15th June 2012
We live in a zoologically impoverished world, from which all the hugest, and fiercest, and strangest forms have recently disappeared; and it is, no doubt, a much better world for us now they have gone. Yet it is surely a marvelous fact, and one that has hardly been sufficiently dwelt upon, this sudden dying out of so many large mammalia, not in one place only but over half the land surface of the globe.
(Wallace, 1876: 150)
Charles Darwin (2003:350) wrote, “The extinction of species has been involved in the most gratuitous mystery… No one can have marveled more than I have at the extinction of species… when I found the tooth of a[n extinct species of] horse embedded in the remains of a Mastodon… I was filled with astonishment”. In Australia, like many other parts of the world, the impact early hunter-gatherers had on the flora and fauna, especially as it relates to Australian Pleistocene ‘megafaunal’ extinctions, has long intrigued the scientific community. However, due to its geographical isolation from much of the world and for many millions of years, Australia has, until recently, been considered peripheral in this debate. This is primarily because of the complete absence of direct evidence for either predation or habitat modification (Wroe, 2004), but also because there has been no consensus over the exact timing of human arrival or megafaunal extinction (Martin, 1984). Central to more recent arguments for anthropogenic causation are claims to have demonstrated concurrence between, the disappearance of megafauna and human colonization (Miller, 1999; Roberts, 2001) that were not correlated with significant climatic change. In addition, the research into Australian megafaunal extinction is further complicated by relatively poor chronology, and the frequent retrieval of newer information.
So the important question to be asked becomes, what role, if any, did humans play in the demise of these large creatures? “Looking at the whole subject again, with the much larger body of facts at our command”, Wallace (1911:246) wrote “I am convinced that the rapidity of . . . the extinction of so many large mammalia is actually due to man’s agency, acting in co-operation with those general causes which at the culmination of each geological era has led to the extinction of the larger, the most specialized, or the most strangely modified forms”. It is perhaps these comments that set the stage for modern discourse pertaining to this phenomenon. Although contemporary scientists understand considerably more about the timing of these extinctions, the number of species involved, and the geographic patterns of extinction, much of the mystery remains, and the debate over the cause(s) of the Late Pleistocene megafaunal extinction continues.
Researchers, in their division regarding the notions of causation continue to ascribe simple unicasual models to the processes of extinction (Barnosky, 2004). Traditionally, like at the time of Wallace, there are some who believe that the humans played a primary role in these extinctions, either directly or indirectly, and others who claim that climate driven processes were responsible. However, it is increasingly clear that single causation models cannot explain the current extinction crisis, and that discerning the relative importance and contribution of multiple factors is a major challenge (Didham, 2005 cited in Wroe, 2006). Indications of this can be seen in recent discourse regarding Pleistocene extinction and increasing conformity to the notion that reasons for causation cannot be derived from a single mechanism (Burney, 2005). Despite this, new research continues to be offered in support of proposals for either climatic alteration or anthropogenic causation.
