At various times in history, the continents of Eurasia and North America have been connected by the Bering Land Bridge which is formed when water levels recede and the Bering Sea is filled in (click HERE for a dynamic map that shows changes in sea level over the last 21,000 years).
When conditions allowed, humans (with their dogs) migrated across the Bering Land Bridge moving from Eurasia to North America. It is not certain exactly when this human migration began and when it ended but a typical estimated range is from 20,000 years ago to 10,000 years ago. It has also been put forth that some of these people resorted to boats to ferry them across this challenging, changing area.
DNA analysis has verified that these new arrivals came from Siberia. It has also refuted Thor Heyerdahl’s “Kon Tiki Hypothesis” that Polynesia was settled by rafters from Peru – Polynesian DNA and American DNA do not overlap at all. On the other hand, there is DNA evidence that some of the trekkers from Siberia had cousins who lit off in the opposite direction and became aboriginal people of New Guinea and of Australia!
The Los Angeles area is famous for its many attractions. Among them is the site of the La Brea Tar Pits, click HERE for a classic image. This is the resting place of countless animals who were sucked in to the primeval tar ooze here over a period of thousands and thousands of years. What is most striking is that so many of them have gone extinct, especially large animals such as the camel, the horse, the dire wolf, the giant sloth, the American lion, the sabre-toothed tiger, … . In fact, with the exception of the jaguar, the musk ox, the moose, the caribou and the bison, all the large mammals of North America disappeared by 8,000 years ago. Humans arrived in North America not so many years before and we know they were successful hunters of large mammals in Eurasia. So, as in radio days, the $64 question is: did humans cause the extinction of these magnificent animals in North America? Mystère.
The last Ice Age lasted from 110,00 years ago to 11,700 years ago (approximately, of course). During this period, glaciers covered Canada, Greenland and states from Montana to Massachusetts. In fact, the Bering Land Bridge was created when sea levels dropped because of the massive accumulation of frozen water locked in the glaciers. The glaciers didn’t just sit still; rather they moved South, receded to the North and repeated the cycle multiple times. In the process, they created the Great Lakes, flattened Long Island, and amassed that mound of rubble known as the Catskill Mountains – technically the Catskills are not mountains but rather a mass of monadnocks but the Rockies and Appalachians are mountains, created by mighty tectonic forces that thrust them upward. In any case, South of the glaciers, North America was home to many large mammal species.
As the Ice Age ended, much of North America was populated by the hunting group known as the Clovis People. Sites with their characteristic artifacts have been excavated all over the lower 48 and northern Mexico. For a picture of their characteristic spear head, click HERE. The term Clovis refers to the town of Clovis NM, scene of the first archaeological dig that uncovered this culture; luckily this dig preceded any work at the nearby town of Truth Or Consequences NM.
The Clovis people, who dominated North America south of the Arctic at the time of these mass extinctions, were indeed hunters of large and small mammals and that has given rise to the “Clovis overkill hypothesis” – that it was the Clovis people who hunted the horse and other large species to extinction. The hypothesis is supported by the fact that Clovis sites have been found with all kinds of animal remains – mammoths, horses, camels, sloths, tapirs and other species.
For one thing, these animals did not co-evolve with humans and had not honed defensive mechanisms against them – unlike, say, the Zebra in Africa (descendants of the original North American horse) who are aggressive toward humans; the Zebra have neither been hunted to extinction nor domesticated. In fact, the sociology of grazing animals such as the horse can work against them when pursued by hunters. The herd provides important defense mechanisms – mobbing, confusing, charging, stampeding, plus the presence of one or more bulls. But since these animals employ a harem system and the herd has many cows per bull, the solitary males or the males in very small groups who are not part of the herd are natural targets for hunting. Even solitary males who use speed of flight as their first line of defense can fall victim to persistence hunting, a tactic known to have been used by humans – pursuing prey relentlessly until it is immobile with exhaustion: a horse can run the 10 furlongs of the Kentucky Derby faster than a human in any weather but on a hot day a sweating human can beat a panting horse in a marathon. An example of persistence hunting in more modern times is stag hunting on horseback with yelping dogs – the hunts ends when the stag is exhausted and immobile and the coup de grace is administered by the Master of the Hunt. As for dogs, they were new to the North American mammals and certainly would have aided the Clovis people in the hunt.
