Around the time of the transition ~12,000 years ago, between a world with continental glaciers as much as 3km deep, extending, in North America to below the USA-Canadian border, and our present more benign climate, mega-fauna disappeared from the Americas. So was this due to the change in the climate as we left, what is popularly called an Ice Age to our present warmer climate. First we have to establish some terms and a little background.
Over the past 2.75m (million) years, the world has gone through about 40 cycles of Continental glaciers and warmer periods, such as we are living in at present. During the first half of this 2.75m year period, the cycles were about 41k (thousand) years while over the past half of this two and three quarter million year Ice Age, cycles have been about 100k years. The exact timelines are hard to establish on the land, because each cold period, when kilometer-thick glaciers grind across the landscape, they wipe out the record of previous glacial an interglacial periods.
All information was not lost, though. Ice itself is one path to documenting past fluctuations. The Antarctic has been glaciated for at least 34m years and the ice is built up in layers which are much like tree rings. Generally (not always) one layer is laid down per year*. Glaciation has fluctuated but there is a reliable, continuous record over at least 800,000 years. In addition there are ocean bottom sediments and a peculiar meteor crater in Northern Siberia called El'Gy'Gytgyn that has a sediment record which extends over 3.6m years.
*Sometimes a layer is not laid down in a given year and a warm period can wipe out a number of layers. A unseasonable warm period in the middle of a year can result in two rings for a year. But there are work-arounds for these problems.
As mentioned something interesting, has been observed from these records. It is that for the first half of this 2.75m year period of alterations between icy and less icy times, the cycle length has been about 41k years while for the second half of this period, the cycle is closer to 100k years*. Incidentally, the previous warm period goes by various names but the one that many 'westerners' use is the Eemian interglacial period. The one we are in now is called the Holocene Interglacial Period.
*For some reason different Milankovitch Cycles dominated at the beginning of this
Ice age than more recently. Note that the Ice Age has not ended. We are just in one of the interglacial periods and by now should have been, sliding toward the next Glacial period (but are not).
The popular literature and even some scientists call the most recent glaciated period an Ice Age. Even in the above video this is so. I have no objection to this term (not much!!!) but if we use it to describe the recent glaciated period then we have to use a different name for the 2.75m year period during which we have had many alternations between warm and cold periods.
So for the rest of this diatribe, I am going to call the last 2.75m year ice age, an Ice Age and will call glaciated periods 'Glacial Periods' and the warmer interglacial periods, 'Interglacial Periods'. At present we are in the so called Holocene Interglacial Period and as mentioned, the previous one is called the Eemian Interglacial period*.
*Previous Interglacial Periods have different names but in order from the most recent to more ancient ones they are often called, Eemian, La Bouchet, Parfleet, Hoxnian, MI13, Cromerian, Holstein, Waalian, Bramertenian, Passtonian etc.
Now we come to two problems if we want to insist on the theory that the transition from the recent Glaciated Period to the present Holocene Interglacial period caused the demise of the immensely varied mega-fauna of the Americas.
I think everyone would agree that, based on our present knowledge of how long it takes to create a new species, that all this megafauna didn't evolve over the past 100k years. To put it another way, the ancestors of the elephant types, Camel types, Giant ground sloths, Giant beavers, Equinine species and so forth existed at the start of the 2.75m Ice age that we are still in. Undoubtedly they evolved special adaptations to glaciation over the glaciated periods of the 2.75m year ice age. For instance, greater harryness or thickness of blubber might have increased as we slid into an Glacial Period and decreased as we left a Glacial Period*.
*If you think that evolution is always a very very slow process, look at how the Galapagos finches change their bill shape over El Nino, La Nina cycles as the food supply changes. Or for that matter look at the proliferation of Dinosaur types following the end-Triasic extinction of the existing reptiles when the Ecological niches suddenly were empty.
The critical point is that they didn't go extinct, during previous slides from a warm period into a cold period or from a cold period into a warm period. And there were at least 40 such cycles. So why would we think that the change in the climate, which happened many times before caused their demise only this time.
