* This is the latest installment of “Problematica.” It is based on new research that digs into the history of the “Cambrian explosion”— the expression, that is, not the event. With luck, a proper article on this topic will soon be appearing in a journal near you. The image behind the title card was created by Mark Witton (with my apologies for lack of accreditation in the original post). Problematica is written by Max Dresow…

Peter Vickers is no oracle; he's just a philosopher with a weakness for astrobiology. Still, in his recent book he ventures a list of thirty scientific claims “we can be confident will last forever” (Vickers 2023, 1).* Some are taxonomic: the sun is a star; the Milky Way is a spiral galaxy. Others are more contingent: the Moon causes the tides; red blood cells carry oxygen through the body. Then there are theoretical claims, including “Numerous chemical facts about elements and how they relate to each other”; “The germ theory of disease, including numerous things we know about the properties and behavior of various different bacteria and viruses”; “Much of cosmology, including the large-scale structure of the universe, the expansion of the universe, and the properties of various entities such as quasars, pulsars, and galaxies”; and “Numerous facts coming under the broad heading of ‘climate science’.” Vickers even includes some findings from the paleosciences, including “A large body of thought concerning the geological history of the Earth”; “Detailed knowledge of the history of human life”; and “Detailed knowledge of many dinosaurs, including at least some aspects of how they lived and interacted.” Then there’s this: “There was an explosion of life on Earth approx. 540 million years ago.” Obviously this refers to the “Cambrian explosion,” the apparently rapid increase in animal diversity and abundance that took place between about 540 and 520 million years ago (Wood et al. 2019).

[* The list is meant to put some meat on the conceptual bone of future-proof science, not to provide an exhaustive catalog of those parts of science that are immune to revision. For Vickers, a scientific claim can be regarded as future-proof “when the relevant scientific community is large, international, and diverse, and at least 95% of that community would describe the claim as a ‘scientific fact’.”]

Among the “future-proof” facts Vickers lists, this last one is the most unusual. The reason is that it includes the word “explosion”— Vickers does not just say that there was a major biodiversification event about 540 million years ago (although presumably this is what he has in mind). Of course, there was a major biodiversification event around 540 million years ago, just as there were major events in the Ordovician and Cenozoic (to name just two others). So this much is probably future-proof. But to say, as Vickers does, that there was “an explosion of life on Earth about 540 million years ago” is arguably to say something stronger, or more theory-bound. Just what is “an explosion of life” anyway? And why do we talk about the Cambrian radiation (to use a different metaphor) as an evolutionary explosion?

The Cambrian explosion in a children’s picture book, *Out of the Blue: How Life Evolved out of Prehistoric Seas*

These are the questions I want to explore in this essay. And one more too. How likely is it that scientists in a hundred years will agree with the claim that there was “an explosion of life on Earth approx. 540 million years ago”? Just how future-proof is the concept of a Cambrian explosion?

* * *

The term “explosive evolution” is sometimes credited to the American paleontologist George Gaylord Simpson (1944), but this is all wrong. Lots of people talked about explosive evolution during the early twentieth century, including Richard Goldschmidt, who described evolution on volcanic islands as “a kind of explosive evolution” in his much-maligned Material Basis of Evolution (1940). And here is Simpson’s mentor at the American Museum of Natural History, William Diller Matthew, writing in The Quarterly Review of Biology (1926):

The paleontologist… often speaks of rapid evolution and even of the “explosions” of phyla. But an explosion which took at least many centuries to explode would not seem to the lay mind to be very sudden; and in fact I doubt whether there is any real evidence of more rapid evolution than the change from Eohippus to Equus, and that, if the radium calculations are correct, took some fifty million years. (Matthew 1926, 178)

As this quotation illustrates, the term evolutionary explosion was already widespread in the 1920s, and was used whenever evolution seemed to proceed with great haste in particular lineages. (Great haste, as ever, is in the eye of the beholder.)

