Two paleontology outsiders, a father and a son, changed the narrative around mass extinctions. But did they remake paleontology?
At first, Luis Alvarez could not understand why his son Walter was so interested in rocks.
The elder Alvarez was a particle physicist who spent his days inventing devices, revealing undiscovered subatomic particles, using cosmic rays to probe the interiors of Egyptian pyramids, and putting forward theories that fundamentally transformed the way physicists thought about matter.
It was Walter Alvarez’s mother, Geraldine Alvarez, who brought out her son’s fascination with rocks through frequent road trips to see rock formations.
While Luis was initially dubious about geology, Walter’s interest in ancient rocks would spark a scientific collaboration between father and son that would last over a decade and yield a theory that would shake up geology, paleontology, and pop culture.
In the 1960s, Walter Alvarez was a grad student at Columbia’s Lamont Geological Observatory, the recent epicentre of the Plate Tectonics Revolution. Excitement over the new paradigm was still palpable. Shortly after finishing his PhD, he traveled to the Apennine Mountains in Italy, hoping to gather geomagnetic data that would reveal the Mediterranean’s tectonic history.
A deceptively simple-looking layer of clay in a rock formation near Gubbio, Italy caught his attention. The limestone below it abounded with fossils of large, hard-shelled plankton called forams. Above the 65-million-year-old clay, there were only microscopic forams. Something had decimated the plankton around the time the clay layer formed. That date also happened to mark the boundary between the dinosaur-dominated Cretaceous and the Tertiary.
“The more I thought about the KT [Cretaceous-Tertiary] boundary, the more it fascinated me,” Walter later wrote in his 1997 book T. rex and the Crater of Doom. “Much of the work we do as scientists is about filling in the details of about matters that are basically understood already, or applying standard techniques to new specific cases. But occasionally there is a question that allows an opportunity for a really major discovery.”
The Cretaceous die-off as one such mystery, so he set off to solve it. Soon, he was discussing the problem with his dad via long distance phone calls and brainstorming ways to find out what ended the dinosaurs’ world.
In the early 1970s, proposing sudden cataclysms to explain fossils was considered a bit gauche. Geology’s dominant paradigm was Uniformitarianism, which held that geologic processes, such as erosion and mountain formation, were slow and steady. Natura non facit saltum — “Nature does not make sudden leaps!” — was its motto. Most paleontologists thought that the dinosaurs and other Cretaceous creatures had died off gradually, over a span of millions of years.
Uniformitarian doctrine asserted sedimentary rocks formed at a steady rate, so geologists estimated the age of stones and fossils based on their depth in sedimentary deposits. Writings by early Uniformitarian Charles Lyell strongly influenced Darwin’s work on evolution, and plate tectonics fit with the slow and steady hypothesis. The strength of this paradigm in the 1970s and 1980s meant that geologists were still distrustful of hypotheses featuring sudden catastrophes, because, in some ways, it hearkened back to apocalyptic and decidedly unscientific religious explanations for natural phenomena. Such hypotheses were often labeled “Catastrophism”.
But slow, steady Uniformitarianism didn’t explain the sudden foram die-off in the Gubbio rocks.
The Alvarez collaboration kicked into high gear when Walter landed an assistant professorship at Berkeley, where his father taught physics. Neither Alvarez was a paleontologist per se, but that didn’t stop them from digging into the KT die-off.
Measuring concentrations of platinum-group metals in rocks from the KT boundary was the physicist’s idea. Platinum group elements, such as iridium and platinum, are incredibly rare at the Earth’s surface. The traces of these metals that are near Earth’s surface come from slow, gradual deposition space dust from meteorites. Maybe, the Alvarezes reasoned, the amount of iridium in the KT clay layer would reveal whether the clay layer had been deposited suddenly or slowly. It was a very Uniformitarian plan.
But when they teamed up with trace-metal-measuring specialists from Berkeley’s chemistry department, the results seemed impossible. Iridium levels in the KT clay were about 30 times higher than expected. The clay, these results suggested, was full of space dust.
A 1971 paper had proposed that a massive dose of radiation from a nearby supernova explosion could have killed the dinosaurs but hadn’t gotten much traction due to the total absence of geological evidence for it. Luis Alvarez pointed out that a supernova would also have released massive amounts of plutonium isotope 244, which could have fallen to Earth and left detectable traces in the clay. (which is a substance that is almost enitrely man-made, but a small amount has been found on Earth having arrived from outer space), along with the iridium. If they found plutonium 244 in the KT boundary clay, it would support the supernova hypothesis. Once again, they teamed with Berkeley chemists Frank Asaro and Helen V. Michel.
Walter recounts hearing the results rather dramatically in his book: “As the light of of dawn was gathering outside, the results were finally ready, and…And there was plutonium-244 in the KT boundary clay!”
“Dad and I were nearly jumping up and down with excitement — a nearby supernova had killed the dinosaurs... Helen and Frank were too tired to do more than nod with satisfaction,” he adds. The elder Alvarez wanted to publish right away, but Walter was skeptical. His caution proved founded.
But when Michel and Asaro retested the samples, they found no trace of plutonium.
Disappointed but undeterred, the Alavarezes brainstormed again and realised that an impact from an asteroid or a comet might deposit iridium but not plutonium. However, an impact, even a large one, seemed like it would only cause a local extinction. Without a clear cause of global death, impact wasn’t much good as an extinction-causing hypothesis.