Also there is a domino effect as an animal is hunted to extinction: the predator animals that depend on it are also in danger. By way of example, it is thought that the sabre-toothed tiger disappeared because its prey the mammoth went extinct.
Given all this, how did the bison and caribou survive? In fact, for the bison, it was quite the opposite. At this time, there was a bison population explosion; given that horses and mammoths have the same diet as bison, some scientists postulate that that competition with the overly successful bison drove the others out. Another thing is that bison live in huge herds while animals like horses live in small bands. It is theorized that the caribou, who also travel in massive herds, survived by pushing ever further north into the arctic regions where ecological conditions were less hostile for them and less hospitable to humans and others.
However, all this is happening at a dramatic period, the end of the Ice Age. So warming trends, the end of glaciation and other environmental changes can have contributed to this mass extinction: open spaces were replaced by forests reducing habitat, heavy coats became a burden, … In fact, the end of the last Ice Age is also the end of the much longer Pleistocene period; this was followed by the much warmer Holocene period which is the one we are still in today. So the Ice Age and the movement of the glaciers suddenly ended; this was global warming to a degree that would not be seen again until the present time. The warming that followed the Ice Age would also have changed the ecology of insects, arachnids, viruses et al, with a potentially lethal impact on plant life and on mega fauna. Today we are witnessing a crisis among moose caused by the increase of the winter tick population which is no longer kept in check by cold winters. We are also seeing insects unleashed to attack trees. Along the East Coast, it is the southern pine beetle which has now reached New England – on its Shermanesque march north, this beetle has destroyed forests, woods, groves, woodlands, copses, thickets and stands of once proud pine trees. It is able to move north because the minimum cold temperature in the Mid-Altantic states has warmed by about 7 degrees Fahrenheit over the last 50 years. In Montana and other western states it is the mountain pine beetle and the endless fires that are destroying forests.
Clearly, the rapidly occurring and dramatic transformations at the end of the Ice Age could have disrupted things to the point of causing widespread extinctions – evolution did not have time to adjust.
And then there is this example where the overkill hypothesis is known not to apply. The most recent extinction of mammoths took place on uninhabited Wrangel Island in the Arctic Ocean off the Chukchi Peninsula in Siberia, only 4,000 years ago, and that event is not attributed to humans in any way. The principal causes cited are environmental factors and genetic meltdown – the accumulation of bad traits due to the small size of the breeding population
In sum, scientists make the case that climate change and environmental factors were the driving forces behind these extinctions and that is the current consensus.
So it seems that the overkill hypothesis is an example of the logical fallacy “post hoc ergo propter hoc” – A happened after B, therefore B caused A. By the way, this fallacy is the title of the 2nd episode of West Wing where Martin Sheen as POTUS aggressively invokes it to deconstruct his staff’s analysis of his electoral loss in Texas; in the same episode, Rob Lowe shines in a subplot involving a call-girl who is working her way through law school! Still, the circumstantial evidence is there – humans, proven hunters of mammoths and other large fauna, arrive and multiple large mammals disappear.
The situation for the surviving large mammals in North America is mixed at best. The bison are threatened by a genetic bottleneck (a depleted gene pool caused by the Buffalo Bill era slaughter to make the West safe for railroads), the moose by a climate change and tick borne diseases, the musk ox’s habitat is reduced to arctic Canada, the polar bear and the caribou have been declared vulnerable species, the brown bear and its subspecies the grizzly bear also range over habitats that have shrunk. The fear is that human involvement in climate change is moving things along so quickly that future historians will be analyzing the current era in terms of a new overkill hypothesis.