If it is wrong to say that Simpson invented the term “explosive evolution,” and wrong even to say that he popularized it, it is perhaps not wrong to say that he legitimized it, at least in the eyes of Darwinism biologists. The legitimation happened in that great book, Tempo and Mode in Evolution, published in 1944. The aim of the book was to characterize rates and styles of evolution using data from the fossil record. To do this, Simpson introduced terms like bradytely, horotely, and tachytely to indicate slow, normal, and rapid evolution, respectively. Indeed, the term “explosive evolution” appears only infrequently: once in a discussion of the causes of evolution, once in a discussion of tachytelic lines that become bradytelic, and once in a discussion of “quantum evolution.” But these discussions were influential out of all proportion with their frequency, and after 1944, when an author spoke of “explosive evolution” there was a good chance they had Simpson’s model of quantum evolution in mind.

Simpson regarded the model of quantum evolution as “[p]erhaps the most important” part of his book, and also the most speculative (Simpson 1944, 206). By “quantum evolution” he intended “the relatively abrupt shift of a biotic population in disequilibrium [with its environment] to an equilibrium distinctly unlike the ancestral condition.” This can occur at any scale, but is “the dominant and most essential process in the origin of taxonomic units of relatively high rank, such as families, orders, and classes.” Paleontologists had long puzzled over how to explain the apparently rapid origin of major groups of organisms. Why did major taxonomic groups seem to spring into existence fully formed, as opposed to emerging gradually over long periods of time? For Simpson, the answer was evolutionary disequilibrium. In ordinary phyletic evolution, the evolving population is at every point in equilibrium with its secular environment, or very nearly so. Every now and then, however, equilibrium is lost, permitting a population to move into a new adaptive zone by a combination of drift, preadaptation, and natural selection. It is drift, or else changes in the physical environment, that lead to disequilibrium, usually spelling doom for the affected population. However, if “nearby” adaptive zones are unoccupied and if the population has or manages to acquire the right preadaptations, then natural selection will rapidly return the population to equilibrium. This is quantum evolution. Simpson illustrated the process with a figure that shows a single population explosively radiating into many new adaptive zones, as well as some regions of no man’s land.

Simpson’s diagram of quantum evolution. From Simpson (1944)

So there you have it: a population biological model of explosive evolution, fit to explain the apparently rapid origin of major taxonomic groups. But who first thought to apply this model to the events of the early Cambrian, undoubtedly the most iconic explosion in the history of life? The answer, I think, is Preston Cloud (1948), then an assistant professor at Harvard University about to resign his position to return to the U.S. Geological Survey. Writing in Evolution, Cloud credited Simpson with painting “a vivid word picture of what might transpire at times of eruptive evolution.” (Cloud prefered the term “eruptive” to “explosive.”) He first applied this model to an episode of rapid evolution in terebratuloid brachiopods, the group he had studied for his dissertation research. Then, he turned his attention to “[a] more speculative field of thought”: the origin of the Cambrian fauna (Cloud 1948, 346).

In Cloud’s view, “the seemingly sudden appearance of representatives of a number of the more important Phyla of animals” represents “one of the great unexplained facts of the geologic record.” Of course, it was possible to argue that the whole thing was a stratigraphic mirage. That was Charles Doolittle Walcott’s position, and it led him to postulate a long period of “Liparian” time during which the Cambrian fauna slowly evolved without leaving any traces in the geological record (Walcott 1910). Alternatively, “[the] appearance of diversified multicellular animal life in the Cambrian may actually have been almost as sudden as the record suggests, an instance of eruptive evolution of the root stock of animal life.” In support of this scenario, Cloud observed that the pre-eruption ocean would have been mostly free of competitive pressure and abounding in niches waiting to be snatched up. Abundant plankton probably existed near the surface of the ocean “to start the nutritive cycle running.” If, in addition, “paedomorphic influences should have been common in the early multicellular stocks [predisposing lineages to produce novelties distinctive of major groups], it is not too much to suppose that a diversity as marked as that of the Early Cambrian faunas might have been produced within a few tens of millions of years or less” (347). BANG!

