Lost Giants

By Stephen Wroe, Judith Field & Richard Fullagar
Nature Australia, Volume 27, no 5: 54 - 61

Make no mistake, the protestations of a vocal minority aside, size matters. Big is good. Big animals get first access at the local watering hole and can monopolise food resources. They also have fewer predators and live longer. Consequently, it surprises many to learn that large animals are rare. The vast majority of species, both in the past and present, are small.

Superficially, this seems counterintuitive, but scrutiny reveals there is an extensive downside to large size. Big animals need more food, more water and they need more time to grow and reproduce. Thus, their populations are typically small and, when the going gets tough, they are the first to bite the dust. It remains cruelly ironic that the Lion is far more vulnerable to extinction than the louse.

Still, for all their evolutionary fragility, there can be no doubt that large animals have charisma and when big animals go extinct people demand an explanation. Such issues become even more vexatious when many species go extinct within short periods (mass extinctions). Of course, most species that have ever lived are extinct and most of these weren't extinguished in mass events. These unsung extinctions remain largely unexplained. But mass extinctions always attract comment and, for some at least, consensus has been reached. Sixty-five million years ago a very large wayward rock took out the dinosaurs. With big names, big body counts and a bona fide extraterrestrial, this theory has always had real Hollywood-appeal and, not surprisingly, it's had wide coverage. Everyone knows an asteroid killed the dinosaurs.

With dinosaur extinctions wrapped up, in recent years another mass extinction has come to the fore - the late Ice Age extinction of the megafauna. Over the last 100,000 years around half the genera exceeding 44 kilograms or so have disappeared. Lost giants include everything from mammoths and sabre-toothed tigers to marsupial lions and the rhino-sized, wombat-like Diprotodon. Although most were not as massive as dinosaurs, a twist to these extinctions gives them special appeal: Homo sapiens was at the scene.

Exposing the causes of late Ice Age extinctions has occupied scientists for centuries. Humans and climate change have both been strongly implicated, but neither model has gained clear ascendancy. That is until recently. Over the last few years a succession of articles and books has placed the blame squarely on humankind. Proposed mechanisms range from human-induced habitat modification to the introduction of disease, but a model involving human causation called 'blitzkrieg' has received particular attention.

Developed more than 30 years ago to explain late Ice Age extinctions across the planet by Paul Martin (University of Arizona), blitzkrieg is characterised by tantalising simplicity. Proponents of Martin's best-selling global model argue that, wherever late Ice Age humans invaded pristine environments, they violently and almost instantaneously eliminated most of the megafauna. Support centres largely on proposed dates for human arrivals and megafaunal extinctions, but two supposed behavioural phenomena are central. Foremost is the concept of naivety, that is, that the large animals of previously unoccupied lands were easy prey because they were ignorant of humans. The second principle is that, regardless of culture, if people can exploit a food source they will. Furthermore, such exploitation will only cease if the resource is squeezed into extinction or becomes uneconomical to obtain.

Today's blitzkrieg hit-list includes the Americas, Australia, Madagascar, New Zealand and various smaller islands. Some reports have presented global blitzkrieg as fact.

However, the issues here are important and of more than strictly scientific significance. Politicians and special-interest groups have misused and misrepresented interpretations of late Ice Age extinctions. In Australia some have argued that, because we now 'know' Aborigines wiped out the megafauna, farmers are in fact simply applying a replacement therapy by stocking our continent with large hard-hoofed animals. Others have used this 'fact' to attack the credibility of Aborigines as environmental custodians. Thus, it is especially important that scientists distinguish fact from hypothesis and that all sides of the debate are heard.

Here we give our case and, far from accepting that recent investigations have proven global blitzkrieg, we feel that some actually represent compelling evidence against it, especially with respect to Australia. Consequently, we will focus on Australia as the spanner in the works for those who advocate blitzkrieg as a worldwide phenomenon.

But first we wish to point out that, despite the high profile developed for universal blitzkrieg, there remains not a single landmass exceeding 150,000 square kilometres (South Island of New Zealand, where the Maoris wiped out the moas) for which human causation is generally accepted, and even here it is not clear that predation was the primary factor. Although many scientists clearly don't subscribe to global blitzkrieg, their protestations have largely fallen on deaf ears. But then perhaps this is not surprising. The mostly climate-based counter-hypotheses were always going to look shop-worn against the sensational imagery of spear-wielding hordes hacking their way through herds of startled megafauna. Yet while universal blitzkrieg has simplicity and pizzazz, it has obvious problems. Fundamental to these is the fact that each of the landmasses involved are so very different - different sizes, different biological histories, different climatic histories and different cultures, to list a few. These differences are often overlooked by advocates of blitzkrieg, who focus on the timing of extinctions and human migrations, together with the fact that more megafauna went extinct in places such as the Americas and Australia than in Africa, where humankind evolved.

