Ancient apocalypses are so disappointing

Earth has suffered some pretty major catastrophes during the seven million years since the hominin lineage got its start. Humans live on a changing planet today, but ancestral hominins experienced some shocks that we haven't seen in recorded history. Here's a small sampler:

• The Messinian Salinity Crisis. Starting just under six million years ago, the Mediterranean Sea was cut off from the Atlantic Ocean, causing a 640,000-year-long evaporative period during which the Mediterranean had a much lower level and deposited a kilometer-thick bed of salt at its bottom. This abruptly ended with the opening of the Strait of Gibraltar and the rapid infill of the entire basin.
• The Eltanin impact. Around 2.5 million years ago, a large meteorite struck somewhere in the southern ocean, causing massive tsunamis and possibly enduring atmospheric effects.
• Meteorite impacts resulted in several craters of 10 km or more during the Pleistocene, including Bosumtwi, Ghana (1.1 million years), Zhamanshin, Kazakhstan (900,000 years), and Pantasma, Nicaragua (815,000 years). The Australasian tektite strewnfield resulted from another large impact around 780,000 years ago. Any of these may have had global climate effects.
• The Yellowstone caldera. A series of massive eruptions of the Yellowstone geological hotspot include events 2.1 million, 1.3 million, and 640,000 years ago. The largest of these 2.1 million years ago ejected 2450 cubic kilometers of magma. More than a half dozen supervolcano eruptions are known to have occurred during the span of human evolution. All of these would have had immediate atmospheric effects and longer-term climate effects as ashfalls increased the reflectivity of Earth's surface.

Each of these events was a very big deal, and scientists have naturally wondered how they mattered to climates and hominin populations. But despite their apocalyptic appearance, it has been extremely hard to trace their effects.

Consider the Eltanin impact. Traces of its tsunamis may be found in South America, Antarctica, New Zealand and Australia, and the footprint of the impact is thought to be present in the deep ocean 1500 km from South America. James Goff and coworkers in 2012 modeled the event, noting that it happened very near the time we now recognize as the boundary between the Pliocene and Pleistocene epochs. That seems like fascinating timing considering the long-term changes in global climates marked by this boundary. But those climate changes were extremely long-term, and—big as it was—the Eltanin event may not even have made a difference for decades.

Scientists' attempts to look at the possible effects of other catastrophes are just as speculative. A lower Mediterranean would have changed some of the dispersal possibilities for the earliest hominins, but we've never yet found any hominin fossils from this era in the regions neighboring the Mediterranean. No doubt a meteorite gouging out a 10-kilometer crater a million years ago would have induced a serious cold spell, but hominin populations were sparse and we have hardly any ability to assess their numbers or range dynamics.

The truth is, we don't know what effects such events may have had because we know so little about the species that existed at any of these times, even into the early Middle Pleistocene.

We have a lot more evidence about climates and hominin populations of the Late Pleistocene and Holocene, within the last 130,000 years. The biggest catastrophic event during that time was the eruption of Mount Toba, Indonesia, around 74,000 years ago. Toba ejected some 2800 cubic kilometers of magma, or even more by some estimates. During the 1990s, geologists and some archaeologists speculated that this massive eruption may have caused a decades-long volcanic winter. They suggested that human populations in Africa and Eurasia had suffered a bottleneck, taking them down to a few thousand individuals, and proposed that survivors had dispersed rapidly from Africa sometime after the event.

Over the last 25 years, geologists, geneticists, and archaeologists have shown that the Toba event did not have such massive impacts on human populations. As I wrote several years ago, the Toba bottleneck simply didn't happen. Multiple lines of evidence, including high-resolution cores from ice sheets and lake bottoms, show that the climate impacts were much less than 1990s-era geologists imagined. Better archaeological and genetic records now show that populations of humans and other hominins survived without observable cultural or demographic changes.

That doesn't mean that the supereruption was a walk in the park. Surely it really did make the world a much less pleasant place for several years.

