level of complexity has remained unchanged.

Gould argues that the typical lineage is just as likely to become simpler over geologic time as it is to become more complex. This is because, as per the paradox of the first tier, natural selection adapts creatures to their immediate environments, and complexity does not necessarily convey greater adaptive fitness than simplicity. The prime examples of lineages simplifying over evolutionary time are of those adopting a parasitic way of life. Since parasites typically live on the body of other organisms, they tend to become simplified eating structures, losing many of the complex features of their ancestors.

Why, then, are there more complex species alive today than there were one billion years ago, before the rise of metazoans? Simply because over time, there are more chances to get successive net shifts toward complexity in some lineages (as there are, likewise, more chances to get successive net shifts toward simplicity). In the same sense, if you flip many different coins (metaphoric lineages) many times (with each flip being a metaphorical unit of time), some particular coins, compared to other coins, will almost surely come up substantially more heads than tails, simply due to chance. And the more times each coin is flipped (the more time that passes), the more opportunities exist for a particularly large absolute number of heads (shifts toward complexity) in excess of tails (shifts toward simplicity) on some coins. Note that this process does not lead to a change in the central tendency of the proportion of heads and tails (that is, it should remain around 50 percent), but does lead to a much wider range (variability) in the absolute number of heads in excess of tails (complexity).

It is important to note that Gould does not argue that organisms evolve due to chance processes alone. He is only pointing out that, in adapting creatures to their local environments, natural selection is approximately as likely to make any particular lineage more simple as more complex. That is to say, there is no particular reason to believe that complex organisms are generally fitter, in the Darwinian sense, than simple organisms, as evidenced by the remarkably enduring success of single-celled organisms. In one set of circumstances, selection pressure may favor simple organisms, and in another, it may favor complex organisms.

So, although the most complex organism extant at any one time is generally more complex the closer we are to the present (due to more opportunities for metaphoric flips of the complexity coin), there is no general trend toward complexity, only increasing variability in the level of complexity across organisms. The specific twist to the history of life is that since, by necessity, life originates as the simplest form, there is no room to vary in the direction of even less complexity at the starting point of the frequency distribution of complexity. However, other than at the very simplest end, the complexity of any particular lineage is approximately as likely to drift in one direction as in another.

Gould’s position is best understood when contrasted with thinkers who are prone to interpret evolutionary history as directional, notably a diverse group including Richard Dawkins, Daniel Dennett, Robert Wright, and Simon Conway Morris. These thinkers have varying backgrounds and credentials, but agree that there is some degree of progress in evolution. What is telling here is that they agree on the idea of progress in evolution while sharing fundamentally different assumptions about the nature of the world.

The general ethos of the progressive position is well represented by journalist Robert Wright’s argument that both social and natural history have a trajectory that moves from simple to complex. His belief in what we have called meta-convergence is so strong and his view that society moves in a progressive direction is so deeply held that the subtitle of one of his books is The Logic of Human Destiny.29 He writes: “When you look beneath the roiled surface of human events…you see an arrow beginning tens of thousands of years ago and continuing to the present. And looking ahead, you see where it is pointing,” which of course is to a more advanced (in the sense of being further along the imagined road of progress) society, since “change in the structure of societies…is more likely to raise complexity than to lower it.”30 Wright has little original to say and mostly draws on the work of Dawkins and other scholars, but he is noteworthy for his popularity in certain circles, demonstrating that he presents a position that fits well with the preferences of mainstream culture and the social elite.

Richard Dawkins, undoubtedly one of the most distinguished contemporary evolutionary theorists, clearly represents the Darwinian tradition in his views on the nature of evolution. He is a strict materialist and a believer that, due to convergence, the larger patterns, although not the particulars, of evolutionary history were to a large extent inevitable. He explains that “a progressive trend is one in which there are no reversals; or if there are reversals, they are out-numbered and outweighed by movement in the dominant direction,” following a particular anatomical trend from early to intermediate to late. He proposes that “evolution exhibits progress” that is “value-free” (neutral) as well as “value-laden.” The latter entails a progressive evolutionary direction that is desirable, in some general sense, as far as “somebody’s value system.” Relying on the metaphor of the arms race, which is very similar to Darwin’s wedge argument, Dawkins argues that species generally progress through history due to competition.31 Here micro-level events generate evolutionary changes that can be seen at the macro level, with a tendency toward convergence.

