MEGAFAUNAL EXTINCTIONS | Slaying Giants:

An Introduction to the Megafaunal Extinctions

What a world it was in the late Ice Age. The Earth at this time was truly the Planet of the Giants, and we have previously seen that Ireland had its own complement of massive mammals, such as the woolly mammoth and the giant Irish deer. However, these ancient Irish behemoths were but a drop in an ocean of mega-beasts that lumbered, prowled, waddled, and slithered across the many lands of the Earth, the continents ruled by an endless army of giants. Giant elephants, giant marsupials, giant bears, giant snakes, giant geese, giant sloths, giant armadillos, mammoths, woolly rhinos, cave lions, sabretooth cats, dire wolves, and many, many more. But, of course, the ascendancy of these towering titans was not to last, and by the end of the Ice Age a great many had been laid low, and more were to follow in the Holocene (c. 11,700 years ago–Present). Many causes have been proposed for these great megafaunal extinctions, among them the rise and spread of a line of mammals with unprecedented intelligence and adaptability – modern humans.

The possibility of a human hand in ancient extinctions has direct relevance to the present, as in recent years it has become clear that we are in the middle of one of the great extinctions of life on this planet – one which is being driven by human activities. Whereas once we spoke of the greatest mass extinctions as the ‘Big Five’, the last of which wiped out around 65–75 per cent of species 66 million years ago, including the non-avian dinosaurs, many scientists now hold that a Sixth Extinction is under way.

Species are currently going extinct at a rate of 100–1,000 times the normal background level, of about 10–25 species a year, and are being wiped from the face of the Earth by hunting, pollution, deforestation, and habitat destruction. This global mass extinction, along with other examples of human alteration of our planet, have driven a growing consensus that we are now no longer living in the Holocene Epoch but have crossed into a new one, the Anthropocene, a name derived from the Greek for ‘human’ (anthropos) and ‘new’ or ‘recent’ (kainos). Yet, some believe the evidence for human-driven extinctions goes back much further, all the way back to the Pleistocene Epoch (c. 2.6 million years ago–11,700 years ago) – the Ice Age.

As mentioned previously, in the late Ice Age there were a series of extinctions of large-bodied mammals across the world’s continents which carried on to some extent into the Holocene – the megafaunal extinctions. ‘Megafauna’ are defined as mammals weighing 45 kilograms or more, but many of the Ice Age beasts to suffer extinction at this time were far larger than this, the greatest being possibly over three times heavier than the largest land mammal on Earth today, the African elephant.

During this incredible extinction, most of the continents lost all of their truly large mammals, with all species over one tonne (1,000 kilograms) disappearing from everywhere except Africa and southern Asia, while around 80 per cent of those weighing 100–1,000 kilograms were also lost. And though we now know that the giant Irish deer, which belonged to this second category, survived longer than previously thought, with its last members not suffering extinction until the early Holocene, it did disappear from many lands in the late Ice Age, including Ireland.

This dramatic loss of so many large species was strikingly evident to one of the fathers of evolution, Alfred Russel Wallace, who remarked in 1876 that ‘we live in a zoologically impoverished world from which all the hugest and fiercest and strangest forms have recently disappeared.’ But exactly why we now inhabit such a zoologically impoverished world has been a matter of extremely heated debate – what did cause the extinction of the mighty megafauna?

Theories regarding the extinction of the megafauna have generally related it to one of two things, either human hunting or climate change. Proponents of the view that humans caused the megafaunal extinctions through hunting, a theory known as the ‘overkill hypothesis’, argue that climate change cannot be responsible as none of the previous episodes of mammal extinction related to climatic or environmental factors discriminated so heavily against mammals of large size. Furthermore, they contend, the megafaunal extinctions took place at different times and in different ways on different continents, and that human hunters would have focused their energies more heavily on large mammals as these would have been easier to locate and kill, and would have been a far richer source of meat and materials than smaller mammals.

One of the central arguments in the overkill hypothesis is that the extinction of the megafauna in each region mostly coincided with the arrival of humans, which occurred at a different time on each continent. To support this view, overkill enthusiasts cite the evidence of much more recent megafaunal extinctions which occurred after the arrival of humans on islands like New Zealand and Madagascar.

