This isn't a new question; it's been posed by all types of persons ranging from virtually no education through topmost scholars. Theories are as varied as the people asking, ranging from ostensibly far-fetched hypotheses to seemingly "commonsensical" ones. Some of the proposed theories from scholars include:
- Hyperdisease impacting only certain species
- Comet impact
- Solar radiation
- Blitzkrieg (rapid overkill)
- Sitzkrieg (fire, habitat fragmentation, and introduction of invasive species and diseases)
- “Simple” climate and environmental change
Instead of reallly boring you with full studies on theories, how about I summarize various hypotheses and provide an opinion based on general plausibility from questions or challenges left unanswered by each author. Trust me, you like this option better because it means I do the hard work and you get the snapshot! Win-win, eh?!
Extraterrestrial Impact Event
The extraterrestrial impact event theorizes that the onset of the 1,000+ years of Younger Dryas cooling event coincided and the Pleistocene megafaunal extinctions (as well as extinctions of the Clovis people) because of an extraterrestrial (comet) impact (Firestone, 2009). The author cites a carbon-rich black material (referred to as “black mat”) which can be found immediately covering Clovis kill sites. Furthermore, the author states that no megafaunal fossil or Clovis artifacts are discovered above (stratified) the black mat. The composition of the black mat appears to coincide with extraterrestrial components as well as carbon components which allude to a superheated shockwave and high temperature fires. He points to the Carolina and Great Lakes “craters” as substantiation for impact areas. You caught that, right? The Great Lakes are craters. That right there is going to cause me a lot of research sometime in the near future.
Anyhow, the back mat referenced by the author is seen in various articles and, as such, does seem to play some role in the overall extinction. In regard to the proposed impact craters: the author argues proof of impact based on shallow "crater pools" in the 15 Carolina Bays as well as elliptical shallow lakes in Georgia, Virginia, Nebraska, Kansas, New Mexico and Texas; the inexplicable depth of the Great Lakes is why he states they were also formed due to some interstellar cause. He provides ample imagery and ‘impact-related mineral data’ to explain the extraterrestrial-caused formation of the numerous shallow areas and Great Lakes. In specific regard to the Great Lakes, he agrees that a large comet strike would have left craters but believes these craters were diminished in standard identifiable features. He believes the comet struck the Laurentide Ice Sheet, which greatly absorbed some of the impact energy. The impact caused the ice sheet to fail and, upon failure, provide a sudden release of both rushing water and ice ejecta. Therefore, with circumstances being as they may, there would be no ‘tell tale’ crater but, instead, “a great scar in an otherwise featureless landscape.” The author further links the formation of the Great Lakes to coincide with the formation of Charity Shoal, a 1km crater in Lake Ontario known to have formed near the time of the theorized Younger Dryas impact. The author uses an image with an overlaid line to express that 3 of the 4 deepest areas of the Lakes are in a line similar to crater ‘chains’ previously observed on the Earth, moon, and Jupiter after the Shoemaker-Levy comet impact.
While many of the data appear solid, my questions on this theory are many. If the Pleistocene extinction - which included both highly adaptable people and animal species - was caused by an extraterrestrial impact, why were only select species eradicated? Furthermore, the explanation may somehow work into the North American extinction event, but how does this tie into extinctions on other continents like Australia? Unfortunately for the author, other scholars also questioned the theory. Firestone’s original journal entry apparently appeared in a National Academy of Science journal wherein his collection methods and theory garnered a rebuttal by Haynes, Jr. et al. While the authors of the refutation do not negate the potential of an impact, the evidence was found to be lacking (and I'm pretty much in agreement). The future article by Firestone in the Journal of Cosmology is, for all intents, the author’s second effort to provide substantiation for his impact-caused allegation for extinction.
