Sabre-toothed cats, wooly mammoths and rhinos, ground sloths and even giant beavers. These are just a few of the large animals that walked the Earth alongside our ancestors for most of the last ice age, between 10,000 and 100,000 years ago.
Many of these massive beasts — the Ice Age 'megafauna' — went extinct during the Pleistocene, a geological epoch that lasted some 2.6 million years, up until 12,000 years ago. Known as the 'Great Ice Age', the Pleistocene consisted of several cycles of ice ages, during which huge ice sheets covered much of Europe and North America. Of the species that died out, many did so right at the end — coinciding with the final glacial period, the 'last Ice Age' — perhaps pointing to climate as the key factor in the Pleistocene extinctions.
But the ice ages were also punctuated by warmer interglacial periods, and palaeontologists point out that the ancestors of some species of megafauna had managed to survive previous ice ages and climate fluctuations, suggesting that the appearance of humans was the key factor.
This idea is supported by archaeologists, as the dating of megafauna remains shows that the extinctions matches the arrival of our ancestors in different locations: Homo sapiens started to emerge out of Africa 50,000 to 80,000 years ago, spreading throughout Europe and Asia 40,000 years ago, then on to the Americas by around 15,000 years ago.
Although dig sites have unearthed evidence that humans ate big beasts, this doesn't prove they were hunted, as meat can also be obtained by scavenging. Ethnographic studies show that hunter-gatherers are good at driving big cats off kills, but those predators then have to find something else to eat, putting further pressure on a prey population. Ultimately, it doesn't really matter how humans acquired megafauna for food because the overall effect is similar.
While the coincidence with the arrival of humans is compelling, unravelling the cause of the Pleistocene extinctions is difficult because the period involved rapid climatic and environmental change.
After the peak of the last ice age 11,500 to 18,000 years ago, temperatures warmed, glaciers melted and rainfall patterns changed, with extinctions being more extensive on continents that experienced the greatest climatic changes. Perhaps some species simply found it difficult to adapt to these changing environments.
It's a lively and ongoing debate. Traditionally, most of the evidence to support the various theories came from looking at a few bones from archaeological sites. But recently, powerful new scientific techniques have been deployed to generate new types of evidence. And in combination, the results are providing deeper insights into the timing and causes of the Pleistocene extinctions.
A 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, we can now determine what the atmosphere and climate were like hundreds of thousands of years ago. The ice cores show that climate change could be incredibly rapid, with major fluctuations over just a few decades.
Scientists are also looking for links between the ancient climate and evolutionary history. Last year, a team of biologists, led by Eske Willerslev of the Centre for GeoGenetics at Copenhagen University, used multiple techniques to approach the problem, such as analysing ancient DNA, species distribution and archaeology.
Willerslev's team investigated the roles of both climate and humans on large prehistoric mammals — including the woolly mammoth, reindeer and Eurasian musk ox — and found a relationship between the size of their available habitat (estimated from reconstructions of ancient climate) and their genetic diversity. Looking at the levels of genetic diversity and matching this with human fossil records, the biologists found that several of these species were under pressure thousands of years before the arrival of humans.
The picture that's emerging is more complex that we once thought: some megafauna seem to have been affected more by climate change, others by humans, and the distinction between these two factors may not be so cut and dry. And hunting isn't the only way in which our ancestors might have affected these species.
Although we tend to think of anthropogenic climate change as something that's only emerged in the last few centuries, palaeoclimatologist William Ruddiman of the University of Virginia has argued that it actually goes back thousands of years.
The conventional view is that human actions first began having a warming effect on the climate as a result of releasing carbon dioxide (CO2) and other greenhouse gases during the earliest industrial revolutions of the 18th and 19th centuries.
But Ruddiman has discovered something unexpected within tiny bubbles of ancient air trapped within Antarctic ice cores. For most of the past 400,000 years, the bubbles pointed to concentrations of CO2 and methane rising and falling in predictable patterns corresponding to the periodic changes to the Earth's 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 CO2 and 5,000 years ago for methane.
Humans began farming in the eastern Mediterranean around 11,000 years ago, in northern China not long after, and in the Americas several thousand years later. Ruddiman concluded that farming and associated forest clearing caused extra CO2 and methane to be pumped into the atmosphere, triggering other mechanisms that released even more CO2, making the planet warmer and preventing ice sheets from beginning to form in the northern hemisphere.
In other words, human activity has helped 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 civilisations to flourish.
Does this mean we needn't worry about anthropogenic climate change, and that we should even see it as a positive development? Ruddiman has always urged caution: although tinkering with the atmosphere goes back thousands of years, humans are having a greater effect now than ever before, and we don't fully understand the long-term consequences.
Recent insights into the late Pleistocene extinctions, plus the effects of early farming on our climate, suggests 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 mammals and warm the climate enough to avoid an ice age, what will be the longer-term impacts of today's seven billion individuals?
If anything, the lessons from the last ice age show that we need to inject greater urgency into efforts to counter the current rapid rise in greenhouse gas concentrations and to further reduce our impact on endangered animals.
Human civilisations have flourished in the unseasonably stable climate of the last 10,000 years, and we could easily upset that balance with disastrous consequences — not only for the future of other species, but for our own as well.
This is an edited extract from The Eden Articles, a free e-book available from Amazon and iTunes from 20 July, which also includes essays by Adam Rutherford, Kevin Fong, Maggie Aderin Pocock and Ben Miller. Alice is appearing in The Incredible Human Journey on Eden (Sky TV channel 532 and Virgin 208).