But neither archaeology nor genetics provide enough resolution to see stress, mortality, or changing environments on the scale that the Toba eruption had. We probably have not found a single bone or tooth from any hominin individual who was alive at the time of the eruption, much less a sample of individuals that might provide evidence of health or mortality. What archaeologists can look for is signs of abrupt change in cultural evidence—layers flanking the time of eruption with different archaeological material. This they have not found; to the contrary, in the few places where evidence occurs both before and after the Toba eruption, there is no evidence of change. But such evidence is not known from very near Toba itself, leaving any localized effects on hominin populations entirely invisible.

It's sobering to realize that the largest catastrophic event of the last 100,000 years left no clear trace of any effects on humans, other hominins, or other terrestrial animals. Toba may have seemed apocalyptic, but we haven't seen anything that it ended.

That hasn't stopped scientists from looking for possible catastrophic explanations for changes in the past. For example, in early 2021 a team led by the geneticist Alan Cooper proposed that a weakening of the Earth's magnetic field around 42,000 years ago may have had atmospheric effects resulting in megafaunal extinctions and the final extinction of the Neanderthals. The authors even suggested that the atmospheric effects caused human populations to take refuge in caves, leading to a florescence in cave painting traditions.

Scientists outside the team of authors, including me, reacted to the paper by pointing out that neither extinctions nor evidence for cave use were concentrated at 42,000 years ago. The geological and archaeological records from 42,000 years ago are better than the time of the Toba eruption in many ways, with many more skeletal and archaeological remains and dense arrays of radiocarbon dates for many sites. As I wrote, “Megafaunal extinction in Australia, Neanderthal extinction in Eurasia, and the beginnings of pigment marking in caves all involved factors that began thousands of years before the Laschamps excursion or associated geomagnetic field changes.”

What makes so many scientists gravitate toward catastrophic explanations for long-term events? The idea of sudden, catastrophic change in Earth systems was out of fashion through most of the twentieth century. The 1980 work of Luis and Walter Alvarez, who provided evidence for a large meteorite impact that coincided with the Cretaceous-Paleogene boundary 65 million years ago, revived the idea that other mass extinctions might have been shaped by singular events. This was just the most well-known of other geological observations of rapid change, especially the massive floods that followed the melting of the great ice sheets near the end of the Pleistocene. The paleontologist Derek Ager in 1985 called this growing awareness of singular events The New Catastrophism: an assumption that  many patterns in geology and the history of life might have resulted from massive forces operating in a geological instant.

The reason I was thinking about this topic today was a new paper by Vance Holliday and twelve coauthors in the journal Earth Science Reviews. The group has put together a massive overview of evidence related to the idea that some meteorite or comet had a major effect on the Earth's climate and human populations around 12,900 years ago. That idea is widely known as the “Younger Dryas Impact Hypothesis”, with the name Younger Dryas referring to the 1200-year-long period of colder climate that started around 12,900 years ago. Holliday and coworkers spend 293 manuscript pages reviewing evidence that shows that the meteorite idea doesn't hold water.

The idea of a meteorite or comet impact 12,900 years ago has gotten a lot of press since 2007 when the idea was introduced by the chemist Richard Firestone and twenty-five coauthors. The hypothesis is that an extraterrestrial impact caused global cooling, which had long-term impacts on humans and other species, including the end of the Clovis technocomplex in North America and extinctions of some megafaunal species. The initial work proposed that signs of an impact are present in archaeological sequences spanning the beginning of the Younger Dryas, including an abundance of charcoal, tiny spheres of magnetic material, nanodiamonds, and fullerene molecules containing extraterrestrial helium. They proposed that these were present at many sites within the context of a narrow stratigraphic horizon of dark or gray material which they termed the “black mat”. Firestone and collaborators also found such markers within rounded depressions in the coastal plain of the southeastern United States, known as “Carolina Bays”, although they also pointed to results that exclude these landscape depressions as possible impact-created features. The authors suggested several reasons why an impact crater from 12,900 years ago has never been found, including the possibility that the impactor was a comet or other “soft” body that struck the Laurentide Ice Sheet.  