Cloud’s diagram of explosive evolution in terebratuloid brachiopods. In Cloud’s description, the figure “represents the approximate known ranges in time of the earlier terebratuloid genera and their inferred relationships. It shows within the main eruptive phase several marked eruptive nodes that probably coincide with the opening up of new ecologic niches as a result of paleoecologic changes or biotal readjustments or both.” (And notice how it looks kind of like Simpson’s diagram turned on its side.) From Cloud (1948)

Cloud concluded that, while its causes are inferential, “the process of eruptive evolution is demonstrable, for its effects may be seen,” if not in the Cambrian then elsewhere in the fossil record (Cloud 1948, 347). “It seems inescapable that the concept should be brought to bear on speculation concerning that most puzzling of all paleontological phenomena— the relatively sudden appearance in Cambrian rocks of diversified multicellular animals.”

Cloud’s discussion of eruptive evolution in the Cambrian has a modern ring to it. But what stands out in hindsight is just how unusual it was. Most contemporary discussions of explosive evolution did not emphasize the Cambrian Period, and many did not even mention it. This included perhaps the most extensive discussion of explosive evolution in the paleontological literature at the time: a 1949 symposium jointly sponsored by the Paleontological Society, the Society for Vertebrate Paleontology, and the Geological Society of America.

* * *

The symposium was called “Distribution of Evolutionary Explosions in Geologic Time,” and had as its purpose “to examine the foundation in paleontology of the diastrophic theory” (Henbest 1952, 299). This was the theory of periodic diastrophism developed by Thomas Chrowder Chamberlin, which exercised a profound influence on American geology and paleontology during the first half of the twentieth century (Greene 1982).

The influence was impossible to separate from Chamberlin’s own. A longtime professor at the University of Chicago, Chamberlin made his name as a survey geologist in Wisconsin, where he worked amid the glacial deposits of the Upper Mississippi Valley. There he demonstrated that there had been multiple Pleistocene glaciations in North America— a major discovery— and with that as a recommendation was appointed chief of the new glacial division of the U.S. Geological Survey in 1881. Always a synthetic thinker, he proceeded to investigate the role that carbon dioxide played in stimulating glacial advance and retreat during the ice ages (Fleming 2000). It all began with diastrophism. Following a diastrophic revolution (a large-scale vertical movement of the crust), the continents are left high and dry. Because more rock is then exposed to the air, more carbon dioxide than usual enters into chemical combination with rocks. This causes temperatures to plummet, stimulating the growth of glaciers, like those that periodically flowed through the Upper Mississippi Valley.

Thomas Chrowder Chamberlin (1843–1928) at his desk at the University of Chicago

Chamberlin worked out this model around 1900, the culmination of nearly three decades of work in glacial geology. Yet his interests had by this point expanded to embrace larger stretches of space and time. An especially important publication bore the title, “The Ulterior Basis of Time Divisions and the Classification of Geologic History.” In it, Chamberlin declared “[the] most vital problem [facing] the general geologist” to be “the question of whether the earth’s history is naturally divided into periodic phases of world-wide prevalence, or whether it is but an aggregation of local events dependent upon local conditions uncontrolled by overmastering agencies” (Chamberlin 1898, 449). The former position was close to the one advocated by the great Viennese geologist Eduard Suess. The latter resembled that of Charles Lyell. Anyway, Chamberlin’s purpose was to suggest that the history of the earth was indeed measured out in rhythmic pulses: episodes of diastrophic disturbance— causing sea levels to drop— followed by base-leveling and oceanic deposition— causing them to rise. This, he suggested, was the ultimate basis of stratigraphic correlation, and the reason the major divisions of the geological column were the same the world over.

But that was not all. According to Chamberlin, life has responded to these diastrophic pulses. When sea levels drop (a consequence of foundering ocean basins), the restriction of the total extent of the shallow seas places pressure on shallow water organisms, causing “repressive evolution” (Chamberlin 1898, 457). Likewise on land, the rapid emergence and deformation of continents puts pressure on terrestrial organisms, resulting in a winnowing of diversity. All this happens together— the foundering of ocean bottoms, the withdrawal of shallow seas, the evolutionary response— when stresses on the crust trigger a diastrophic revolution. Then things settle down. Erosion and sedimentation set to work raising sea levels, restoring shallow water environments, and relieving the pressure on organisms, causing bursts of evolution “in which new ground and rich opportunities constitute the dominant condition” (458). Finally, mounting stresses again trigger a diastrophic revolution. This brings the geological cycle full-circle.