Daniel Dennett, a philosopher of science and biology, presents a similar argument:

There aren’t global pathways of progress, but there is incessant local improvement. This improvement seeks out the best designs with such great reliability that it can often be predicted by adaptationist reasoning. Replay the tape [of life—a reference to Gould’s metaphor] a thousand times, and the Good Tricks will be found again and again, by one lineage or another.32

Dennett’s argument is that there are a limited number of ways for organisms to survive in the world, and over time natural selection will explore all of these options, finding again and again the best solutions to the challenges the environment throws up. Thus, although the particularities of the history of any lineage may be largely contingent, there will be considerable meta-convergence, where the larger patterns of history are constrained to have unfolded generally as they did. And this pattern is characterized to some degree as being one of progress to more finely adapted organisms. Dawkins and Dennett are materialists who make sensible arguments. However, they do not effectively counter Gould’s case since they do not demonstrate a proper understanding of the contingent nature of many instances of convergence.

For example, Dawkins has argued that the independent evolution of the eye in multiple lineages is a prime example of evolutionary convergence, suggesting that eyes are to some degree inevitable features of the natural world.33 It is indeed true that various types of eyes have emerged in divergent species and that the characteristics of these eyes are constrained by the physics of light, and thus similarities in eyes across distantly related species to some degree reflects convergence. However, what Dawkins fails to appreciate is that the convergence of eyes is also to some degree a contingent convergence, in that eyes across species, even those from different phyla, are built using some of the same developmental genes, indicating that eyes in different phyla are in part homologous, not simply analogous. As Gould explains, discoveries in the 1990s showed that different phyla use homologous developmental pathways to build eyes, controlled by shared Pax genes, most notably the Pax-6, and that amino acid identity shared between mammalian and insect Pax-6 sequences is over 90 percent.34

Thus, although complex eyes emerged independently in different lineages, they are built using a shared underlying structure, and that structure is an evolved one—that is, a product of history. The Pax genes did not evolve originally to produce eyes, since the eyeless common ancestors of multiple phyla shared them. The Pax genes could not have been produced by natural selection for their ability to build complex eyes. However, once they emerged, for whatever particular reasons that led to their development at the time, they provided a structure that could be co-opted to produce eyes. Without this historically produced structure, complex eyes as we know them may not have evolved. After all, distantly related species use the same developmental structure, rather than each having a uniquely evolved one.

This example demonstrates Gould’s point that structures constrain developmental pathways and can lead to convergence, but the structures themselves may be contingent products of history. Eyes, while clearly good examples of convergent evolution, are also good examples of the contingent nature of that convergence. The Pax genes, along with the Hox genes, illustrate the importance of historically constructed structures and—counter to Dawkins’s interpretation—show the profound effects of contingency on subsequent evolution. If, early in the evolution of metazoans, developmental structures other than the Pax and Hox genes had evolved and become dominant, the subsequent history of life on Earth may have been dramatically different.

Simon Conway Morris, who was one of the paleontologists who did important work on the Cambrian Shale fossils, makes an argument for meta-convergence that goes beyond the more mundane (and fairly sensible, if limited) one made by Dawkins and Dennett. Bizarrely, for a modern scientist, Conway Morris is actively hostile to materialism and takes a stance in favor of a kind of theistic evolution.35 Although he does not argue for divine intervention in evolution, which separates him from the more absurd supporters of “intelligent design,” he clearly assumes that there is some design in the universe that structured it to lead to the emergence of humanity. One of his most prominent books carries the rather extraordinary subtitle Inevitable Humans in a Lonely Universe, clearly demonstrating that he not only sees the broad features of evolution as largely inevitable (similar to Dawkins and Dennett) but even the particulars of humans as inevitable.36 In this book, Conway Morris itemizes the many examples of convergent evolution throughout history, including eyes across phyla, but also many particular features across different branches of the arthropod phylum, arguing that the ubiquity of convergence points to the inevitability of our present world. This is an extreme example of viewing history as unfolding in a progressive manner leading to a predetermined outcome. Conway Morris’s views come close to the absurd caricature of faulty reasoning that Twain mocked, which would lead one to conclude that the Eiffel Tower was built so that it could have a layer of paint on its top. According to Conway Morris, if the tape of life were replayed, evolutionary structures and constraints would lead to the inevitable return of Homo sapiens or a form very similar to present humans. In other words, from a specific evolutionary starting point, given the existing material and the course of development, there is a highly restricted range of options as far as evolutionary pathways.

