* T.J. Perkins is a graduate student at the University of Utah advised by Extinct co-founder Joyce Havstad. His research examines socio-cultural trends and their influence on scientific progress to extend avenues of philosophical analysis of the historical sciences. He writes…

One of my favorite places to hunker down and read during the summer is the patio in my backyard. Not only is the mountain view a delight, but the patio is situated under a large apricot tree which provides some cover from the hot Utah sun. In the first couple of weeks of July, however, this tree becomes hazardous for the unsuspecting reader: As the apricots ripen and transition from a deep green to a vibrant orange, they fall from their branches with a distinctive splat! This is both distracting and awfully messy at times.

Just as (the mythical version of) Isaac Newton was inspired by falling fruit, so too is this post! What I will argue here is that science resembles my apricots in one specific way. Both apricots and science can be said to “ripen.” I am not the first person to propose the metaphor of ripening as a way of understanding some aspects of science. Indeed, I have been inspired by a piece written by the paleontologist David Fastovsky, who invokes the metaphor to characterize some recent episodes in the history of paleobiology:

I consider three case studies: the paleobiology of the large theropod T. rex, the discovery of dinosaur maternity, nests, eggs, and embryos, and the dinosaur extinction. In each case, my thesis is that the work gained a foothold not only because the interpretations were supported by discoveries, but because the social climate was ripe for these kinds of inferences. (Fastovsky 2009, 240, emphasis added)

In this post, I am going to focus my attention on the third of Fastovsky’s case studies—the debate about the cause of the end-Cretaceous (K-Pg) mass extinction. (I know, I know.) In 1980, Walter Alvarez and colleagues published a now famous paper featuring novel evidence for a bolide impact around 66 million years ago, sparking a large interdisciplinary debate about the merits of impact as a cause of a mass extinction. I will show, using some recent history of the end-Cretaceous mass extinction, that features of ripening, like time, favorable conditions, and maturation, played a role in the receptivity of the impact theory, in addition to new evidence. More specifically, I will argue (as I have elsewhere argued), that we should understand the impact hypothesis for the K-Pg mass extinction to be the product of what I term “cultural readiness.” The giant splat made by the impact hypothesis was due in part to cultural factors, in addition to evidential ones.

Fastovsky’s aim is to explore the relationship between culture and the impact hypothesis. He begins with a puzzle: “despite the absence of any real data about dinosaurs and the pace of their extinction, the theory invoked a deus ex machina ending for dinosaurs (and other organisms) at the end of the Cretaceous. On the face of it, it was absurd—so why did it catch fire?” (Fastovsky 2009, 249). Why indeed. This is one of the questions at the heart of my current research.

To get a handle on this, it is useful to look at an earlier impact hypothesis, proposed several decades before what is now called “the Alvarez hypothesis.” This was the work of a biologist named Max Walker de Laubenfels, and instead of dropping like a ripe apricot it sank like a stone.

Impact before Alvarez

In 1956, De Laubenfels published an article titled, “Dinosaur Extinction: One More Hypothesis,” which he put forward a novel proposal for the cause of the dinosaur extinction. At the time, discussions of the dinosaur extinction were a free-for-all with many hypotheses vying for attention. De Laubenfels lists several of these, including the idea that dinosaurs went extinct because egg-eating species were on the rise, as well as the notion that dinosaurs suffered from pituitary complications (all of them?). The purpose of his paper was to float meteor impact as a hypothesis worthy of scientific attention—or at least as worthy of attention as its often ridiculous rivals.