This is too simple, but even at this level, one fact is often overlooked. In Europe there were also major megafaunal extinctions over the last 100,000 years. Yet these were clearly staggered over tens of millennia and there is no evidence for rapid mass extinction, despite both major climate change and human activity. Recent discoveries demonstrate that technologically sophisticated humans had extended their range to the European Arctic nearly 40,000 years ago (see "Arctic Pioneers" in Nature Strips, this issue) and at such high latitudes these people were almost certainly big-game hunters (in this extreme environment there was little else for humans to eat). Yet they had no immediate or obvious effect on the megafauna. This inconsistency will require some special pleading by proponents of blitzkrieg.

There are other elemental difficulties. Typically flagged as support for the linchpin of blitzkrieg - naivety - is humankind's ability to eliminate species from previously uninhabited islands. Examples include everything from Dodos to moas. But ignored are the facts that island species are uniquely vulnerable to extinction and that the animals in question had no prior experience with any large terrestrial predators, human or otherwise. It is misguided to transfer this model across to continents, orders of magnitude larger and dominated by ferocious carnivores the calibre of sabre-toothed tigers and marsupial lions.

Differences among continents themselves further eat into arguments for universal blitzkrieg. Australia is particularly conspicuous. Among the three continents for which blitzkrieg has been invoked, Australia is the smallest, the flattest, the driest and the most isolated. The history of human occupation in Australia is also unique and an intractable thorn in the side of global blitzkrieg theorists is the fact that no direct evidence exists for even a single Aboriginal kill. Even more importantly, the large stone spear-points typically associated with Ice Age big-mammal hunting elsewhere don't appear in Australia until 6,000 years ago, long after the megafauna perished. The significance of this absence is evidenced by investigations of more recent cultures. A study of 70 traditional American societies by Christopher Ellis (University of Western Ontario) demonstrated that stone points were used almost exclusively for hunting large game (over 40 kilograms). Large game was sometimes hunted with wooden-tipped projectiles, but never systematically. We don't argue here that a lack of big-game-specific hardware excludes the possibility that Australia's first humans killed megafauna. In fact we think that they almost certainly did, on occasion. But the lack of such a tool-kit strongly suggests that, where the megafauna was hunted, it was done so opportunistically. This does not sit well with blitzkrieg, which demands efficient, systematic and continent-wide persecution.

Technological differences are closely associated with another weakness in the argument - the assumption that all colonising cultures must treat all big game not just as food, but as the primary food source. The elimination of 20 megafaunal genera within 1,000 years from a landmass the size of Australia, by small bands of people that only occasionally ate big game, truly stretches credulity to the limit. Stone Age societies were not a homogenous, culturally impoverished rabble, and not every society has systematically hunted megafauna, even when such fauna was abundant. Indeed, such societies have always been in the minority. Among many examples, the Early Stone Age Gravettian people of southern Europe relied heavily on small game, not because large prey or necessary hardware were absent, but because they had technology (such as nets) that made smaller prey easier to get. We know almost nothing about the culture of first Australians, but if they were anything like all known societies from low latitudes, then their diet comprised around 70 per cent vegetable matter and most meat eaten was in the form of small game.

The type of culture imported by colonising humans may have profoundly affected their approach to the big game they encountered. Debate still rages over whether the Clovis Indians of North America exterminated 'their' megafauna. But we do know they had a megafauna-specific tool-kit, they definitely used it to kill at least some species and, importantly here, they hailed from a proud tradition of high-latitude, big-game hunting, honed on the vast Siberian tundra. However, for the first Australians, there is no smoking gun in the form of murdered megafauna, there are no specialised weapons, and their immediate ancestors were almost certainly not systematic hunters of big animals.

One last important factor distinguishes Australia. On other landmasses that were allegedly blitzkrieged, humans arrived in company. For example, when humans entered North America around 13,000 years ago, they were joined by a suite of essentially Eurasian megafauna, including Moose, Brown Bears and, quite possibly, the Grey Wolf - one of the most successful and efficient big-game hunters the world had ever seen. Demonstrable climatic upheaval aside, the devastation wrought on the endemic North American megafauna by these non-human assailants must have been significant. Singling out Homo sapiens as the sole culprit under such circumstances is indefensible. Most accept that the extinction of North America's megafauna happened quickly, but if humans played a role it was done with assistance. In Australia we don't know whether megafaunal extinction was rapid, but we do know that, if humans contributed, they had no help from other non-endemic megafauna.

For these reasons and others, Australia has long been the weakest link in the case for global blitzkrieg. Indeed, the strongest argument for any human culpability in Australia has been evidence suggesting that a big bird (Genyornis) went extinct from three sites in arid-semiarid southern Australia around 50,000 years ago, without evidence for climatic catastrophe. The team that forwarded this data, headed by Gifford Miller (University of Colorado), further inferred that, if people did in Genyornis, then maybe they exterminated other megafauna. But younger dates for Genyornis at Cuddie Springs (New South Wales) indicate that Genyornis extinction wasn't continent-wide at 50,000 years and, even if it was, the great majority of extinctions can't be attributed to obvious calamity. So, a lack of major climate change at this time doesn't automatically implicate humans in the extinction of Genyornis, let alone the rest of the megafauna. Lastly, if less extreme climate change was responsible, then arid-semiarid Australia is the first place we'd expect megafauna to disappear.

