26 - The Late Pleistocene extinction
Overkill, Overgrill, Overchill, or Overill
This substack series has largely used a uniformitarian approach - the present is the key to the past - to explain the events of the end of the last glacial period. Up to this point only the Oldest Dryas Lake Missoula megafloods can truly be classified as catastrophic, and even these had a fairly uniformitarian cause - basal glacier melting due to an increase in geothermal heat - and were only catastrophic due to a geographical fluke: the North American meltwaters had to cross 2 drainage divides to flow into the Pacific, and ran into an ice dam along their way.
But a uniformitarian approach cannot explain the Late Pleistocene Extinction (LPE), whereby 77 American genera in total - 3/4 of all American mammals weighing 100 pounds or more - went extinct over a period of a few thousand years. A smaller, size-based extinction happened around the same time in Africa and Eurasia, while in Australia all the large mammals vanished 30,000 years earlier. The main question is of course why these genera disappeared. But other questions present themselves: why then? why were these extinctions fairly drawn out affairs? why did some survive?
What happened in the Americas?
Dr. D. Grayson, one of the most prolific scientists investigating the LPE, created an excellent, absorbing video, which is well worth watching if you have an hour to spare.
Large‐bodied animals such as mammoths, camels, and saber‐tooth cats still roamed the Americas during the Late Pleistocene, but went extinct some time before 11.4 ka. There is still much debate on the details of the timing and causes of the extinctions. For example, Faith and Surovell[1] claim:
The extinction chronology of North American Pleistocene mammals therefore can be characterized as a synchronous event that took place 12,000–10,000 radiocarbon years B.P [13.8 ‐ 11.4 cal ka]. Results favor an extinction mechanism that is capable of wiping out up to 35 genera across a continent in a geologic instant.
while Grayson and others claim 37 North American genera went extinct over a slightly longer period.
What happened in Australia?
A similar catastrophic extinction of Australian genera occurred roughly 30 ka before the American extinction, resulting in the complete loss of all animals heavier than ~100 kg, together with all megafaunal reptiles (6 genera)[2]. This extinction event coincided with the Laschamps geomagnetic excursion as well as the (Eurasian) demise of the Neanderthals.
Proposed causes of the Megafaunal extinction
The most often cited causes of the megafaunal extinction are:
Overkill: human hunters
Overchill: climate change
Overgrill: a comet airburst
Note that leading expert Grayson (video) doesn’t believe in any of these explanations.
Overkill: human hunters did it
It beggars belief that a small population of humans could, or would want to, wholesale slaughter multiple species in a short period over entire continents. Channell & Vigliotti[2] :
The human population of Australia at ~40 ka was likely no more than a few tens of thousands, with no evidence of an increase in population at this time. According to Webb (2013), "the “overkill” hypothesis “is more sensational, granted, but the arguments are unrealistic and the evidence for it, at least in Australia, is non‐existent.”
Still, many studies argue the case for “overkill” in North America as the extinctions around ~13 ka concur with the brief Clovis‐tool period (13,050 - 12,750 BP). However, the demise of the North American herbivores began around ~14.5 ka[2], ~1.5 ka prior to the Clovis culture, and most mammals were already firmly in decline by Clovis time. Grayson claims Clovis tools have only infrequently been found in association with megafaunal remains, i.e. no “kill sites” have been found, and that less than half of the extinct animals co-existed with the Clovis hunters: only 17 out of 37 extinct genera were still alive when the Clovis hunters arrived in North America. The overkill hypothesis cannot explain why Eurasian mammals - albeit to a lesser extent - disappeared around the same time, nor the Late Pleistocene disappearance of 10 species of birds in North America[3].
Overchill: climate change did it
Another popular yet very likely inadequate explanation is that colder temperatures in parts of the Northern Hemisphere during the Younger Dryas (YD) caused animals to die out. Grayson claims there is no coherent and compelling argument that climate change caused the extinctions. The greatest proportion of North American extinctions occurred in the US southeast, an area that previous posts have demonstrated warmed rather than cooled during the YD. There were no physical barriers to large mammals migrating from cooling Pennsylvania to warming Florida, or vice versa. The North American herbivores started declining around ~14.5 ka, during the relatively warm Bølling‐Allerød, ~1.7 ka before the Younger Dryas[2], and only 17 out of 37 genera were still alive at the start of the YD. In addition, climates had been changing all through the Last Glacial Period, so why would the YD climate change be any different?