Thomas Chamberlin died in 1928. But his geologist son Rollin kept the diastrophic flame alive for a further two decades. This is why Lloyd Henbest could say, in his introduction to the 1949 symposium, that periodic diastrophism “represents the prevailing concept in historical geology… [its] tenets… widely accepted and taught even in their extreme form” (Henbest 1952, 299). These tenets were that “(1) diastrophism is periodic and synchronous on a world-wide scale; (2) diastrophism is a major control, if not the principal stimulus of organic evolution; and therefore (3) diastrophism is the ultimate basis for correlating events in earth’s history.” But was it true? Testing was required, and it was the purpose of the symposium on evolutionary explosions to put the theory to the test. Seven papers followed by experts in paleontology and geochronology. One, on “Periodicity in Vertebrate Evolution,” was authored by Simpson and found little support for “the theory of simultaneous, world-wide physical and biological climaxes at the period and era boundaries” (Simpson 1952, 359). Another, on “Periodicity in Invertebrate Evolution,” was authored by Simpson’s colleague at the American Museum, Norman Newell.

Newell’s paper had the same aim as Simpson’s: to assess the evidence for the synchronicity of diastrophic movements and evolutionary explosions. He displayed this evidence in a series of graphs that depicted invertebrate diversity through time. Together these suggested that fluctuations in the evolutionary activity of major groups are not “random” but instead cluster around peaks of low activity. These are followed by times of accelerated extinction, which in turn are followed by “renewed radiation into vacant or uncrowded ecologic niches”— evolutionary explosions. Newell observed that “[the] first of these widespread evolutionary expansions was during the Cambrian and Ordovician” when rates of generic diversification were at all-time high levels. He spoke of the Cambrian and Ordovician together because his “synthetic graph” of diversification rates showed more orders and classes originating in the Ordovician than in the Cambrian. But elsewhere he singled out the Cambrian as “the most critical time in invertebrate history,” when life radiated into largely unoccupied ecological space (Newell 1952, 381).

Newell’s “synthetic graph showing highest rates of generic diversification, and times of first appearance and extinction of major groups of invertebrates.” Rates of generic diversification were high in the Cambrian, only topped by the succeeding Ordovician Period. But the number of groups at the peak of their evolutionary activity was low. From Newell (1952)

Despite these remarks, it is worth noting just how little Newell has to say about the Cambrian. His comments are limited to just a few sentences, including one in which he notes that invertebrate life in the Cambrian “was relatively undiversified as compared with later times” (Newell 1952, 383). Newell clearly thought that the Cambrian was an important period, corresponding to the original diversification of multicellular life. Yet in other ways it was undistinguished. For example, the Cambrian saw fewer groups at the peak of their evolutionary activity than any period besides the Permian: the time of the “Great Dying.” In this sense, the Cambrian could not be regarded as a terribly explosive period in the planet’s history. Indeed, no other author in the symposium even mentions what would later become known as the Cambrian explosion— in a symposium devoted to evolutionary explosions!

* * *

So how did the Cambrian explosion come to overshadow all other evolutionary explosions to become “the Cambrian explosion”?

The term “Cambrian [evolutionary] explosion” did not appear in the scientific literature until 1965. Curiously, it made its debut in the Journal of Atmospheric Sciences, in a paper written by a pair of physicists (Berkner and Marshall 1965). The purpose of the paper was to construct a model of the oxygenation of the early atmosphere. Simpson’s model was brought in to explain the polyphyletic radiation of major groups once a threshold of minimal oxygenation had been breached: the “Cambrian evolutionary explosion.” The Princeton paleontologist Alfred Fischer called the whole thing “a brilliant contribution” and “the first plausible explanation of this milepost of the fossil record [the Cambrian radiation]” — although he quickly added that he was no fan of explosive evolution (Fischer 1965, 1210). In 1965, most paleontologists preferred their Cambrian sans explosion.

