So – What Happened?

Between 10 and 12,000 years ago, the world changed.  In North America, 35 genera of large mammals became extinct.  Not species, but genera – whole groups of closely related species.  Thirty five genera:  Mammoths, mastodons, giant sloths, horses, saber-toothed cats, the short-faced bear (but not grizzlies), the American lion (but not the Mountain lion), camels (but not llamas and vicunas in South America), the giant beaver, but not the regular little guy.

Bison_herd_grazing_at_the_National_Bison_Range

So – what happened?  Where’d everybody go?  And why?

For years, paleontologists and archeologists have tried to figure this out.  There have been five main theories to explain this wholesale extinction:

  1. Overkill – to much hunting by the newcomers to North America, the so-called Clovis people;
  2. Environmental change;
  3. Disease;
  4. Some extraterrestrial impact, akin to the comet which is believed to have killed the dinosaurs;
  5. Some combination of any of the above.

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One issue that has come up, because of the paucity of fossil remains, and gaps in the fossil record, is the timing of these extinctions.  Did these genera go extinct at roughly the same time, or were these extinctions staggered, spread out over time? Was this a slow catastrophe, or a fast one?

In 2009, two scientists, J. Tyler Faith and Todd Surovell, took a look at this issue, and published an article titled Synchronous Extinction Of North America’s Pleistocene Mammals, in PNAS, vol. 106, no. 49.  No slouchy journal, either – PNAS means Proceedings of the National Academy of Sciences of the United States of America.

By doing a careful statistic analysis of the fossil remains associated with the extinctions, they concluded that “the combination of these lines of evidence suggests that North American late Pleistocene extinctions are best characterized as a synchronous event.”  Specifically, “our analyses demonstrate that the structure of the chronology of North American late Pleistocene extinctions is consistent with the synchronous extinction of all taxa between 12,000 and 10,000 radiocarbon years. B.P.”

Okay, so what does that mean?

It means that most of these animals all became extinct in a two thousand-year span.  The authors note that:

         “Our simulations do not rule out the possibility that some extinctions may have occurred before 12,000 radiocarbon years B.P. The biogeographic simulation suggests that anywhere from 0 to 8 genera could have disappeared before the terminal Pleistocene . . . Even so, 23–31 genera abruptly disappeared at approximately the same time. Our results leave open the possibility for a small level of background extinctions (0–8 genera) followed by a surge in extinction rates that wiped out the remaining taxa (23–31 genera) between 12,000 and 10,000 radiocarbon years B.P.”

So it is possible, they acknowledge, that of the 35 genera that became extinct, maybe as many as 8 of them had gone extinct earlier that 12,000 years ago.  That still means, however, that 27 of them became extinct in that short – remarkably short – period of time.  As the authors put it,

           “Whether or not background extinctions took place, that a catastrophic event or process occurred at the end of the Pleistocene is abundantly clear.”

The implications for this are important.  Whatever happened, it happened very fast, and was continent-wide.  Europe experienced what the authors call a “long-term, piecemeal extinction process.”  Not so, here.  It happened all across the continent, in what they call “a geologic instant.”

BisonBones

This conclusion doesn’t necessarily eliminate any of the five possible causes of the mass extinction, but it does put certain constraints on them.  An environmental change, for example, if it was the primary cause of these extinctions, must have been nation-wide and very rapid.  But intriguingly, even those limits – speed and breadth – still fit with three of the possible causes for the extinctions:  “This time period encompasses the earliest secure evidence of human foragers in North America . . . the Younger Dryas cold interval . . . and a possible extraterrestrial impact.”

Well, science marches on.  We still don’t know why these extinctions occurred.  And while two thousand years may be a geologic instant, in the lives of these animals, it encompassed tens or hundreds of generations.  A drought that lasted five hundred years, or seasons so cold that plants wouldn’t grow, could certainly have caused some of these extinctions.  And there may have been a cascade effect, too:  If a given herbivore becomes extinct or vanishingly rare, then the predator that preys on it is in trouble, too.  And family structures may have been disrupted by hunting, too, for that matter:  If the matriarch of the mammoth herd is killed, maybe the young ones don’t know how to survive a particularly harsh winter, or a dry summer.

But as to what happened?  We still don’t really know.  As Faith and Surovell put it, “further research on the biogeographic histories of individual species in relation to detailed paleoclimatic, paleoecological, and archaeological data could help to finally pin down the cause of North American end-Pleistocene extinctions.”

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