Overgrill: a cosmic impact did it
This theory relies on all of the extinctions happening at exactly the same time, while numerous studies[1,2,3] indicate the extinctions were multi-centennial to multi-millennial, long, drawn out affairs (Grayson). The Australian megafaunal extinction happened roughly 30 ka before the American extinction, and would therefore require a second unproven cosmic impact. In addition, the usual arguments against the so-called “Younger Dryas Impact Hypothesis” (YDIH) can be invoked:
No large impact crater dating to the early YD has been found
Several of the YDIH sites, e.g. Blackville, South Carolina, lie in areas that warmed during the YD, indicating any cosmic impact did not cause climate change there.
A cosmic impact should have significantly raised marker molecules, such as NO3, in North American ice cores, which to date not been observed[3]
the North American herbivores started declining around ~14.5 ka, during the Bølling‐Allerød, so ~1.7 ka before the Younger Dryas[2].
Adding Overill: space radiation did it
LaViolette[3] argues a super-sized solar particle event (SPE) around the start of the YD overwhelmed the geomagnetic field to deliver a lethal radiation dose of up to 6 Sv to the Earth's surface, thereby terminating numerous genera. His explanation however does not take into account the long, drawn-out nature of the extinctions, the decline of the North American herbivores from 14.5 ka onwards, why the SPE would show a preference for large animals and some birds, nor the Australian megafaunal extinction 30 ka earlier.
Channell & Vigliotti[2] present a similar, yet more comprehensive explanation. The American and Australian die‐offs are contemporaneous with minima in Earth's magnetic field strength. Earth surface-incident solar Ultra Violet Radiation (UVR) flux was very likely higher during geomagnetic minima, when Earth’s low geomagnetic field strength was inadequately deflecting the Earth-incident Galactic Cosmic Ray (GCR) flux that generally depletes atmospheric ozone. Atmospheric ozone in turn blocks much of the harmful UVR from reaching Earth’s surface.
Earth-incident UVR disassociates oxygen molecules (O2) into oxygen radicals that combine to form ozone (O3), which in turn absorbs UVR. Ozone‐depleting agents (such as nitrogen oxides) are produced naturally by highly energetic space radiation particles (SPE, GCR) that are able to penetrate the geomagnetic field. High UVR flux in turn has been correlated to higher genetic mutation rates and evolution[2]:
UVR reaching Earth's surface influenced mammalian evolution with the loci of extinction controlled by the geometry of stratospheric ozone depletion.
C&V also propose a hypothesis on how some species survived the extinction events:
UVR causes two classes of DNA lesions: cyclobutane pyrimidine dimers and 6‐4 photoproducts. Both lesions distort DNA structure, introducing bends or kinks and thereby impeding transcription and replication […]. One “hot spot” for UV‐induced damage is found within a commonly mutated oncogene TP53 [..], which in normal function has an important role in tumor suppression.
Their theory can be summarized as: Low geomagnetic field strength -> more GCR → lower ozone → more UVR → more mutations & more cancer among some genera
Space Radiation Health Hazards
NASA has identified four generic categories of risk from space radiation exposure:
carcinogenesis
degenerative tissue risk (e.g. cardiovascular disease)
degeneration risks to the central nervous system (CNS)
acute radiation poisoning
whereby more leads to worse. Their categories mainly deal with the hazards encountered by astronauts in space: UVR/mutations are not relevant in this regard as astronauts can successfully be shielded from harmful electromagnetic radiation. High-energy SPE & GCR particles however are a different matter, and pose a formidable problem for space travel outside the protection of Earth’s magnetic field.
The protection provided by Earth’s magnetic field and atmosphere
The geomagnetic field deflects low-energy charged particles (e.g. solar wind) around Earth and its atmosphere: the Lorentz force diverts a charged particle in a direction perpendicular to its movement as well as the local geomagnetic field direction. The geomagnetic field however can only inadequately deflect high energy particles, so cannot prevent them from entering and interacting with Earth’s atmosphere.
The intensity of primary GCR and SPE particle fluxes decreases with altitude due to the particles’ interactions with atmospheric molecules and the geomagnetic field: at present only allows the highest-energy particles > 1 GeV reach Earth’s surface.
Most of the primary Earth-incident space radiation particles collide with atmospheric molecules to generate an “Air Shower” of secondary cosmic ray particles (above). A few of these secondary cosmic ray particles are detrimental to megafaunal health in high doses, though some are worse than others. Most Earth-incident particles are relatively harmless 3rd to 7th generation cascade particles, though some - e.g. protons - interact with materials through Coulombic (charge–charge) interactions to create dense linear “tracks” of ionizations, while others - e.g. neutrons - can induce radioactivity in most substances they encounter, such as bodily tissues.