Interestingly, contrary to Dawkins and Dennett, and half in line with Gould, Conway Morris sees evolution as controlled by underlying structures, which lead to the ubiquity of convergence. However, unlike Gould, he fails to recognize the historical emergence of many of these structures, assuming that they represent a more general underlying order to nature. In a more recent book—the title of which is The Deep Structure of Biology: Is Convergence Sufficiently Ubiquitous to Give a Directional Signal? (a question clearly answered in the affirmative)—Conway Morris and his fellow contributors rely on examples of convergence based on underlying structures to make the case for progress in evolution and, sometimes implicitly, sometimes explicitly, design in the universe.37 In his own contribution to this volume he is clear that he sees metaphysical implications stemming from convergence.38 In fact, in the closing essay in this volume, theologian John F. Haught ends with the claim that the history of life may be “pregnant with the promise of ultimate meaning. If so, there may still be abundant room, alongside of science, for a theology of evolution.”39 It is worth noting that this book is published by Templeton Foundation Press, which, as expressed on the back jacket of the book, aims to help “intellectual leaders and others learn about science research on aspects of realities, invisible and intangible. Spiritual realities include unlimited love, accelerating creativity, worship, and the benefits of purpose in persons and in the cosmos.”

It is particularly remarkable that Dawkins, a prominent atheist and vocal critic of theistic arguments, largely endorses Conway Morris’s argument, without acknowledging that it is based on anti-materialist premises.40 Dawkins does acknowledge that Conway Morris’s structuralism sits in tension with his own functionalism, but declares himself in this instance somewhat convinced of Conway Morris’s argument. Dawkins’s willingness to accept an argument based on theistic premises, since it agrees with his own conclusion about directionality in evolution, is perhaps indicative of a strong bias in favor of seeing progress. Gould often noted, in his examination of the history of science, that many metaphysical interpretations of nature remain unchanged even when the scientific understanding of the relevant phenomena fundamentally changes. This persistence in interpretation despite radical change in evidence suggests a cultural bias rather than a logical inference from the facts of nature.

For example, Gould has noted that Alfred Russel Wallace, who discovered natural selection independently of Darwin, believed, unlike Darwin, that the universe was pervaded by mind and that human life was no accident. Wallace made the all too common error of imposing his desire for meaning onto nature, which is basically the same error Conway Morris makes in finding meaning in the history of life. Gould writes:

Wallace examined the physical structure of the earth, solar system, and universe and concluded that if any part had been built ever so slightly differently, conscious life could not have arisen. Therefore, intelligence must have designed the universe….Wallace’s argument had its peculiarities, but one aspect of his story strikes me as even more odd. During the last decade, like cats and bad pennies of our proverbs, Wallace’s argument has returned in new dress. Some physicists [notably Freeman Dyson] have touted it as something fresh and new [the “anthropic principle,” the idea that intelligent life lies foreshadowed in the laws of nature and the structure of the universe]….To me it is the same bad argument….The central fallacy of this newly touted but historically moth-eaten argument lies in the nature of history itself. Any complex historical outcome—intelligent life on earth, for example—represents a summation of improbabilities and becomes thereby absurdly unlikely. But something has to happen, even if any particular “something” must stun us by its improbability.41

Gould then goes on to explain that Wallace’s understanding of the universe was radically different from our modern understanding, and thus Wallace and Dyson, like Conway Morris, manage to reach the same conclusion—that there is some form of design in the universe—based on profoundly divergent conceptualizations of the natural world. Gould suggests: “If the same argument can be applied to such different arrangements of matter, may we not legitimately suspect that emotional appeal, rather than a supposed basis in fact or logic, explains its curious persistence?”42

We find it particularly interesting that both Conway Morris and Dawkins, the theist and the atheist, who have fundamentally different conceptions of the universe—the former believing that it reflects an underlying intelligence and the latter being an ardent materialist—both see some form of progress and inevitability in the history of life. Conway Morris sees this as a sign of divine intent, while Dawkins sees this as a reflection of deterministic natural laws. Following Gould’s perspective, we suggest they are both wrong. Perhaps they both see progress in evolution and believe that the present was inevitable because of a pervasive cultural bias rather than because of a clear signal from nature. Hope as we may, intelligent life was not inevitable, but rather it is a contingent outcome of history.

