Human Arrival and the Demise of Massive Mammals in the Americas
As we saw last month, the megafaunal extinctions in the Americas were extensive, culling a great many mighty mammals both north and south. So, a profound change must have occurred on these continents in the late Ice Age to cause such a dramatic loss of life, and, as with the megafaunal extinctions in Australia, there is some evidence to suggest that these vanished greats may have perished due to more than simply climatic or environmental factors, with the arrival of modern humans once more in the frame. However, there has long been heated debate over when humans first arrived in the Americas, and over recent decades a flood of new evidence has poured plenty of fuel on the fire. We can now compare this evidence to that we have already encountered regarding the patterns of the megafaunal extinctions in the western continents to see how the claims of human involvement really stack up – that is, to see if the demise of massive mammals was triggered by modern humans coming to America.
Our investigation, then, will sift through the latest evidence to try and find answers to a number of key questions: When did humans first arrive in the Americas? How did they arrive? And how long did it take for them to spread across these continents? Also, how does the timing of their arrival and spread fit with the pattern of megafaunal extinctions that was outlined last month, and what is the evidence for the nature of their interactions with the megafauna in terms of predation and co-existence?
Well, to begin with, the arrival of humans in the Americas occurred in a way that simply could not happen today. Because sea levels were far lower during the last glacial, Siberia and Alaska were not cut off from each other by the waters of the Bering Strait, but joined together by a great landbridge that was twice the size of Texas, appropriately named Beringia. Thus, at this time, people and animals could just walk from Asia to North America.
The earliest certain evidence of humans on the North American continent, published in a study in January 2017, dates from around 24,000 years ago from the Bluefish Caves in the Yukon, which lie on what is now the border with Alaska. But though this proves a human presence on the North American continent earlier than previously thought, it is still too late to account for the extinctions which took place in the Alaska/Yukon region before this, the short-faced bear Arctodus simus, a species of horse, and the sabretooth cat Homotherium serum all having disappeared by this time.
The arrival of humans in the Alaska/Yukon region, then eastern Beringia, was not followed by a swift peopling of the Americas, though. Because of the vast ice sheets blanketing the northern half of the continent, these humans, these Beringians, could not penetrate any deeper into North America, and so a true colonisation of the Americas could not take place until the ice sheets began to recede. This mighty invasion of humans from the north was once thought to have been a relatively straightforward affair, but mounting evidence in recent times has indicated that this was a far more complex process than once thought.
The old model for human arrival in the lower regions of the Americas envisaged a late migration of people through a 1,500-kilometre-long ice-free corridor which opened up as the great northern ice sheet began to melt and split into two retreating ice sheets, facilitating a southwards movement from eastern Beringia. The earliest evidence of humans was thought to come from very distinctive stone tools, including spear points, associated with the ancient Clovis culture, named after the town in New Mexico where they were first found. The oldest Clovis artefacts date to only 13,400 years ago, and so such a late arrival of humans seemed to fit quite neatly with the idea of the megafaunal extinctions being caused by overkill, as most massive mammals went extinct at around this time or not long after it.
On top of this, direct evidence of human hunting of megafaunal species has been found, with Clovis spear points having been discovered in direct association with mastodons, the Columbian mammoth, and yet another relative of modern elephants, the spiral-tusked gomphothere Cuvieronius. Furthermore, the very shape of the spear points themselves suggests that they would have been very efficient at dispatching large prey.
However, we now know that this ‘Clovis first’ model is no longer correct. To begin with, a recent study has shown that the ice-free corridor in the north could not have acted as a gateway for humans to the south until around 12,600 years ago, as before this the conditions would have been too inhospitable for human survival. Also, over recent decades, a body of evidence has been growing for a pre-Clovis human presence in the Americas.
Surprisingly, some of the earliest evidence comes from a very southerly location, from a site called Monte Verde in Chile, which is thought to date from at least 14,500 years ago, and possibly as early as 18,500 years ago. Pre-Clovis sites have also been discovered in the southern half of North America, the oldest being the site of Debra L. Friedkin in Texas, which dates to around 15,500 years ago, but there are a number of others in widely scattered locations, including in Florida, Pennsylvania, Wisconsin, and Oregon.
This evidence, taken together, tells us a number of very important things about the peopling of the Americas. First, that humans must have invaded the Americas below the ice sheets initially along the west coast, possibly by boat, and only later did humans also invade from the north via the ice-free corridor. Second, that this human invasion seems to have begun thousands of years earlier than once thought, long before the tools of the Clovis culture turn up in the archaeological record less than 13,500 years ago.
Thus, just as we have recently seen with Ireland, in the Americas human colonisation began much earlier than once thought, and was also a far more complex affair than once envisaged. And this, of course, has deep implications for our understanding of the possible part played by humans in the megafaunal extinctions in the Americas.
Although this new evidence brings some of the extinctions which occurred before 13,400 years ago within the frame of human habitation of the Americas, it simultaneously undermines any notion of a swift overkill scenario, with the majority of the megafaunal species going extinct quite rapidly after the arrival of humans. At the Page-Ladson site in Florida, for instance, where the oldest human artefacts have been dated to around 14,500 years ago, there is evidence of humans having co-existed with megafaunal species for around 2,000 years, up to their extinction about 12,600 years ago.
In the end, though, with the current level of evidence it is impossible to be definitive about the possible human involvement in the megafaunal extinctions in the Americas, and debate continues to rage. Regarding North America, for instance, some experts argue that the dates for earlier extinctions do not reflect the true, later extinction of these mammals and are due to inadequate sampling so that the late Pleistocene extinctions in North America more closely correlate with an overkill scenario or extraterrestrial impact. However, in his review of these extinctions, Dr Anthony Stuart concludes that ‘the dates available at present from North America outside Alaska/Yukon are inadequate, not only in number but in many cases also in quality, for constructing a reliable chronology’, so the question of causes cannot be resolved at present.
But that is not to say that humans did not play a part, perhaps even a pivotal part, in the extinction of megafaunal species in the Americas. Perhaps ancient Americans whose ancestors had lived beside megafaunal species for thousands of years were forced to prey on them to an even greater degree with the onset of the climatic downturn of the Younger Dryas around 12,900 years ago, and for these large, slowly-reproducing mammals this was simply the straw that broke the camel's (or the mastodon's) back. As with the extinctions of megafaunal species in other regions, it is hard to put their disappearance down solely to changes in climate or vegetation, as these factors had never resulted in the wholesale loss of so many large species at any earlier time in the Ice Age. However, ultimately, as with Australia, although there is certain evidence that implicates humans in the disappearance of the American megafauna, it is far from conclusive. We still cannot say if the giant beasts of the western continents vanished due to ancient humans coming to America.