De Laubenfels makes two arguments in his paper, which mirror those Alvarez et al. would make several decades later. First, he argues on the basis of the fossil record that the extinction was caused by a brief episode of intense heat that killed “exposed large animals in certain parts of the world” (208). Next, he argues that a meteor impact could produce such an episode. As evidence for the first claim, he examines a variety of taxa that thrived in the Cretaceous and asks if they made it through to the Paleogene (then the “Tertiary”). For example, De Laubenfels notices that birds, mammals, and some reptiles coexisted with the mosasaurs, pterosaurs, and dinosaurs, yet only the former taxa made it to the Tertiary (as evidenced by their extant descendant species). He posits that the dinosaurs, mosasaurs, and pterosaurs were just the kinds of animals that were most likely to be exposed to the killing heat, while mammals, smaller reptiles, and birds were likely to have been sheltered, or otherwise to have occupied a geographic range spreading beyond the reach of the brief heat wave.

He also observes that the flora of the Cretaceous did not undergo any radical change as a result of the K-Pg extinction. Early in the Cretaceous a revolution did occur with the evolution of angiosperms, but no such revolution took place at the opening of the Paleogene. Further, in the event of an intense brief heat the mature flora would have been decimated, but the roots and seeds may have survived to propagate once again as we see today in forest fires and areas destroyed by volcanoes.

De Laubenfels discusses some competing hypotheses including the two I mentioned above; but two others are likely to strike modern readers as more plausible, since they involve climate change. One holds that a global cooling event caused the dinosaur extinction, but De Laubenfels dismisses it based on the fact that other apparently tropical species like palm trees and snakes failed to perish. (There is also the problem that no stratigraphic evidence of cooling was then known.) The other proposed climate event was a prolonged episode of global warming, but Laubenfels dismisses this too since warming would have forced many species toward the poles, and again, there is little evidence for this.

These assorted inferences led De Laubenfels to conclude that an episode of intense heat rapidly engulfed an area of the earth large enough to decimate whole lineages, while leaving other lineages unscathed to persist into new periods. Here, then, is his first conclusion: that a brief, intense heat wave caused the extinction of the dinosaurs. But what was capable of producing such an event?

De Laubenfels identified a meteor strike as the most likely culprit. The dinosaurs, pterosaurs, and mosasaurs would have borne the brunt of the impact by immediate vaporization in the destruction zone or by the slower route of starvation. The smaller animals hidden within the destruction zone or able to live outside it could still feed on the seeds, roots, and other remains left by the impact, or so De Laubenfels infers.

De Laubenfels relies heavily on the data from recent meteor strikes, notably the strike at Tunguska in Russia to support the idea. On June 30, 1908, a meteor entered Earth’s atmosphere over Siberia and, as it approached the surface, exploded in the atmosphere creating a massive shockwave. Small fragments of the meteor were able to make craters between 50 and 200 meters in diameter. Reportedly, the glow from the impact could be seen from England, some 5,000 kilometers away.

De Laubnefels describes the destruction caused by the impact at Tunguska:

In a circle of 20 kilometers diameter, the trees still stood, although scorched and carbonized by intense heat. Here the shockwave struck nearly vertically. Around this, to a diameter of about 60 kilometers, the trees were prostrate as well as carbonized, tops directly away from the center of impact, flattened by the terrific wind. An area of nearly a thousand square kilometers had been heated so that all animal life perished. This included several families of human settlers, at least 1,500 reindeer, and numberless smaller animals. This is one of the most sparsely inhabited regions on earth, or the human deaths would have been far greater and worldwide attention demanded. (De Laubenfels 1956, 210)

Felled trees after the Tunguska event of 1908

He goes on to list other known meteor strikes on Earth including a much larger impact from 5,000 years ago in Arizona that left a crater 1.2 kilometers in diameter. The destructive potential of such an impact must have been immense. “It is only necessary to postulate a larger event of exactly the same kind, to implement the present hypothesis” (210).