The bottom line is that a significant increase in primary or secondary particles due to a weak geomagnetic field will harm fauna and flora.
Lessons from the saga of the Saiga
Grayson mentions (video) that the Saiga survived the Late Pleistocene extinction event (but not in North America) only to be recently decimated by illness. Wikipedia states:
In May 2015, uncommonly large numbers of saigas began to die from a mysterious epizootic illness suspected to be pasteurellosis. Herd fatality is 100% once infected, with an estimated 40% of the species' total population already dead.
Recent studies indicate that bacterial mutation rates are strongly linked to primary and secondary cosmic particle incidence rates, e.g. neutrons. This suggests a second path whereby UVR & space particles cause extinctions, that is through the mutation of bacteria and viruses into deadly new pathogens that decimate entire herds.
Australian Extinction
The graph above suggests that large extinction events occurred around ~13 ka and ~40 ka. During the latter fourteen out of 16 Australian genera of Pleistocene mammalian megafauna went extinct, together with all the megafaunal reptiles (6 genera)[2], as well as the (Eurasian) Neanderthals. Channell & Vigliotti[2] date the Neaderthal extinction to 41‐39 ka (95% CI), that is concurrent with the Laschamps geomagnetic excursion as well as the huge Miyake Event (ME) that occurred around ~41 ka. This huge ME was almost certainly powerful enough to overwhelm the weakening geomagnetic field and/or to significantly heat the Outer Core and cause further geomagnetic field weakening. Earth-incident space radiation particle flux would have increased dramatically during and after this event, thereby directly causing any number of radiation illnesses or indirectly causing higher mutation rates among pathogens and flora and fauna. The mystery is therefore not why the Neanderthals (and mammals, reptiles, etc.) went extinct - a smorgasbord of lethal possibilities exists - but how some species such as Anatomically Modern Humans (AMH) survived. Channell & Vigliotti[2] suggest genetic differences in UVR-defense mechanisms may have played a role, but other possibilities include:
Geographical fluke: the Neanderthals lived in an area that received the full space radiation dose, while AMH lived in an area that didn’t
Cultural: Neanderthals lived in larger groups that were more susceptible to new pathogens
Housing: AMH were more often sheltered from space radiation by their caves
Genetic: AMH were not as susceptible to the new pathogens
Intelligence: AMH had learned to shelter in caves during severe Auroral displays, while the Neanderthals were amazed and bedazzled by the lightshows.
Overkill! AMH warred with the Neanderthals and exterminated them
It’s hard enough to prove why a species went extinct, let alone why one didn’t. Interestingly, Homo Sapiens arrived in Europe from the Near East around the time (~42 ka)[5] the Neanderthals went extinct, suggesting that all options may have played a role: the Neanderthals and their main habitat very likely suffered catastrophic space radiation damage from the Miyake Event, opening the door for lesser-impacted Near East Homo Sapiens to expand their range.
The North American LPE
The graphic above demonstrates that North American extinctions were more severe in the southeast than in the north and northwest. A comparison to the reconstructed ~14 ka paleogeomagnetic field demonstrates the good inverse geographical correlation between paleogeomagnetic field intensity and species extinctions: more extinctions occurred in areas where the geomagnetic field protection was lower. Note that the geomagnetic field currently varies from ~40 µT at the equator to ~60 µT at higher latitudes, so a field strength of < 20 µT is extremely weak.
The previous post highlighted the numerous Miyake Events that occurred just before and during the period the 14.5-11.4 ka period the megafauna went extinct. The geomagnetic field strength was fluctuating significantly during this time: the previous post identified a 500-700 year strength minimum around 13 ka, that is around the start of the Younger Dryas (YD). Earth-incident space particle radiation almost certainly increased dramatically during this minimum, thereby directly causing any number of space radiation illnesses or indirectly causing higher mutation rates among pathogens and flora and fauna alike. Space radiation and ozone depletion would have been especially severe in areas, such as southeastern US, where the geomagnetic field was extremely weak. Once again, the mystery is not why the megafauna went extinct, but how some genera survived. Possibilities include:
Fecundity: some species, e.g. feral pigs and rabbits, reach sexual maturity after only a few months, and can have numerous litters per year, so are able to reproduce before space radiation illnesses strike
Housing: some surviving species, e.g. rabbits, bears & jaguars, spend significant amounts of time in burrows, caves or under dense jungle foliage that offer protection against UVR and space particle radiation.