Fischer’s diagram of the fossil record plotted against the evolution of the atmosphere. Although Fischer depicts a polyphyletic radiation near the Cambrian boundary, his model is an “explosion of fossils” model, since the radiation does not correspond to the origin of metazoans: just their entry into the fossil record. From Fischer (1965)

Interest in the early evolution of metazoans grew during the next fifteen years. Throughout this interval, the term “Cambrian [evolutionary] explosion” remained a relatively rarity. But this began to change during the 1980s. The key, I wish to suggest, was the adoption of the term by scientists who were agnostic or even antagonistic to the suggestion that there had been an evolutionary explosion in the early Cambrian. As Bruce Runnegar noted in an important review, there were two ways of interpreting the apparently rapid diversification of metazoan taxa in the early Cambrian. The first was to allow that there really had been an explosion of diversity during this interval, which the fossil record dutifully preserves (Runnegar 1982). This was the account favored by Cloud (1948) and Berkner and Marshall (1965); but it was probably a minority position in 1982. The other was to regard the explosion as an “explosion of fossils” (Towe 1981): an explosion of pre-existing groups into visible evidence. Runnegar favored this second interpretation (with some caveats: he answered his own question— animals or fossils?— with a confident “both/and”). Still, he did not shy away from talking about a “Cambrian explosion,” and even titled his article, “The Cambrian explosion: animals or fossils?” This had the effect of decoupling the term from the controversial model of explosive evolution. And this greased the wheels of its further— if always incomplete and controversial— acceptance.

To put the point differently, during the 1980s, it became possible to talk about a “Cambrian explosion” without implying that there had been an episode of explosive evolution at the base of the Cambrian. Or at least an episode of explosive evolution corresponding to the origin of metazoan phyla from a common ancestor. Also on the table was a polyphyletic radiation of pre-existing groups, perhaps in response to a trigger like oxygen accumulation (Fischer 1965; Runnegar 1982). Or perhaps the explosion was just a phase in a longer process— still an “explosion” of sorts, but not a discrete thing. Anyway, the term “Cambrian explosion” was no longer attached to a single model of explosive evolution and could encompass divergent opinions. Animals or fossils, or animals and fossils— something had exploded during the Cambrian, which could be intelligibly discussed under the heading of the “Cambrian explosion.” (The same diversity was harder to accommodate under the expression “Cambrian evolutionary explosion,” since this wore its commitment to the Simpsonian model of explosive evolution on its sleeve.)

The 1980s were the key decade for consolidating “the Cambrian explosion” as a term of art. Prior to this, the term had been a rarity. By the middle of the decade, however, it had become a relative commonplace, at least within paleontology. Then came Stephen Jay Gould’s bestseller, Wonderful Life (1989). Gould was not an expert on the Cambrian explosion. Indeed, he never performed any direct research on it at all. Still, his book made a splash far beyond the workbenches and seminar rooms of academic paleontology. (It even netted Gould a Pulitzer Prize nomination.) This had the effect of setting the “Cambrian explosion” loose on broader scientific discourse, and even on popular culture. In the terminology of Gould’s great rival, it was a highly infectious— and so, highly successful— meme.

The original dust jacket of *Wonderful Life,* and its author, Stephen Jay Gould, at the Walcott Quarry

The same can’t be said about the concept of explosive evolution. When Wonderful Life hit bookshelves in 1989, the Simpsonian model of evolutionary explosions was in decline. Unmoored from any population biological anchor, the expression “explosive evolution” had also fallen from favor, surviving as a relic of an earlier era of evolutionary theorizing. This had the effect of rendering the Cambrian explosion the last explosion standing. Whereas, once, the canonical example of explosive evolution had been the Cenozoic mammalian radiation, today one rarely hears about evolutionary explosions outside of the Cambrian. It is a remarkable change of affairs since the 1949 symposium on evolutionary explosions, in which the Cambrian was a marginal presence at best.