Other scientists soon raised questions about these observations. Some took new samples to attempt to reproduce the results showing nanodiamonds and other possible impact markers were present in the key layers. These attempts had varied outcomes; skeptical researchers didn't find the same pattern of evidence, while proponents of the impact hypothesis suggested that the skeptics had sampled the wrong locations. Some of the images of purported carbon microspherules were later identified as structures produced by soil fungus. Many skeptics challenged the stratigraphic interpretations by Firestone and coworkers, in particular the nature of the “black mat” and the claim that the Younger Dryas originated all at once, in an single event. Meanwhile, proponents of the impact hypothesis have claimed to find evidence on the other side of the world, from Abu Hureyra, Syria (excavated in the 1970s and now under a reservoir).

These scientific debates about the Younger Dryas Impact Hypothesis have gone on for 16 years and I doubt they will end anytime soon. The successive arguments have taken many twists, turns, and scenic detours. In their long manuscript just released, Holliday and coworkers provide long tables of claims and counterclaims, and anyone writing a history of our understanding of extraterrestrial impacts and their effects on Earth's climate will benefit greatly from these lists. On balance I find the critical perspective compelling: a lot of the claimed evidence to support an impact was misidentified or did not replicate, and the supposed effects of the Younger Dryas cold period on human populations have been exaggerated.

Still, I just can't imagine that this work will quiet anyone who thinks a comet hit the earth 12,900 years ago. A review from 2021 by Martin Sweatman gives an account of evidence that tends to confirm that idea of a Younger Dryas impact, and while Holliday and coworkers take on each of Sweatman's points, there probably will be rebuttals for many of their criticisms.

Which brings me back to the point I started with. The known catastrophic events of the last seven million years seem to have produced very little evidence of impacts on hominins or other species. For the most part, that's because paleoclimate, genetic, and archaeological evidence doesn't have the resolution to show change on the short timescales necessary. The one unquestionable catastrophe with multisite archaeological records flanking its date is the Toba eruption, and it didn't create change in archaeological or genetic records.

The paleoclimate, archaeological, and ancient DNA records are stronger in the period around 12,900 years ago than any earlier times. Each of these records manifests several visible changes within particular regions of the world. From the archaeological perspective, some early villages in the Fertile Crescent really did burn, and the Clovis technocomplex did come to an end. From the genetic perspective, some Y chromosome lineages had massive demographic growth in this period of time.

But those events unfolded over centuries, and were not unique: every archaeological tradition and village occupation ends sometime, and each of today's common Y haplogroups has a common ancestor, all of which lived at varied times. What I would love to see: any evidence that the events in the neighborhood of 12,900 years ago were unusual, and any evidence that unusual events were clustered in the short time following 12,900 years ago.

In other words, the evidence of an apocalypse should look apocalyptic. From the human perspective, the Younger Dryas doesn't seem very much like a global catastrophe. As in the case of the Toba eruption, that doesn't mean the Younger Dryas was an ideal time for everyone. It's just that the kinds of events that happened—some good and some bad for particular communities—were also happening beforehand and afterward.

Notes: A great review of the science concerning Mount Toba's effects on climate can be found in the 2018 article by Chad Yost and collaborators. For meteorite impacts, a 2020 review by Martin Schmieder and David Kring gives a list of large events during Earth's history with visible impact craters or ejecta.

I'm pointing to the reviews by Sweatman (2021) and Holliday and coworkers (2023) because they are the most recent and much information can be found in both of them. Readers interested in the details can dive into them there.

References

Ager, D. (1995). The New Catastrophism: The Importance of the Rare Event in Geological History. Cambridge University Press.