Later, Luis remembered reading about the 1883 Krakatoa eruption, where the volcano released enough dust and ash to turn the sky dark at mid-day. Maybe, the physicist reasoned, a large comet or asteroid would produce an even bigger dust cloud, which could block out the sun. Without sunlight to feed plants, even ecosystems far from the impact site would be thrown into chaos, with death cascading down the food chain in the absence of life-giving plants. The, lack of photosynthesis caused by this dust was key to their hypothesis about the mass extinction event.
Finally, in June 1980, the Alvarezes and their co-authors published their iridium findings and impact hypothesis in Science. The paper set off an immediate scientific frenzy, and the impact hypothesis, with its proposed sudden and fiery end for the mighty “terror lizards”, became an immediate media darling.
The paleontological community remained split on the issue throughout the 1980s. A widely-cited 1982 analysis estimated that about three-quarters of megafaunal species on Earth died in the KT extinction. But when a New York Times reporter conducted an informal poll of 118 paleontologists at a conference in 1985, almost a third maintained there hadn’t been a catastrophic extinction at the end of the Cretaceous at all.
Interestingly, 90% said that a large asteroid or comet might have collided with Earth 65 million years ago, but only 4-5% thought the asteroid impact was the likely “cause-of-death”.
Many objectors saw impact theory as merely a headline-grabbing scenario concocted by outsiders. “Despite their ignorance, the geochemists feel that all you have to do is crank up some fancy machine and you've revolutionised science,” paleontologist Robert T. Bakker opined to The New York Times in 1985. “In effect, they're saying this: 'We high-tech people have all the answers, and you paleontologists are just primitive rock hounds.'''
In actuality, Luis Alvarez thought paleontologists were “more like stamp collectors”. His son frequently tried to keep peace between camps.
But impact theory’s biggest problem was the lack of an impact site. None of the known large impact craters on Earth were the right age.
Some impact proponents thought the impactor landed in the ocean, in which case it might already have been melted down and subsumed into the planet’s magma. If that was so, no one would ever find it, but scientists kept looking.
In the end, only one Alvarez lived to see impact theory’s confirmation. Luis Alvarez died of cancer in 1988, just three years before the crater’s identification. Walter wrote, “For the past ten years, he had been at the center of some of the most exciting research on Earth’s history...Dad would have loved the discovery of the Crater of Doom.”
The son continued to search, but it was a young Canadian geologist named Alan Hildebrand who pieced together a set of geological anomalies. These sites, including torn-up ancient creek beds in Texas and strange rock formations in Haiti, were places where the massive tsunami created by the impact left scars on the rock.
Hildebrand also realised that the Yucatán peninsula in Mexico might have been the tsunami’s point of origin. He dug through the literature and found an obscure 1981 abstract by geologists Glen Penfield and Antonio Camargo Zanoguera. They proposed that the Yucatán’ might be home to an impact crater but didn’t publish a formal paper. In 1991, Hildebrand, along with several co-authors including Penfield and Camargo, finally published a paper proposing the Yucatán’’s Chicxulub crater as the smoking gun.
The 1991 paper touched off another scramble to confirm or deny the impact site’s validity. There are still disputes over how important the impact was and whether other causes, such as a super-volcano in India, contributed to the mass extinction, but the Chicxulub Crater is widely accepted as the site of a late Cretaceous impact.
Impact theory certainly addressed previously unexplained observations, provoked new questions, and enabled further research. However, solving the KT extinction mystery didn’t change the line-up of suspected causes behind mass extinctions as dramatically as one might expect. Super-volcanoes and climate change still remain the prime suspects for other mass extinctions such as the Permian-Triassic “Great Dying”.
In Structure of Scientific Revolutions, Kuhn draws an admittedly porous and blurry distinction between normal science, where scientists solve existing puzzles, and revolutionary science, where the questions scientists ask fundamentally change. Though Impact Theory had plenty of behind-the-scenes drama, the question remained the same from start to finish: What killed those charismatic Cretaceous megafauna?
So did impact theory fundamentally transform geology and paleontology? Or was it an especially influential normal science puzzle that yielded a startling answer? Paleontologists were reluctant to embrace a catastrophic explanation, and I’m also not convinced that Catastrophism has actually become a governing principle for modern paleontology and earth science.
I’d argue that the most revolutionary aspect of Impact Theory wasn’t its Catastrophism, but rather its interdisciplinarity. Walter Alvarez himself highlights this point in describing a 1981 conference where physicists and geologists gave each other tutorials. “The first Snowbird meeting gave birth to a unique scientific culture, in which a scientist in one field is not afraid to ask the most basic questions about a remote discipline...What might otherwise have been considered scientific trespassing became the expected thing to do.” Impact theory papers integrated geologists’ and paleontologists’ meticulous and systematic cataloguing, physicists’ models, astronomers’ observations of comets and meteorites, and chemists’ careful analyses of rocks.
Impact theory wasn’t the only big scientific hypothesis promoting collaboration between these disciplinary silos — biogeochemical research on the pre-Cambrian Earth and controversial Gaia Theory were also prominent in the 1980s — but it was the one that literally lent itself to movie posters. And unlike Impact Theory, the trend toward increased interdisciplinarity in geology and paleontology has gone largely unchallenged.
Impact Theory was certainly a groundbreaking idea that changed the way scientists think about mass extinction, but it was also part of a much larger revolution in how we study Earth’s ancient history.
Edited by Tessa Evans and Matthew Soleiman