For instance, in New Zealand, giant moas – massive flightless birds which were bigger than our modern ostriches – suffered extinction within a century after the ancestors of the Maori first made the island their home in the late thirteenth century.

And up until very recently the picture in Madagascar was thought to be quite similar. On that great island, even larger flightless avian forms called ‘elephant birds’ – which could be up to three metres in height and weighed half a tonne or more – suffered extinction as part of a megafaunal extinction that lasted from around 2,400 years ago up to around 500 years ago, which also wiped all of the large lemurs from the island. Madagascar was once home to some truly gigantic lemurs, with one, Archaeoindris, weighing as much as 200 kilograms – as large as a male gorilla! However, by 500, or maybe even 300, years ago the last of the giant lemurs had been driven to extinction.

This great Madagascan megafaunal cull is thought to have been driven by human hunting and alteration of the island’s natural habitats, and it was long thought that this process began not long after humans arrived on the island, with evidence for their first presence dating to around 2,500–4,000 years ago. However, in September 2018 the discovery of evidence for a far earlier human arrival in Madagascar was announced, with humans now known to have been present on the island around 10,000 years ago.

Interestingly, this evidence consists of cut marks on the bones of elephant birds, showing that they were being hunted for their meat by the earliest human settlers of the island and that it may indeed have been human hunting which ultimately wiped these great birds out altogether, And yet, equally, this evidence also showed that the elephant birds and other Madagascan megafauna like the giant lemurs co-existed with humans on the island for a far, far longer period of time – over 9,000 years – than had been envisaged previously. Thus, even if humans were indeed responsible for the extinction of the island's megafauna, this had not been a case of a very swift demise following human arrival in Madagascar.

The evidence from recent megafaunal extinctions in New Zealand and Madagascar, then, provides some support for the overkill scenario for the late Pleistocene–Holocene megafaunal extinctions across the world, although the new evidence from Madagascar also demonstrates that modern humans have managed to live alongside megafaunal species in some places for thousands and thousands of years before the latter went extinct.

Thus, this new evidence from Madagascar could perhaps also be cited by proponents of the climate change view, who believe that the possible influence of humans on ancient megafauna has been overstated. They argue that human populations in the late Ice Age were too small, and their technologies too simple, to have had such a profound effect on the large mammals they shared the continents with. Others, perhaps wisely, have attributed the megafaunal extinctions to a mixture of these two causes, while the human impact is noted by some to possibly have been due not only to hunting but to the modification of the landscape using fire.

So, what caused the extinction of the giant Irish deer in Ireland? Was it human hunting? Was it climate change, with the descent into the freezing conditions from around 12,900 years ago which immediately preceded the beginning of the Holocene? Or was it a combination of these factors?

Evidence to support each of the main views regarding the causes of the megafaunal extinctions can be found on different continents, but to see how these inform our understanding of the extinction of the giant Irish deer, and if the first Irish played a part in their demise, it may be useful to look at each continent in turn over the coming months to determine what the latest evidence tells us about the possible role humans played in each case. Along the way we will meet a cast of mammalian characters and other animals of truly mind-blowing dimension and diversity, opening our eyes to the strange and beautiful world inhabited by the first Irish and all of the other ancient humans of the late Ice Age.

Finally, it is worth re-stating why such an in-depth investigation of this issue is warranted. The questions relating to the possible human role in the megafaunal extinctions strike at the very heart of our nature as human beings and our interactions with the life on this planet and the planet itself. They challenge us to ask ourselves if the first Irish were more similar to the modern Irish than we would care to admit – if their impact on the natural world was different to that of the humans in Ireland today only in degree, not in nature. In other words, has the footprint of unsustainable human activity been pressed into the earth only over recent centuries, or can its outline be discerned as far back as the Ice Age? Or, as Tom Kemp neatly phrases the question in his 2005 book The Origin and Evolution of Mammals: ‘Was the end-Pleistocene megafaunal extinction the last of the many acts of biotic reorganisation by unbridled Nature, or the first of the many acts of global devastation by unconstrained Humanity?’