In addition, the record from the previous 4 cycles, for which we have the best information, were very similar to the most recent one. A great treatment of the past handful of glacial cycles is found in the book Plows Plagues and Petroleum by Ruddiman*
*Ruddiman makes a very convincing argument that man has been interfering with the climate for at least the past 8000 years and that the level of interference was just about enough to counter what should have been our next slide into a glacial period. Now we have too much of a good thing.
The second problem is the evidence from other continents. The incredibly varied population of mega-Marsupials and Monotremes in Australia went extinct about 45k years ago, around the time that man arrived in Australia. The amazingly varied mega-fauna of the Americas disappeared around 12k years ago, just as the first people spread through the Americas and the mega fauna of, of New Zealand, consisting of giant ground living birds, plus a giant eagle and many species of 'dickie birds' disappeared 700 years ago just as the first people arrived in New Zealand. Coincidence?? I don't think so.
The only conclusion that seems reasonable to me is that in every case, the first people who arrived in a new land multiplied and ate their way through all the megafauna. There are at least three reasons that many people find this conclusion is hard to accept.
Leaving aside the nutters who believe that the world is only ~6000 years old, It is very hard to grasp deep-time. Even grasping the length of time back to the previous warm period, which is 62 time as long as from the time of Christ to the present, is hard. Grasping the idea of millions of years is even more daunting.
A secibd problem is our difficulty in accepting exponential growth. It is hard to grasp how fast a small starter population of people can multiply in an environment when all those edible animals are completely unwary of people and all that energy rich meat is there for the taking. Lets be really conservative and say that a population of humans can double each generation and a generation is 25 years. I think you would agree that in the absence of birth control methods, this is ridiculously conservative. Take a starter population of two and see how many individuals we would have in 20 generations (20 x 25 years = 500 years). Raise 2 to the power of 20 and you get 1,048,576 people. One more generation (25 years) and you have 2,097152 people. One more and you have 4,194,304. You see where this is going. Exponential growth which only added a handful of people at the beginning each generation is now adding 2m people every generation.
Of course, a starter population would likely be a small tribe which for genetic reasons would be far more viable.
The further reason is PC (political correctness). We have got so used to maintaining the legend that the first people, whether from the Americas, from Australia or from New Zealand are great ecologists and couldn't possibly be responsible for the demise of the mega fauna of their home range. In fact they are just people like we are and find it hard to moderate their motivations for the greater long term good of themselves and their societies. We Europeans have only recently realized what damage we have done to the environment and are far from catching up and modifying our actions to preserve what we have left. Why do we think that an earlier population of humans would do any better when faced with unbelievable abundance.
The third reason is hubris. For some reason we see ourselves as more intelligent than our distant ancestors when recent findings show that this is, pretty clearly not the case. They just applied their intelligence to different problems. Just imagine how you would fair if transported back into a glacial period in Europe in a period in which Neanderthals and our ancestors survived and thrived.
And fourth, we can't imagine that they were capable of killing such huge animals. If you have this bias, just look at how the pigmies in the jungles kill jungle elephants or how the bushmen of the Kalahari, kill Kudu.
This leaves a contrary problem. How did some of the mega-fauna of Africa survive until Europeans arrived on the continent. After all this is where man evolve. The only explanation I can think of is that the animals evolved a fear of man as man became more and more able to kill them and to kill them at greater and greater distances (first thrusting spears, then throwing spears, then the throwing sticks (atiatl) for throwing spears and then bow and arrow) not to forget the sling for throwing stones. Animals would have learned that when the strange two legged animal was around, they kept getting hit by heavy and pointy objects. Thus the animals probably evolved avoiding behaviors as man became more and more able to kill them and to kill them at greater distances. A process just like the antelope learning to avoid predatory cats.
Perhaps, as we Europeans apologize for the wrongs we did to the ancestors of the first people, we should ask for an apology for the destruction of the mega-fauna at the hands of their ancestors.