Cooper, A., Turney, C. S. M., Palmer, J., Hogg, A., McGlone, M., Wilmshurst, J., Lorrey, A. M., Heaton, T. J., Russell, J. M., McCracken, K., Anet, J. G., Rozanov, E., Friedel, M., Suter, I., Peter, T., Muscheler, R., Adolphi, F., Dosseto, A., Faith, J. T., … Zech, R. (2021). A global environmental crisis 42,000 years ago. Science, 371(6531), 811–818. https://doi.org/10.1126/science.abb8677

Firestone, R. B., West, A., Kennett, J. P., Becker, L., Bunch, T. E., Revay, Z. S., Schultz, P. H., Belgya, T., Kennett, D. J., Erlandson, J. M., Dickenson, O. J., Goodyear, A. C., Harris, R. S., Howard, G. A., Kloosterman, J. B., Lechler, P., Mayewski, P. A., Montgomery, J., Poreda, R., … Wolbach, W. S. (2007). Evidence for an extraterrestrial impact 12,900 years ago that contributed to the megafaunal extinctions and the Younger Dryas cooling. Proceedings of the National Academy of Sciences, 104(41), 16016–16021. https://doi.org/10.1073/pnas.0706977104

Goff, J., Chagué-Goff, C., Archer, M., Dominey-Howes, D., & Turney, C. (2012). The Eltanin asteroid impact: Possible South Pacific palaeomegatsunami footprint and potential implications for the Pliocene–Pleistocene transition. Journal of Quaternary Science, 27(7), 660–670. https://doi.org/10.1002/jqs.2571

Gvirtzman, Z., Heida, H., Garcia-Castellanos, D., Bar, O., Zucker, E., & Enzel, Y. (2022). Limited Mediterranean sea-level drop during the Messinian salinity crisis inferred from the buried Nile canyon. Communications Earth & Environment, 3(1), Article 1. https://doi.org/10.1038/s43247-022-00540-4

Hawks, J. (2021). Comment on “A global environmental crisis 42,000 years ago.” Science, 374(6570), eabh1878. https://doi.org/10.1126/science.abh1878

Holliday, V. T., Daulton, T. L., Bartlein, P. J., Boslough, M. B., Breslawski, R. P., Fisher, A. E., Jorgeson, I. A., Scott, A. C., Koeberl, C., Marlon, J., Severinghaus, J., Petaev, M. I., & Claeys, P. (2023). Comprehensive refutation of the Younger Dryas Impact Hypothesis (YDIH). Earth-Science Reviews, 104502. https://doi.org/10.1016/j.earscirev.2023.104502

Rochette, P., Alaç, R., Beck, P., Brocard, G., Cavosie, A. J., Debaille, V., Devouard, B., Jourdan, F., Mougel, B., Moustard, F., Moynier, F., Nomade, S., Osinski, G. R., Reynard, B., & Cornec, J. (2019). Pantasma: Evidence for a Pleistocene circa 14 km diameter impact crater in Nicaragua. Meteoritics & Planetary Science, 54(4), 880–901. https://doi.org/10.1111/maps.13244

Schmieder, M., & Kring, D. A. (2020). Earth’s Impact Events Through Geologic Time: A List of Recommended Ages for Terrestrial Impact Structures and Deposits. Astrobiology, 20(1), 91–141. https://doi.org/10.1089/ast.2019.2085

Sweatman, M. B. (2021). The Younger Dryas impact hypothesis: Review of the impact evidence. Earth-Science Reviews, 218, 103677. https://doi.org/10.1016/j.earscirev.2021.103677

Yost, C. L., Jackson, L. J., Stone, J. R., & Cohen, A. S. (2018). Subdecadal phytolith and charcoal records from Lake Malawi, East Africa imply minimal effects on human evolution from the ∼74 ka Toba supereruption. Journal of Human Evolution, 116, 75–94. https://doi.org/10.1016/j.jhevol.2017.11.005