Part 1: The Sixth Extinction and the Anthropocene’s Pleistocene origins

Tens of thousands of years ago, the first wave of a worldwide tsunami now known as the “Sixth Extinction” swept across the planet as Homo sapiens, preceded in some cases by its kin, journeyed from the Cradle of Humankind in Africa and wiped out numerous species of mostly large mammals.

This prehistoric removal of megafauna – keystone species that play an outsized ecological role – by human hunters would have an earth-shattering environmental impact. These ecological consequences, in turn, would have historical consequences, including the rise of the Anthropocene.

Yet awareness of the ecological legacy of these extinctions hardly extends beyond a small band of trailblazing scientists and barely registers in the wider public discourse around the Anthropocene – Earth’s current geological epoch, which bluntly speaks to humanity’s effects on the environment.

Exceptions include the recent and excellent book, Nature’s Ghosts, by Sophie Yeo.

Read more: Spectre of the Anthropocene haunts insightful journey into our wild past

It’s also the case that while a growing number of scientists in this field embrace the notion of human “overkill” as the most compelling explanation for the prehistoric megafaunal extinctions, surprisingly little attention has been paid to answering why this occurred.

I have previously raised the possibility – if one accepts the overkill hypothesis – that human/wildlife conflict may at least have been a contributing factor. In all the scientific literature that I have reviewed, no one has made this link.

Read more: Africa’s beastly burden – the case of shrinking the faunal poverty line

Recent debates about the Anthropocene’s origins – and an emerging field of historical enquiry that looks at the role that animals, and animal agency, have played in history – provide fresh and revealing ways to reframe this issue, bringing it sharply into focus.

Human/wildlife conflict suggests agency on both sides. The term “conflict” implies more than one protagonist. It takes two to tangle in the bush.

And out of such entanglements, the multispecies origin story – red in tooth, tusk, claw, club and spear – of the Anthropocene would emerge.

The Anthropocene’s genesis

There is much debate about when the Anthropocene began and when to date the obituaries of the Holocene, which replaced the Pleistocene 11,700 years ago. Crawford Lake in Milton, Ontario, was proposed by the Anthropocene Working Group (AWG) in 2023 as the marker based on the plutonium levels found at its bottom.

This places the Anthropocene’s detonation in the mid-20th century. A wide range of other dates have been suggested. The late 18th-century Industrial Revolution is a candidate, while some have argued that the Anthropocene’s seeds were first planted 8,000 years ago with the Neolithic or agricultural revolution.

But others have pointed to the initial round of the Sixth Extinction as the start, or at least a progenitor, of the Anthropocene. The previous five mass extinction events – the fifth being the demise of the dinosaurs 66 million years ago – were caused by natural calamities.

The Sixth Extinction, by contrast, is regarded as human-caused – though some scientists now question the notion that such an event is currently under way.

In September last year, researchers drawing on the comprehensive database of the International Union for Conservation of Nature (IUCN) concluded in a paper published in PLOS Biology that recent extinctions were rare and localised – with most occurring on islands where species are particularly vulnerable – and that the pace of extinction has been slowing.

This view can perhaps be viewed as an island of scepticism in what many regard as a sea of scientific consensus, and there are swirling currents of debate on this issue. But pointedly, the scientists who say the evidence does not point to a current mass die-off of species do not deny that human activities have been the cause of recent extinctions.

I want to draw attention to some points that are missing in these debates.

The first is that the initial extinctions caused by humans were the disappearance of dozens of species of mostly very large herbivores – but also some carnivores – around the world that occurred between 132,000 and a few thousand years ago.

The second is that this extinction event would, in turn, shape the subsequent course of global history.

The reasons for this megafauna vanishing act have, for decades, been a highly contested scientific terrain. Theories advanced include a lethal zoonotic disease exchange from early humans to animals and the fallout from a meteorite that crashed into the Earth around 13,000 years ago.

But these are sideshows to the main debate, which broadly pits those who regard climate change as the main driver against those who track the archaic spoor of the extinctions to Paleo-hunters. Some scientists straddle a middle path, maintaining that a changing climate and human hunting combined in ways that proved particularly lethal to large animals.

Archaeologists tend to dig the climate hypothesis while conservation biologists lean toward human overkill, first raised in the 1960s by a pioneering American scientist named Paul Martin.

Both explanations reflect current concerns about the Anthropocene, underscoring the point that neither science nor history are created in a bubble.

