Imagine vast plains grazed by herds of woolly mammoth, with their long, curving tusks much larger than those borne by any modern elephant. With two giant horns to attract mates and fend off attackers, woolly rhinos also roamed the steppe. Giant sabre-toothed cats used their huge canines, each more than six inches long, to tear at the necks of their prey after wrestling them to the ground. Over-sized cave lions preyed on cave bears.
These are just a few of the now-extinct large, exotic and sometimes ferocious animals that walked the Earth alongside our ancestors for most of the last Ice Age, between 100,000 and 10,000 years ago. It was something of a golden age for large mammals. Alongside the better known species like the woolly mammoth and sabre-toothed cat, there were many others American lions and dire wolves, mastodons, ground sloth, giant beavers, the Eurasian steppe bison and wild horse.
Finds at archaeological sites across Europe and North America tell us that many of these species became extinct between 50,000 and 11,000 years ago. Scientists have long puzzled over the causes of what are collectively known as the late Pleistocene extinctions. Some have pointed the finger at over-hunting or the introduction of infectious diseases by our ancestors, while others have emphasised climate change as the primary cause.
Its a lively and ongoing debate. Traditionally most of the evidence used to support the different theories came from archaeological sites. But its difficult to reach firm conclusions about what was happening to a species at a population level by looking at a few bones and cave paintings. In recent years, powerful new scientific techniques have been deployed to yield new types of evidence, providing deeper insights into the timing and causes of the extinctions.
We are currently living through a critical period in which we are having greater impacts on other animals, our environment and the planets climate than ever before. A better understanding of the role that our ancestors played in previous extinctions and climate change is not only of academic interest, but also has the potential to help us learn how to lessen our environmental and ecological impacts, in order to ensure the survival of many species including our own.
The Pleistocene geological epoch, also known as the "Great Ice Age", lasted some 2.6 million years until about 12,000 years ago. Many of the species that became extinct did so right at the end of this period, coinciding with the last glacial period or "last Ice Age", perhaps pointing to climate as the major factor in their demise. Yet throughout the Pleistocene there were repeated cycles of ice ages during which huge ice sheets covered much of Europe and North America. These were punctuated by warmer interglacial periods. Many researchers have pointed out that the ancestors of the large animals had survived through previous climate fluctuations, perhaps suggesting that humans must be the crucial new factor that pushed all these species into extinction at the end of the Ice Age.
There is some evidence to suggest that the last Ice Age was harsher than previous ones. However, archaeologists have highlighted the dating of remains of these large animals, known as megafauna, which seems to suggest that the extinctions coincided with the arrival of our ancestors in different regions. Our species, Homo sapiens, started to emerge from Africa between 50,000 and 80,000 years ago, spreading throughout Europe and Asia about 40,000 years ago and on to the Americas by around 15,000 years ago.
Our species, Homo sapiens, started to emerge from Africa between 50,000 and 80,000 years ago
Some have proposed a "Blitzkrieg" model in which humans arrived in North America and rampaged across the continent, wiping out everything in their path. In this scenario they would have hunted some species, such as woolly mammoths, to extinction, while undermining populations of predators like sabre-toothed cats by competing with them for prey.
There are many archaeological sites where evidence of humans butchering and eating these animals has been unearthed. But of course that is not the same as finding evidence of hunting itself meat can be acquired by scavenging as well as hunting. Ethnographic studies have shown that hunter-gatherers are very good at driving big cats off kills and taking the meat for themselves. But as the cats would then need to kill more prey, this means that human scavenging could still have a significant impact on prey populations.
While the coincidence with the arrival of humans is compelling, it's frustratingly difficult to uncover the real causes of the megafaunal extinctions, because this period was also one of rapid climatic and environmental change. After the peak of the last Ice Age around 20,000 years ago, the global temperature started to warm, with changing rainfall patterns, melting glaciers and rising sea levels. Perhaps many large mammals simply found it difficult to adapt to these changing environments.
One crucial development that has allowed this debate to move forward is the reconstruction of the ancient climate. By examining tiny gas bubbles trapped in ice cores drilled out of ice sheets in Greenland and Antarctica, scientists can determine what the atmosphere and climate were like, going back hundreds of thousands of years. The ice cores have shown that climate change could be incredibly rapid, with major fluctuations over just a few decades. These discoveries helped to confirm previous work by geologist Russell Coope, who argued in the 1960s that there had been extremely rapid climatic changes during the late Ice Age. Coope's claim was based on the discovery of ancient beetle remains in a gravel pit at Upton Warren, near Bromsgrove. The beetles were adapted to very specific temperature ranges, and showed rapid changes in climate. This flew in the face of prevailing ideas at the time, which favoured a model of very gradual change.
In an impressive study published in the journal Nature last year, Eline Lorenzen and colleagues at the University of Copenhagen used multiple techniques to approach the problem of Pleistocene extinctions: ancient DNA analysis, species distribution models and archaeology. They investigated the impact of both climate and humans on woolly rhino, woolly mammoth, wild horses in North America, reindeer, Eurasian steppe bison and Eurasian musk ox. They found a correlation between the size of the available habitat, as estimated from reconstructions of ancient climate, and genetic diversity. By looking at the levels of genetic diversity and matching these with human fossil records, the group found that several species were under pressure thousands of years before the arrival of humans.
