A new study published in Nature communication suggests that the extinction of North America’s largest mammals was not driven by overhunting by rapidly growing human populations after their entry into the Americas. Instead, the findings, based on a new statistical modeling approach, suggest that populations of large mammals have changed in response to climate change, with drastic declines in temperature about 13,000 years ago that began the decline and extinction of these massive creatures. . Humans may have worked in more complex and indirect ways than simple models of overhunting.
About 10,000 years ago, North America was home to many large and exotic creatures, such as mammoths, giant sloths, large life-size beavers, and enormous armadillo-like creatures known as glyptodons. But about 10,000 years ago, most of North America’s animals weighing more than 44 kg, also known as megafauna, disappeared. Researchers from the Max Planck Extreme Events Research Group in Jena, Germany, wanted to find out what led to these extinctions. The topic has been hotly debated for decades, with most researchers claiming that human overhunting, climate change or a combination of the two were responsible. With a new statistical approach, the researchers found strong evidence that climate change was the main driver of extinction.
Hunting against climate change
Since the 1960s, there has been the hypothesis that, as human populations across the continent grew and expanded, the arrival of specialized “big game” hunters in the Americas about 14,000 years ago quickly drove many giant mammals to extinction. The large animals did not have the appropriate anti-predator behavior to handle a new, extremely social, tool-swinging predator, which made them particularly easy to hunt. According to proponents of this ‘excessive hypothesis’, humans took full advantage of the easy prey and devastated the animal populations and carelessly drove the giant creatures to extinction.
However, not everyone agrees with this idea. Many scientists have argued that there is too little archaeological evidence to support the idea that megafauna hunting was persistent or widespread to cause extinctions. Instead, climate and ecological change were to blame.
Around the time of the extinction (between 15,000 and 12,000 years ago) there were two major climate changes. The first was a period of sudden warming that began about 14,700 years ago, and the second was a cold moment about 12,900 years ago during which the Northern Hemisphere returned to near glacier. One or both of these important temperature fluctuations, and their ecological consequences, have been implicated in the megafauna extinction.
“A common approach was to try to determine the timing of megafauna extinctions and see how they correspond to human arrival in the Americas or a climatic event,” says Mathew Stewart, co-author of the study. “However, extinction is a process – meaning it unfolds over a period of time – and to understand what caused the downfall of North America’s megafauna, it’s crucial that we understand how “Their population has fluctuated to extinction. Without those long-term patterns, all we can see are rough coincidences.”
‘Dates as dates’
To test these contradictory hypotheses, the authors use a new statistical approach developed by W. Christopher Carleton, the other co-author of the study, and published last year in the Journal of Quaternary Science. The calculation of population sizes of prehistoric hunter-gatherer groups and animals that have long since become extinct cannot be counted by the heads or hooves. Instead, archaeologists and paleontologists use the radiocarbon record as a proxy for the population sizes of the past. The reason is that the more animals and humans occur in a landscape, the more dateable carbon is left behind after it is gone, which is then reflected in the archaeological and fossil record. Unlike established approaches, the uncertainty in fossil dates is better accounted for with the new method.
The biggest problem with the previous approach is that it mixes the uncertainty associated with radiocarbon dates with the process that scientists are trying to identify.
“As a result, you may end up seeing trends in the data that do not really exist, which makes this method quite unsuitable for capturing changes in previous population levels. Using simulation studies where we know the actual patterns in the data “is, we will be able to show that the new method does not have the same problems. Consequently, our method is able to do a much better job of capturing continuous changes in population levels by using the radiocarbon record,” explains Carleton.
North American megafauna extinctions
The authors applied this new approach to the demand for the late-quaternary North American megafauna extinctions. In contrast to previous studies, the new findings show that megafauna populations vary in response to climate change.
“The Megafauna population appears to be increasing as North America begins to warm up about 14,700 years ago,” says Stewart. “But then we see a shift in this trend about 12,900 years ago as North America began to cool drastically, and shortly thereafter we see how the extinction of megafauna occurs.”
And while these findings suggest that the return to nearly icy conditions about 12,900 years ago was the closest cause of extinction, the story is probably more complicated than that.
“We need to consider the ecological changes associated with these climate changes at a continental and regional level if we want to have a proper understanding of what drove these extinctions,” explains group leader Huw Groucutt, senior author of the study. “People are also not completely off the hook, as it remains possible that they played a more nuanced role in the megafauna extinction than simple excessive models indicate.”
Many researchers have argued that it is an impossible coincidence that megafauna extinctions around the world occurred regularly during the arrival of humans. However, it is important to scientifically demonstrate that there was a connection, and even if it were possible, the causes may have been much more indirect (such as through habitat adaptation) than a deadly madness when people arrived in a region.
The authors conclude their article with a call to arms, urging researchers to develop larger, more reliable records and robust methods for interpreting them. Only then will we develop a comprehensive understanding of the late-quaternary megafauna extinction event.