Conway Morris and others like him make a common error in equating what we value most in our world with what is necessary or inevitable. As Gould himself commented, “There’s a common tendency to equate importance with necessity. Just because something is important—which consciousness clearly is to the history of the planet—doesn’t mean it was meant to be. There’s never anything in the history of life that’s had such an impact on the earth as the evolution of human consciousness, but that doesn’t mean it was meant to be. It could still be accidental.”43 Gould emphasized the importance of contingency because it is a reality of the natural world and the history of life on Earth, and he criticized the belief in progress, as justified by faith in the powers of meta-convergence, to lead inevitably to the conditions of the present, because it represents an inaccurate characterization of natural history. He sought to show that biology is a historical science that cannot be reduced to a set of mechanical, deterministic laws.

However, Gould also focused on contingency and the critique of progress to make a larger point about science and society. The belief in progress is a prime example of how social biases can distort science. Gould aimed to show that the natural world does not conform to human aspirations. Nature does not have human meaning embedded in it, and it does not provide direction to how humans should live. We live, instead, in a world that only has meaning of our own making. Rather than viewing this situation as disheartening, Gould saw it as liberating because it empowers us to make our own purpose. Gould stressed, similar to Karl Marx and other radical thinkers, that we make our own history and that the future is open.

Notes

1. ↩ Michael B. Shermer, “This View of Science: Stephen Jay Gould as Historian of Science and Scientific Historian, Popular Scientist and Scientific Popularizer,” Social Studies of Science 32, no. 4 (2002): 489–524.
2. ↩ Stephen Jay Gould, The Structure of Evolutionary Theory (Cambridge, Mass.: Harvard University Press, 2002); Ernst Mayr, The Growth of Biological Thought: Diversity, Evolution, and Inheritance (Cambridge, Mass.: Harvard University Press, 1982).
3. ↩ Stephen Jay Gould, Eight Little Piggies (New York: W.W. Norton, 1993), 302–3.
4. ↩Darwin quoted in ibid., 302; see also Charles Darwin, On the Origin of Species (Cambridge, Mass: Harvard University Press, 1964), 67; Charles Darwin, Charles Darwin’s Notebooks, 1836–1844 (Ithaca, N.Y.: Cornell University Press, 1987), 375–76.
5. ↩ Gould, Eight Little Piggies, 303.
6. ↩ Charles Darwin, The Origin of Species (New York: Penguin Books, 1985), 336–37; Darwin quoted in Gould, Eight Little Piggies, 303.
7. ↩Quoted from a letter Gould wrote, see Warren D. Allmon, “The Structure of Gould,” in Stephen Jay Gould: Reflections on His View of Life, ed. Warren D. Allmon, Patricia H. Kelley, and Robert M. Ross (New York: Oxford University Press, 2009), 55; and Richard K. Bambach, “Diversity in the Fossil Record and Stephen Jay Gould’s Evolving View of the History of Life,” in Stephen Jay Gould: Reflections on His View of Life, 70.
8. ↩ Stephen Jay Gould, The Flamingo’s Smile (New York: W.W. Norton, 1985), 448.
9. ↩ Stephen Jay Gould, “The Paradox of the First Tier: An Agenda for Paleobiology,” Paleobiology 11, no. 1 (1985): 2–12, quote on 2–3.
10. ↩ Ibid., 3.
11. ↩ Ibid., 10.