Consequently, when a team led by Richard Roberts (University of Melbourne) recently claimed to have pinpointed the last gasp of six genera of megafauna at 46,000 years ago, it was heralded as a shot in the arm by advocates of blitzkrieg and other models founded on human interference (Roberts's team included proponents of extinction through habitat disturbance, as well as blitzkrieg). This is because mass extinction is more likely to correlate with major catastrophe, and cataclysmic climatic change didn't peak until around 20,000 years ago. Again the inference, in the complete absence of direct evidence, is that if it wasn't climate then it must have been people. In this study Roberts et al. dated six sites with megafauna at significantly less than 46,000 years old, yet they dismissed them, based on the grounds that they were more likely to have been disturbed because they didn't contain skeletal material in neat, anatomically correct positions. But this is not standard practice, and serious doubts will remain until the ages of these six and at least ten other sites dated at less than 46,000 years are individually corroborated or disproved. Moreover, the sample of nine sites that ended up being included in the analysis is insufficient on which to draw confident conclusions. An additional problem is that the results actually reaffirmed previously determined dates of 27,000 to 36,000 years for one of the sites dismissed from their analysis, Cuddie Springs.

However, even if we suspend disbelief and accept that the megafauna hit a brick wall 46,000 years ago, one thing is clear - blitzkrieg wasn't the modus operandi. On the basis of previous studies by Roberts himself, his team argues for human arrival at 56,000 years ago. So a best guess for human-megafaunal coexistence based on their own data is 10,000 years. This is not blitzkrieg.

The authors have since stated that their results don't actually rule out blitzkrieg because, if we stretch the confidence limits on these numbers to their maxima, then the human-megafaunal coexistence of 1,000 years or less demanded by blitzkrieg is just possible. Strictly speaking this is true. Indeed very little can be completely ruled out. But in our view, conclusions should articulate what results show to be most likely, and the results tendered by Roberts et al. demonstrate that blitzkrieg most likely didn't happen in Australia.

Together with the lack of kill sites, specialised weaponry, or a big-game-hunting pedigree, this evidence for sustained coexistence leaves the argument for blitzkrieg in Australia all but dead. The implications are wide reaching. It is possible that extremely localised blitzkrieg happened in the relatively small islands of New Zealand and maybe even on other landmasses, but if blitzkrieg does not apply to Australia then, regardless of wherever else it may have occurred, the global blitzkrieg paradigm is sunk. With it goes the notion that mass extinction is an inevitable consequence of human interaction with naive megafauna.

While blitzkrieg now looks untenable in Australia, this hardly solves the riddle of what happened to our megafauna. Various scenarios remain credible, although in our view the available data are most consistent with extinctions occurring over a long period and at varying rates and times according to location. Roberts et al. showed that many genera of late Ice Age megafauna may have been extinct before humans arrived. We believe that climate change probably drove the final round of extinctions, beginning at around 45,000 years ago in the now arid-semiarid zones. Extinctions accelerated on an expanding but irregular outward front from this point, as Australia careered through the tumultuous lead-up to the Last Glacial Maximum 22,000 years ago. Megafauna probably persisted longest in ever-shrinking wetter regions, especially toward the coast. This is a testable hypothesis and supported by the fact that, with one borderline exception, all younger sites dated by Roberts et al. fall outside the now arid-semiarid zone. Such a climate-driven model does not necessarily absolve humans of any responsibility, but against this backdrop of prolonged climatic change, precise apportioning of blame will be very difficult.

Humans are instinctively attracted to simple explanations, but extinction is generally anything but simple. Complicating factors include fluctuations in temperature and rainfall, habitat destruction by humans, reconfiguration of habitats, fire, introduced predators and competitors, disease, and cascading effects whereby the extinction of one or a few key species forces the extinction of others. Particularly pressing questions that need to be answered include - why did the extinction process in Australia almost exclusively target browsers, why did so many late Pleistocene species evidently disappear before humans even arrived here, and why did the few survivors shrink in size? The role of humans will be very difficult to assess against this intricate canvas and, although we feel that blitzkrieg in Australia can be all but discounted, remaining models, including our own, have yet to be comprehensively tested. The way forward in this debate is to find and date more sites, re-date old ones, and investigate changes in animal populations through time, both before and after the arrival of humans. As Richard Wright (University of Sydney) commented a decade ago, "megafaunal extinction is a topic that now requires rather more digging than talking".

Australian Museum Mammals

Lost Giants

By Stephen Wroe, Judith Field & Richard Fullagar
Nature Australia, Volume 27, no 5: 54 - 61

Further reading

Australian Museum Mammals

Lost Giants

By Stephen Wroe, Judith Field & Richard Fullagar Nature Australia, Volume 27, no 5: 54 - 61

Further reading

By Stephen Wroe, Judith Field & Richard Fullagar
Nature Australia, Volume 27, no 5: 54 - 61