Geography: some species, e.g. Saiga & Moose, (currently) show a preference for cold, high-latitude habitats where the North American paleogeomagnetic field protection was relatively high 14.5-11.4 ka ago. While the Saiga did go extinct in North America and Europe, a remnant population was either able to hang on in (realtively high geomagnetic field strength) northeast Asia, or was able to migrate there from North America. Note that the North American Moose, which at ~200 - 1000 kg body weight definitely qualifies as megafauna, is currently spread around areas of relatively high ~14.5-11.4 ka paleogeomagnetic field strength; Moose density is particularly high along the interglacial “corridor”, the ice-free and suitable Moose habitat that started to widen during the Oldest Dryas, that is before the megafaunal extinctions started.
Correlation between extinction and body mass
There is a strong correlation between LPE extinction rates and body mass. This is consistent with an extinction event directly or indirectly due to UVR or space radiation particles:
Cell mutations are often harmful, so large long‐lived animals, such as most megafauna, suffer more harmful mutations during their lives
Large mammals often have long gestation periods, so their foetus suffer a relatively high risk of mutation and still-birth.
Large mammals often reach sexual maturity much later in life, so have a relatively higher risk of dying before being able to reproduce.
Large megafauna often live in large herds that are highly susceptible to communicable diseases that can wipe out the entire herd.
Megafaunal herbivore herds often graze open plains that are very exposed to UVR and space radiation.
Megafaunal carnivores often feed on megafaunal herbivores, so starve following herbivore extinction.
Why then?
It is much easier to identify some of the causes of the extinctions that did happen then to subset those causes in order to explain why similar extinctions didn’t happen. Most authors agree the LPE centered around two peaks, ~40 and ~13 ka, that concur with two geomagnetic field minima. The previous post highlighted the large Miyake Events (MEs) that occurred during these minima, suggesting that these events may be a prerequisite for extinctions to occur.
A comparison of the Oldest Dryas IntCal20 calibration curve to its Bølling-Allerød (BA) and Younger Dryas(YD) counterpart suggests a much higher frequency of MEs just before (14.8 ka) and during the period of low geomagnetic field strength (13.5-12.9 ka), suggesting large extinction events did not occur during other periods of very low geomagnetic field strength, e.g. the “Iceland Basin” geomagnetic excursion around 195 ka, due to their lack of very large Earth-incident Miyake events (SPEs).
Summary
Two episodes of Late Pleistocene megafaunal extinction can be recognized: one around ~40 ka and one between ~14.5-11.4 ka. These extinctions were almost certainly not mainly caused by “overkill” (human hunters), “overchill” (climate change) or “overgrill” (cosmic impact). A good temporal and geographical correlation exists between megafaunal extinction events, and periods & areas of low geomagnetic field strength, indicating low geomagnetic field strength acted as an important catalyst. The megafaunal extinction events were very likely triggered by very large Miyake Events (SPEs) that caused increases in primary and secondary cosmic radiation and that caused large increases in UVR due to their atmospheric ozone depletion. This space radiation could directly cause any number of radiation-related illnesses or indirectly cause higher mutation rates among pathogens and flora and fauna.
References
[1] Faith J. & Surovell T., 2009, Synchronous extinction of North America's Pleistocene mammals. Proc Natl Acad Sci USA., 106(49):20641-5. doi: 10.1073/pnas.0908153106.
[2] Channell, J. , Vigliotti, L., 2019, The Role of Geomagnetic Field Intensity in Late Quaternary Evolution of Humans and Large Mammals. Reviews of Geophysics, 57, 709–738, doi:10.1029/2018RG000629.
[3] LaViolette, P. A., 2011, Evidence for a solar flare cause of the Pleistocene mass extinction, Radiocarbon 53, p 303-323.
[4] Pavón-Carrasco, F.J., Osete, M.L., Torta, J.M., De Santis, A., 2014, A geomagnetic field model for the Holocene based on archaeomagnetic and lava flow data, Earth and Planetary Science Letters, 388, 98-109, //doi.org/10.1016/j.epsl.2013.11.046.
[5] Hanel A. & Carlberg C., 2020, Skin colour and vitamin D: An update. Exp Dermatol., 29, 864–875. https://doi.org/10.1111/exd.14142
[6] Hu, W. et al., 2023, Genomic inference of a severe human bottleneck during the Early to Middle Pleistocene transition. Science, 381, 979-984, 10.1126/science.abq7487.