Human-induced climate change, linked mostly to the burning of fossil fuels, is widely regarded as the biggest current environmental threat to the planet. This has given rise to a focus on previous climate impacts that has been pursued across a range of disciplines, including history, opening new windows into our understanding of the past.

Overkill as a theory also speaks to contemporary cultural currents, including the misleading view that we humans are somehow natural-born killers and despoilers of the environment.

Overkill: The Anthropocene’s Big Bang

If human agency was involved, then these prehistoric mega-extinctions are indeed ground zero for the Sixth Extinction and represent the Anthropocene’s Big Bang.

A paper published in 2024, lead-authored by Jens-Christian Svenning of Aarhus University – a renowned expert in the field – argues that the megafauna extinctions were a “... progenitor of the Anthropocene”.

“A broad range of evidence indicates that the megafauna extinctions have elicited profound changes to ecosystem structure and functioning ... they represent an early, human-driven environmental transformation at large scale,” Svenning and his co-authors write in the paper entitled, The late-Quaternary megafauna extinctions: patterns, causes, ecological consequences and implications for ecosystem management in the Anthropocene.

One major flaw in the climate causation case for the megafaunal extinctions is that the Pleistocene was marked throughout by periods of dramatic climate change, including the spread and retreat of glaciers – it is popularly known as the Ice Age.

Yet these earlier glacial events did not trigger mass megafaunal extinctions, and there is no evidence that the most recent one was unusually severe, the paper maintains.

Critics of the climate hypothesis have also highlighted continuity rather than change. In previous papers, Svenning and other leading proponents of the overkill hypothesis have noted that South America lost far more megafauna species than Africa, despite both regions having a similar and relatively stable climate when the extinctions occurred.

But the extinctions do coincide with patterns of human biogeography – when Homo sapiens or our close kin rocked up, big animals went extinct, usually within the space of a few thousand years, and there is a rich and growing scientific literature in support of this view.

“... extinction severity is strongly linked to human biogeography, with severe extinctions where Homo sapiens was most novel (i.e., the first hominin present) and least severe in the long-term area of human evolution, Africa and southern Asia,” the paper cited above notes.

‘Co-evolution’ in Africa

Africa was spared extinction on the same scale as what obtained elsewhere because its big animals did not share the fatal handicap of their relations elsewhere: having co-evolved with Homo sapiens, the region’s Goliaths were challenging prey for human Davids.

“Notably ... mass extinctions of megafauna did not take place in Africa,” writes the historian Christopher Ehret in his recent and excellent book, Ancient Africa: A Global History to 300 CE. “In our home continent of Africa, differently from elsewhere in the world, wild animals and our human ancestors had, after all, coevolved and coadapted to each other over the millennia – in Africa we were not an intrusive new challenge to the natural order.”

Pointedly, he also notes that “... the capacities of our species for vastly changing the world and the environment around us is not all a development of recent history”.

Indeed it is not – and viewed in this way, the Anthropocene is also not a “development of recent history”.

Many find it inconceivable that Paleo human hunters could have wrought such carnage with an arsenal fashioned from rock, wood and fish tails.

Telling evidence

But there is telling evidence from kill sites and other archaeological signs that late Pleistocene humans were proficient hunters more than capable of taking on and taking down megafauna, in some cases with the help of recently domesticated wolves that had been transformed into dogs.

Massive adult herbivores probably had no predators until the colonisation of their range by Homo sapiens, and they have slow rates of reproduction, making them especially vulnerable to a new alpha predator.

What is also astonishing is the sheer scale of these megafaunal extinctions, both in terms of the number of species and the geographic range.

Of the 57 species of terrestrial mega-herbivores weighing 1,000kg or more that walked the Earth 130,000 years ago, only 11 survive.

In Australia, the mega-extinction list was also cold-blooded, featuring a terrestrial crocodile, a massive constrictor and a monitor lizard, megalania, that may have reached 1,000kg, dwarfing the largest living monitor, the feared and man-eating Komodo dragon of Indonesia.

Several large warm-blooded carnivores, including cave bears and lions in Europe, and sabre-toothed cats and American lions and cheetahs, also disappeared.

The extinctions extend from the Arctic to the tropics and across all inhabited continents. The woolly mammoth alone had a range of around 33 million square kilometres. That exceeds the historic range loss over the past 500 years of African and Asian elephants – species subjected to the full onslaught of the Anthropocene’s modern stage.

This begs the obvious question: why would ancient humans have devoted so much time and energy to eliminating megafauna?

We will address that question in part 2 of this trilogy. DM