It was also a highly contingent one. There is no obvious reason why the Cambrian radiation, and only the Cambrian radiation, should be described as an explosion. Sure, “the Cambrian explosion” has a nice ring to it, and we now recognize it to be (in some respects) a uniquely important event in the history of life. But as episodes of rapid evolution go, it is perhaps not so unusual to warrant an exclusive claim to the term “explosive.” Why not the Ordovician explosion, or the Cenozoic explosion of mammals? There is, I think, no good answer to this question. The answer is simply this: that only in the case of the Cambrian did it become an entrenched feature of linguistic practice to refer to the event as an explosion. Runnegar had a role to play in this. So did paleontologists like James Valentine and Simon Conway Morris, who were interested in the possibility that something special happened at the beginning of the Phanerozoic (e.g., Valentine et al. 1991; Conway Morris 2000). And of course there was Gould’s book, which spread news of the explosion far beyond the boundaries of paleontology. But it might have gone another way, and it is easy to imagine the same discussions taking place under the rubric of the “Cambrian radiation,” especially after Simpson’s model fell from favor in the 1960s.

* * *

What does this tell us about the future of the term “Cambrian explosion”? By itself, perhaps, nothing. But given the basically contingent— and in some respects arbitrary— nature of the expression, it will come as no surprise that it has come in for criticism. One round of criticism was sparked by the hubbub surrounding Wonderful Life and crested in the mid-1990s. A model antagonist was Richard Fortey, who wrote papers with titles like “The Cambrian explosion exploded?” and “The Cambrian evolutionary ‘explosion’ recalibrated” (notice the scare quotes).* Then things settled down for a while. The key development during this period was the emergence of a new understanding of the “explosion”— one that integrated aspects of the “explosion of life” and “explosion of fossils” models. This pictured the explosion as a polyphyletic radiation of crown-group animals “decoupled” from an earlier diversification of major clades— still an explosion (or major biodiversification event), but one with a long Precambrian fuse (Dresow and Love 2022).

[* This goes to show that not everyone in the “explosion of fossils” camp was happy with the term “Cambrian explosion.”]

A cladistic representation of the Cambrian explosion (that is, a phylogenetic tree of Cambrian arthropods), with diversification “decoupled” from phylogenesis, or the divergence of major clades. From Fortey et al. (1997)

Now, a second wave of discontent seems to be cresting as new models challenge the notion that there was a discrete “explosion.” Here, for example, is Rachel Wood and colleagues, writing in an article called “Integrated records of environmental change and evolution challenge the Cambrian explosion”:

a discrete Cambrian Explosion event is difficult to temporally isolate, or indeed to define. The rise of early metazoans can be more simply and holistically recast as a series of successive, transitional radiation events, perhaps mediated via complex environmental change, which extended from the Ediacaran and continued to the early Palaeozoic. (Wood et al. 2019, 535)

Again: “while the Cambrian Explosion represents a radiation of crown-group bilaterians, it was simply one phase amongst several metazoan radiations, some older and some younger” (Wood et al. 2019, 528). Jacob Beasecker and colleagues agree: “the processes and the time scale over which these [evolutionary] processes acted were more complex than implied by a phrase that signals a single event.” These authors also worry about the misleading connotations of “explosion”:

“Diversification” and “radiation” may not have the visceral appeal of “explosion,” but both alternatives are suitable, fitting, apt, proper, and applicable without carrying the implication of catastrophic rate or otherworldly mechanism. Certainly, biodiversification at the beginning of the Cambrian was unique— all those new body plans— but no evolutionary rules were broken, nor is there mystery or discipline-dividing controversy, as is claimed by anti-science concerns who seize on the term “explosion.” (Beasecker et al. 2020, 26)

They conclude that it is time to “defuse” the Cambrian explosion: “for scientific, semantic, and societal reasons, it is time to lay the term ‘Cambrian explosion’ to rest for any use other than historical reference” (Beasecker et al. 2020, 27).

An image from Wood et al. (2019) showing the broader geochemical context of the Cambrian explosion (or, as they would have it, the '“Ediacaran-Cambrian radiation”)

Was there really “an explosion of life on Earth approx. 540 million years ago”? Perhaps— it depends on what you mean— but it is far from clear that scientists a hundred years from now will regard this as a true claim. Even today, I doubt that 95% of paleontologists regard the claim as a fact. (This is the threshold Vickers settles on for future-proof-ness.) There is simply too much baggage attached to the word “explosion.” And skepticism about whether the alleged explosion was a discrete event.