12. ↩ It should be noted that Darwin, despite struggling with this conclusion, understood that Homo sapiens was the product of contingent events of history and were therefore not foreordained to develop. See Darwin, Origin of Species; Charles Darwin, The Descent of Man, and Selection in Relation to Sex (Princeton, N.J.: Princeton University Press, 1981); John Bellamy Foster, Marx’s Ecology (New York: Monthly Review Press, 2000).
13. ↩ As quoted by Stephen Jay Gould in an interview in Wim Kayzer, A Glorious Accident: Understanding Our Place in the Cosmic Puzzle (New York: W.H. Freeman, 1997), 84; see also Darwin Correspondence Project Database, http://darwinproject.ac.uk, letter no. 8658.
14. ↩ Gould, “The Paradox of the First Tier,” 3.
15. ↩ For a discussion of the role of chance—“bad luck”—in extinction, see David Raup, Extinction (New York: W.W. Norton, 1991).
16. ↩ For an assessment of potential causes of the Permian extinction and a presentation of its consequences, see Douglas H. Erwin, Extinction: How Life on Earth Nearly Ended 250 Million Years Ago (Princeton, N.J.: Princeton University Press, 2006).
17. ↩ Stephen Jay Gould, “The Persistently Flat Earth: Irrationality and Dogmatism Are Foes of Both Science and Religion,” Natural History 103, no. 3 (1994): 12–19, quote on 19.
18. ↩ Stephen Jay Gould, Wonderful Life (New York: W.W. Norton, 1989), 45.
19. ↩ Mark Twain, Letters from the Earth (New York: Harper & Row, 1962), 222–23.
20. ↩ Ibid., 226.
21. ↩ Gould, Wonderful Life.
22. ↩ Gould, The Structure of Evolutionary Theory; Stephen Jay Gould, “Is a New and General Theory of Evolution Emerging?” Paleobiology 6, no. 1 (1980): 119–30.
23. ↩ Gould, “Is a New and General Theory of Evolution Emerging?” 129; Richard Lewontin, The Triple Helix: Gene, Organism, and Environment (Cambridge, Mass.: Harvard University Press, 2000).
24. ↩ Gould, The Structure of Evolutionary Theory.
25. ↩ Ibid., 251–60.
26. ↩ Stephen Jay Gould, Full House (New York: Harmony Books, 1996).
27. ↩ However, this diversification has been based on fewer themes, as there was greater disparity among organisms in the Cambrian, when more fundamental body plans existed, as Gould argues in Wonderful Life.
28. ↩ Gould, Full House.
29. ↩ Robert Wright, Nonzero: The Logic of Human Destiny (New York: Pantheon, 2000).
30. ↩ Ibid., 16–17.
31. ↩ Richard Dawkins, The Ancestor’s Tale: A Pilgrimage to the Dawn of Evolution (New York: Houghton Mifflin, 2006), 597–605.
32. ↩ Daniel C. Dennett, Darwin’s Dangerous Idea: Evolution and the Meaning of Life (New York: Simon & Schuster, 1995), 308.
33. ↩ Richard Dawkins, Climbing Mount Improbable (New York: W.W. Norton, 1996), 138–97.
34. ↩ Gould, The Structure of Evolutionary Theory, 1123–34.
35. ↩ Simon Conway Morris, The Crucible of Creation: The Burgess Shale and the Rise of Animals (Oxford: Oxford University Press, 1998); Simon Conway Morris, Life’s Solution: Inevitable Humans in a Lonely Universe (Cambridge: Cambridge University Press, 2004).
36. ↩ Conway Morris, Life’s Solution.
37. ↩ Simon Conway Morris, ed., The Deep Structure of Biology: Is Convergence Sufficiently Ubiquitous to Give a Directional Signal? (West Conshohocken, Penn.: Templeton Foundation Press, 2008).
38. ↩ Conway Morris, “Evolution and Convergence: Some Wider Considerations,” in ibid., 60–62.
39. ↩ John F. Haught, “The Purpose in Nature: On the Possibility of a Theology of Evolution,” in ibid., 230.
40. ↩ Dawkins, The Ancestor’s Tale, 595–96.
41. ↩ Stephen Jay Gould, The Flamingo’s Smile (New York: W.W. Norton, 1985), 393–95.
42. ↩ Ibid., 397.
43. ↩ Gould, interviewed in Kayzer, A Glorious Accident, 92–93.