As far as the Eurasian musk ox and woolly rhino are concerned it looked as though climate change was the main cause of their extinctions. This fits with the archaeology, which shows little evidence of humans hunting musk ox. These animals are still around in Greenland and the far north of America. They are pretty feisty and tend to hang around together, so youd have to be pretty brave to take them on. The ancient climate reconstruction suggested that the available ranges of the Eurasian steppe bison and wild horse in North America were not dramatically restricted, so, in these cases, it looks as though our ancestors may have played an important role in their extinctions. Archaeological evidence also suggest these species were being eaten in large numbers. Lorenzens study was inconclusive when it came to the causes of woolly mammoth extinction, except that it found that their demise occurred gradually, and they overlapped with humans for at least 10,000 years in Europe and several thousand years in North America a finding that doesn't fit with the Blitzkrieg model.
Musk Ox are pretty feisty and tend to hang around together, so youd have to be pretty brave to take them on
Another fascinating new line of evidence comes from palaeontologist Dan Fisher at the University of Michigan. Unlike human teeth, woolly mammoth and mastodon tusks grew throughout life, with the continual addition of dentine. Just as tree rings provide a detailed record of the good and bad years for trees based on their spacing, theres also an inventory of life events in the tusks. The spacing of the lines in the tusks is more compressed during periods of poor nutrition. The four-yearly cycles of reproductive females, carrying the unborn baby mammoth for two years and feeding the infant with milk for another two years, shows up in the tusks. In males, the period when the animal reached reproductive age leaves its mark on the tusk like modern elephants, it looks like young male mammoths went into what is called musth, a kind of sexual rage during which they forget about eating and rampage about looking for mates.
Fisher believes he may be able to identify a sort of breakdown in mastodon society coinciding with the appearance of humans, based on signs of violent injury that he has identified in the skeletal remains of these animals. Perhaps this social destabilisation and disharmony was induced by the removal of older adult males as a result of hunting, something seen in modern elephant groups because of poaching. The younger, untutored males go into musth early and become violent. There are also some sites where Fisher thinks that mothers and infants may have been killed at the same time. Mammoths and mastodons invested a huge amount of time and energy in raising individual young, so the loss of females of reproductive age would clearly impact on the populations ability to recover from losses.
We shouldn't castigate our ancestors for the extinctions of mammoths and mastodons, as they couldn't have known the impact they were having
Tusk science points towards an interaction in which humans were gradually picking off enough mastodons over thousands of years to stop the mammals replenishing their population. To the individual hunter-gatherers, the decline in numbers would probably have been imperceptible. We shouldn't castigate our ancestors for the extinctions of mammoths and mastodons, as they couldn't have known the impact they were having. Fisher has failed to find evidence of nutritional stress in the tusks that would indicate animals struggling in a changing environment, and he has also identified several animals that died in summer or autumn the least difficult seasons for finding food so he argues that climate change is unlikely to have been a major factor in the extinction of these massive proboscideans.
The picture that is emerging is a complex one. Some of these large animals seem to have been affected more by climate change, others by humans. But perhaps this distinction is too cut and dried. Hunting may not have been the only way in which we have had an impact on these species. Although we tend to think of anthropogenic, or human-induced, climate change as something which has only emerged in the last few centuries, palaeoclimatologist William Ruddiman has argued it actually goes back thousands of years.
The conventional view is that human actions first began having a significant warming effect on the climate as a result of the release of carbon dioxide and other greenhouse gases during the earliest industrial revolutions of the 18th and 19th centuries. But Ruddiman identified some unexpected findings from within tiny bubbles of ancient air trapped in Antarctic ice cores. For most of the past 400,000 years these show concentrations of carbon dioxide and methane rising and falling in predictable patterns corresponding to the periodic changes to the Earths orbit and axis of rotation, and their impact on the amount of sunlight hitting the planet. This pattern appears to have been broken 8,000 years ago for carbon dioxide and 5,000 years ago for methane.
Humans began farming in the eastern Mediterranean around 11,000 years ago, in northern China a little later, and several thousand years later in the Americas. Ruddiman concluded that farming and associated forest clearances caused extra carbon dioxide and methane to be pumped into the atmosphere, triggering other mechanisms that released more carbon dioxide, making the planet warmer and preventing ice sheets from beginning to form in the northern hemisphere. In other words, our forebears helped to delay the onset of another ice age.
So it could be that our ancestors helped stabilise the climate in a way that has sustained an ideal environment for us, allowing populations to grow and civilisations to flourish. Does this mean we needn't worry about anthropogenic climate change, and that we could even see it as a positive development?
Ruddiman himself has always urged caution. Although human tinkering with the atmosphere goes back thousands of years, we are having a greater effect now than ever before, and we dont fully understand the long-term consequences. The recent scientific insights into the large animal extinctions and the effects of early farming on our climate, suggest that we need to be incredibly careful about our impact on environments and ecosystems. If a few million humans with stone axes and spears can kill off large numbers of giant mammal species and clear enough forest to warm the climate and delay an ice age, what will be the longer-term environmental impacts of todays 7 billion individuals? If anything, the lesson from this emerging body of work is that we need to inject greater urgency into efforts to counter the current rise in greenhouse gas concentrations and to further reduce our impact on endangered animals. Our own civilisations have flourished in the unseasonably stable climate of the last 10,000 years. We could easily upset that balance with disastrous consequences, not only for other species, but for the future of our own as well.