Meteor Crater in Arizona (photograph by author)

* * *

We can now return to our guiding question from the beginning and ask, why was De Laubenfels’ hypothesis seemingly ignored, while Alvarez et al.’s hypothesis “caught fire” to use Fastovsky’s language? De Laubenfels himself reflects on why it might be the case that a meteor impact was taken by many to be so unlikely, writing,

In a similarly short time the world has been struck by a meteoritic cluster that devastated a large area in Siberia. If this area had been in the United States, American scientists would have been impressed. They would not regard danger from impact as being preposterous, an assumption which is now common. (De Laubenfels 1956, 212)

In what follows I want to address this suggestion and suggest that De Laubenfels is on the right track. His point is a counterfactual one about the contingency of events playing a role in making hypotheses more receptive to scientific audiences. In this case, had a meteor struck the United States prior to 1956, the scientific reception of the meteor strike hypothesis would have been a lot warmer. Instead, what happened was a rapid chilling of US Soviet relations, leading ultimately to a pervasive anxiety about the potential for nuclear annihilation.

The question still remains, as posed by Fastovsky: why did the Alvarez hypothesis “catch fire” and not De Laubenfels’ proposal?

Catching Fire: The Alvarez Hypothesis

So far what we have on the table is an hypothesis about a meteor-induced mass extinction at the end of the Cretaceous. We now know based on iridium anomalies, shocked-quartz, and a crater in Mexico that this event did in fact occur and likely resulted in the mass extinction that robbed us of any chance of seeing dinosauroids (Derek’s “What If” Prehistory). But, the question here is, why was De Laubenfels ignored if he was basically on the right track, even if his proposal contains some inaccuracies from our modern standpoint? Here is where I will bring us back to the ripening metaphor to talk about cultural readiness, drawing on some recent historical scholarship on the impact hypothesis.

As Fastovsky presents it, the impact hypothesis was debated and adopted in a cultural context that played a significant role in conditioning attitudes toward the impact idea. Specifically, Fastovsky recalls that,

In 1977, the movie Star Wars hit the theaters and rapidly attained cult status. “Star Wars” became the popular name of the antimissile defense program instituted by Ronald Reagan for the protection of the United States from intercontinental missile attacks. The idea, therefore, that destruction could come from above—even space—had reached popular radar [sic] as of the 1980s. (Fastovsky 2009, 250)

More recently, David Sepkoski in Catastrophic Thinking complements Fastovsky’s thesis with an in-depth exploration of the interaction between science and society in extinction debates, largely converging on the story that Fastovsky presents. Sepkoski describes in detail what he terms an “extinction imaginary,” “the complex web of values and beliefs associated with extinction at any given historical period.” As Sepkodki argues,

The way we understand extinction—the extinction imaginary of any given time—is ultimately tied to the way we conceive of the basic stability and security of the continued existence of our own species. (Sepkoski 2020, 9)

According to Sepkoski, the way scientists and society at large think about extinction changes through time based upon scientific discoveries, but also in response to shifting socio-cultural attitudes and preferences, and the interaction of these two domains. Echoing Fastovsky, he notes that it could hardly have been an accident “that catastrophic mass extinction became an object of scientific study and popular fascination at precisely the moment when we imagined a similar fate for ourselves” (Sepkoski 2020, 3). This resonates with De Laubnefels’ suggestion that, had the Tunguska event occurred over the US, the plausibility of an impact-induced mass extinction at the K-Pg might have enjoyed an earlier surge in popularity.

The extinction imaginary took a sharp turn after the Cuban Missile Crisis and ensuing flirtation with nuclear annihilation by the world’s superpowers during the Cold War. No longer was extinction viewed as a passive, inevitable force of nature; rather it came to be seen as a looming catastrophe brought about by human means. The popular culture of the post-war period reflected this anxiety. This had an impact on science too, as “it opened the door for a reconsideration of… extinction, as a potentially catastrophic threat of vital personal concern to every member of the human species” (Sepkoski 2020, 129). Sepkoski continues,

On the one hand, nuclear annihilation provided a vivid image of the reality of world-altering physical cataclysm; on the other, empirical recognition of the reality of geological mass extinctions, which began to take hold in the late 1950s, gave historical validation to doomsday prophecies. And as time went on, models of the mechanisms and ecological consequences of catastrophic extinctions became the basis for predicting the effects of nuclear and ecological catastrophes of the present or future. (Sepkoski 2020, 132)

Sepkoski shares with Fastovsky the view that the air of looming nuclear annihilation influenced the development and uptake of the K-Pg impact hypothesis. That extinction was no longer a distant possibility caused a re-think of the value of the future. Preserving this future at all costs became a popular social goal. The De Laubenfels hypothesis was proposed in an era without this kind of cultural readiness, and as such, saw no uptake. But as the conditions changed, so did the plausibility of the idea, leading to its ripening. The culture was becoming more ready to adopt this way of thinking.