The Super-Sized Solar Proton Event
The "super-sized solar proton event" is essentially a period of heavy, immense solar flare activity which caused enough radiation to impact large or unprotected species. The author, LaViolette, provides proof through the Greenland ice records, carbon-14 and atmospheric carbon indicators in conjunction with 2 successive temperature maxima spaced by 1 solar cycle (one solar cycle is approximately 11 years so this event spans 22 years). This raises his questions surrounding warming due to increased solar output which would substantially impact global climate. It is evidence postulated by Firestone - that comet impact guy from the above study - that seemingly spurs LaViolette’s argument towards increased solar activity (as this active period would account for data reported by Firestone but also accounts for the lack of data to validate the impact theory). With LaViolette’s theory, only larger animals or unprotected (for example, humans as they were unaware of radiation concerns) would be impacted.
LaViolette’s theory is relatively sound but, in and of itself and considering the span of time and the unequal disposition of the extinction but it still leaves a few questions. Some are species specific... such as why wooly mammoth and not African elephants - was it just the external heating features? Why the Smilodons (saber-toothed cats) and atrox (American lion) and not the African lion or many species of tigers or other large cats? Simply put, it's likely this theory could not - even on the most forgiving judgement-lapsed day - be the sole cause of the Rancholabrean extinction.
Blitzkrieg, Sitzkrieg, and Overkill
In the recent past a theory began to take ‘center stage’ surrounding the introduction of humans into previously unoccupied areas of the world as the primary cause for the Pleistocene extinctions (specifically the Rancholabrean extinction). The basis of the theory surrounds an introduction of an exotic and invasive new, highly efficient predator (Clovis people, the original North American Homo sapien sapien) upsetting the balance for the large megafauna which exhibit slow maturation with protracted reproductive rates. Proof for this theory is found at what is known as Clovis kills sites; areas where these people would clean meat and hides from kills. Clovis kill sites often contain megafaunal remains such as mastodon and mammoth. The efficiency of the Clovis people upset the balance as the mega-predators primarily fed upon the mega-herbivores. The difference between “overkill” and “Blitzkrieg” is merely in the speed of the event.
To coincide with this theory is the Sitzkrieg theory, which states that fires and exotic species (such as the Clovis people, for example) allowed for habitat fragmentation and introduction of diseases hereto unseen by the Rancholebrean megafauna.
Limited Scope and Tunnel Vision
But you know what? Regardless of how many articles I read, there seems to be evidence and granules of truth littered through each. Trying to pinpoint the first ripple that started a chain of events in recent history is hard enough - but finding that first drip from 16 thousand years ago?! C'mon, really?
So, let's think for a moment. There are things that are incontestable:
- A large portion of Pleistocene epoch megafauna went extinct in, geologically speaking, a short amount of time.
- Humans were present prior to the extinctions in other areas that still have megafauna.
- There are mineral/cosmic remnants found in relation to the same general period of time.
- There was a period of warming and massive reduction in glaciation.
My love of my Smilodon knows no bounds - and I spend days on end dreaming of him.
Ok. Enough of that. if you're dying to see where I got my info, here's the wonderful world of science:
Brook, Barry W. and David M. J. S. Bowman. (2002) Explaining the Pleistocene megafaunal extinctions: Models, chronologies, and assumptions. Proc National Academy of Science USA 99:14624-14627.
Campos, Paul F. et al (2010) Ancient DNA analyses exclude humans as the driving force behind late Pleistocene musk ox (Ovibos moschatus) population dynamics. Proc National Academy of Science USA 107:5675-5680.
Haynes, C. Vance Jr. et al. (2009) The Murray Springs Clovis site, Pleistocene extinction, and the question of extraterrestrial impact. Proc National Academy of Science USA 107:4010-4015
Firestone, Richard B. et al. (2007) Evidence for an extraterrestrial impact 12,900 years ago that contributed to the megafaunal extinctions and the Younger Dryas cooling. Proc National Academy of Science USA 104:16016-16021
Firestone, Richard B. (2009) The Case for the Younger Dryas Extraterrestrial Impact Event: Mammoth, Megafauna, and Clovis Extinction, 12,900 years ago. Journal of Cosmology 2:256-285.
LaViolette, Paul A. (2011) Evidence for a Solar Flare Cause of the Pleistocene Mass Extinction. Radiocarbon 53:303-323