But forget about technical future-proof-ness. How likely is the expression “Cambrian explosion” to hang around? If I had to wager, I would bet that it fades from polite discourse, replaced by the more neutral “Ediacaran-Cambrian radiation” or something else. Perhaps it will survive in popular parlance— one still hears the end-Permian extinction referred to, occasionally, as “the Great Dying.” But this is not the kind of immortality a scientific concept aspires to. Something happened about 540 million years ago— the rapid diversification of animal crown-groups and the assembly of many extant body plans. But the time may have passed when it made sense to speak of all this as an explosion.

References

Beasecker, J., Chamberlin, Z., Lane, N., Reynolds, K., Stack, J., Wahrer, K., Wolff, A., Devilbiss, C. W., Durbin, D., Garneau, H., & Brandt, D. (2020). It’s time to defuse the “Cambrian explosion.” GSA Today, 30, 26–27.

Berkner, L. V. & Marshall, L. C. (1965). On the origin and rise of oxygen concentration in the earth’s atmosphere. Journal of the Atmospheric Sciences, 22, 225–261.

Chamberlin, T. C. (1898). The ulterior basis of time divisions and the classification of geological history. The Journal of Geology, 6, 449–462.

Cloud, P. E. (1948). Some problems and patterns of evolution exemplified by fossil invertebrates. Evolution, 2, 322–350.

Conway Morris, S. (2000). The Cambrian “explosion”: slow-fuse or megatonnage? Proceedings of the National Academy of Sciences, U.S.A., 97, 4426–4429.

Dresow, M. & Love, A. C. (2022). The interdisciplinary entanglement of characterization and explanation. The British Journal of the Philosophy of Science. http://doi.org/10.1086/720414.

Fischer, A. (1965). Fossils, early life, and atmospheric history. Proceedings of the National Academy of Sciences, 53, 1205–1215.

Fleming, J. R. (2000). T. C. Chamberlin, climate change, and cosmogony. Studies in History and Philosophy of Science, 31, 293–308.

Fortey, R. A. (2001). The Cambrian explosion exploded? Science, 293, 438–439.

Fortey, R. A., Briggs, D. E. G., & Wills, M. A. (1997). The Cambrian evolutionary ‘explosion’ recalibrated. BioEssays, 19, 429–434.

Goldschmidt, R. (1940). The material basis of evolution. New Haven: Yale University Press.

Gould, S. J. (1989). Wonderful life: the Burgess Shale and the nature of history. W. W. Norton & Company.

Henbest, L. G. (1952). Significance of evolutionary explosions for diastrophic division of earth history— introduction to the symposium. Journal of Paleontology, 26, 299–318.

Matthew, W. D. (1926). The evolution of the horse. A record and its interpretation. The Quarterly Review of Biology, 1, 139–185.

Newell, N. (1952). Periodicity in invertebrate evolution. Journal of Paleontology, 26, 371–385.

Runnegar, B. (1982). The Cambrian explosion: animals or fossils? Journal of the Geological Society of Australia, 29, 395–411.

Simpson, G. G. (1944). Tempo and mode in evolution. New York: Columbia University Press.

Simpson, G. G. (1952). Periodicity in vertebrate evolution. Journal of Paleontology, 26, 359–370.

Towe, K. M. (1981). Biochemical keys to the origin of life. In J. Billingham (ed.), Life in the universe (297–306). Cambridge: MIT Press.

Valentine, J. W., Awramik, S. M., Signor, P. W., & Sadler, P. M. (1991). The biological explosion at the Precambrian-Cambrian boundary. Evolutionary Biology, 25, 279–356.

Vickers, P. (2023). Identifying Future-Proof Science. Oxford: Oxford University Press.

Walcott, C. D. (1910). Cambrian geology and paleontology II. Abrupt appearance of the Cambrian fauna on the North American continent. Smithsonian Miscellaneous Collections, 57, 1–15.

Wood, R., Liu, A. G., Bowyer, F., Wilby, P. R., Dunn, F. S., Kenchinson, C., Hoyal Cuthill, J. F., Mitchell, E. G., & Penny, A. (2019). Integrated records of environmental change and evolution challenge the Cambrian Explosion. Nature Ecology & Evolution, 3, 528–538.