Within the geosciences, another shift was taking place at the same time. For most of the twentieth century, geologists and paleontologists had given preference to gradual over catastrophic modes of change (a position sometimes called “gradualism” or “uniformitarianism”). However, beginning in the 1960s and ‘70s, a “new catastrophism” emerged as concerns about the destructive force of humans became increasingly salient. This was not a dominant perspective when the impact idea resurfaced in 1980. But neither was it dormant, and this likely influenced the reception of the Alvarez hypothesis. As Stephen Jay Gould writes, despite initial resistance,

the extra terrestrial impact theory soon proved its mettle in the most sublime way of all – by Darwin’s criterion of provoking new observations that no one had thought of making under old views. The theory, in short, engendered its own test and broke the straitjacket of previous certainty. (Gould 1995, 152)

We get a sense here that Gould is noticing the same kind of ripening or readiness that Fastovsky introduces, and De Laubenfels gestures at. It isn't that the new catastrophism lent evidential support to the hypothesis or anything like that. Rather, it helped to turn the hypothesis into a live option, capable of winning acceptance on its own merits.

* * *

So why did the Alvarez et al. hypothesis “catch fire” in the era of Rubik’s cubes and Reaganomics? Fastovsky’s claim is that the broader social context of the Cold War influenced the culture of the geosciences by making plausible the catastrophism that had been suppressed for so long. Along with a change in theoretical commitments came new standards of evidence and other changes to epistemic norms. Suddenly, the idea of a cataclysmic event was not only possible, but plausible and imaginable. This owed partly to the prevailing political climate and partly to the fact that evidence was viewed differently under a catastrophist than a gradualist framing. The widespread anxieties generated by the threat of nuclear war were probably the dominant factor. Scientists, as members of Cold War society, experienced these anxieties firsthand: something that influenced their theoretical commitments such that a hypothesis more in sympathy with catastrophism than gradualism became more enticing. Thus, cultural readiness helps to account for the ways in which the wider socio-political culture influences the culture of scientific disciplines, such that an initially implausible hypothesis can become a live option. New empirical evidence plays a role, sure, but culture also plays a significant part in determining the fate of ideas.

The broader lesson here is that facets of science like timing, maturation, and context are all philosophically relevant when thinking about how science progresses and is deemed successful. The ripening metaphor may be imperfect, but it points to some aspects of the history of science that must be accounted for when characterizing the trajectory of scientific ideas.

References

Alvarez, L.W., Alvarez, W., Asaro, F., and Michel, H. V. 1980. Extraterrestrial cause for the Cretaceous-Tertiary extinction. Science 208:1095–1108.

De Laubenfels, M.W. 1956. Dinosaur extinction: one more hypothesis. Journal of Paleontology 30:207-212.

Fastovsky, D.E. 2009. Ideas in dinosaur paleontology: resonating to social and political context. In D. Sepkoski and M. Ruse (eds.) The Paleobiological Revolution. Chicago: University of Chicago Press.

Gould, S.J. 1995. Dinosaur in a Haystack: Reflections in Natural History. New York: Three Rivers Press.

Perkins, T.J. 2023. Culture’s impact on the historical sciences. Journal of the Philosophy of History 17:31–52.

Sepkoski, D. 2020. Catastrophic Thinking: Extinction and the Value of Diversity from Darwin to the Anthropocene. Chicago: University of Chicago Press.

Impact before Alvarez

Catching Fire: The Alvarez Hypothesis

References

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