Historically, the case for anthropogenic causation in Australia was based on extrapolation of evidence from other continents and remote islands (Flannery, 1994 cited in Hiscock, 2008). While late Holocene societies clearly may have driven many island species to extinction, “islands are not continents writ small” (Wroe, 2006). It was at this time, arguments for anthropogenic causation in Australia received what many acknowledged as their first empirical evidence in support of their theory, the discovery of a large flightless terrestrial bird, Genyornis newtoni (Miller, 1999). Miller (1999) reviewed the evidence pertaining to its disappearance and dated its extinction to 50 ka, based on ages from fossil eggshells retrieved from Lake Eyre Basin, inland northeastern South Australia (Pate, 2002). It has been suggested that humans had arrived in Australia by this time, and that anthropogenic causation was either directly or indirectly responsible for the demise of Genyornis and other Australian megafauna. The latter argument predicates that the early Australian, human inhabitants set fire to large areas of grassland, in order to increase grassland productivity at the expense of trees and shrubs, and that this caused the collapse of long-established ecosystem elements, including the extirpation and eventual extinction of browsers that depended on tree and shrub vegetation (Elias & Schreve, 2007). Further sampling also suggested simultaneous dietary alterations in sympatric emu populations (Miller, 2005 cited in Hiscock, 2008). Miller (1999) also contended that, the demise of megafaunal predators followed the extinction of their large herbivore prey. In 2001 authors of an alternate study concluded that all Australian megafauna became extinct by around 46ka (Roberts, 2001), and argued that megafaunal extinctions in Australia were therefore anthropogenicly mediated because they predated significant climatic change. Computer simulation have also been interpreted as supportive of anthropogenic causation (Brook & Bowman, 2004 cited in Hiscock, 2008)
For some Australian scientists, a primary objective has been to demonstrate that extinctions predated ‘significant’ climate change, but post-dated human arrival (Roberts, 2001), since fundamental to arguments for anthropogenic causation is the requirement that all, or at least most megafauna were present when humans arrived. Roberts (2001) noted that available data only confirmed the existence of 60 percent of extinct genera at ages older than 80ka. The possible extinction of approximately 40 percent of genera prior 80ka would predate the recognized arrival of humans and coincide with significant climatic change. Wroe (2006) supports this in claiming that “65% of extinct Pleistocene megafauna are absent from deposits younger than 130ka and only eight species were clearly present at the earliest point for which there is a broad acceptance for human arrival. At least four of these taxa survived up to 36-30ka and the onset of full glacial conditions”. Similarly fossil evidence from many continents points to the extinction mainly of large animals at or near the end of the last glaciation. These large animals or ‘megafauna’, were worst affected because they maintained relatively low population densities, matured late, and had few offspring. Furthermore, they were slower to recover numbers following any significant population decline. In many regions of the world, herbivorous mammals, such as the woolly mammoth and woolly rhinoceros, dominated the Pleistocene megafauna. However, in biologically isolated regions of the world such as Australia, the Pleistocene megafauna also included other vertebrate groups, such as large, flightless birds, and giant turtles and lizards (Elias, & Schreve, 2007). Despite Australia’s initial megafaunal diversity, approximately 86% of the Australian megafauna had died out before 16 ka, leaving only three genera of large mammals in the Holocene (Barnosky, 1989; Elias, & Schreve, 2007; Martin, 1984).
The timing of these extinctions, and possible associations with human hunting, are a matter of some controversy. Most recent analysis supporting anthropogenic causation in Australia accepts continent-wide extinction at around 46ka. As such younger ages are erroneous by default. According to Horton (1984 cited in Wroe, 2006), there are in excess of 167 Australian Pleistocene sites bearing megafauna. Of these Wroe (2006) argues that current evidence is plausible for ages of 36ka or less only at four sites, Clogg’s Cave, Seton Rock shelter, Nombe Rock Shelter (New Guinea) and Cuddie Springs. The interpretation of the fossil record at the Cuddie Springs site is a good example of this. Cuddie Springs is an ancient lakebed, near Carinda, in the semiarid zone of north-central New South Wales. Ages for the Cuddie Springs site are particularly well supported and have provided evidence of megafauna, including Genyornis and diprotodon, within sealed strata (Wroe, 2006). Dating by luminescence, AMS and radiocarbon determinations on charcoal and sediments have revealed ages between around 30-36ka (Wroe, 2006). Furby (1996) suggests that many of the animals, like the Diprotodon, disappeared because they were tied to waterholes and were unable to migrate, perishing when the waterhole dried up. Others however have attested that Aboriginal people hunted megafauna to extinction. Excavations at the site in 1994, uncovered Pleistocene megafaunal remains in a clay pan in the center of the lakebed. This work eventually led to the discovery of stone tools and evidence of animal butchery indicated by cut marks on fossilized bones (Furby, 1996). The most significant archeological discovery was made in one of the lower levels at the Cuddie Springs site. A small-flaked stone tool (possibly an arrowhead or scraper) was found lodged between a Diprotodon mandible and a Genyornis femur. The tool had traces of dried blood, and wear patterns consistent with its use in butchering (Furby, 1996). The combination of tools and bone of extinct species, found together, in the deposits at Cuddie Springs is now viewed as the only secure evidence in Australia for the interaction between humans and megafauna (Furby, 1996).
Radiocarbon dating of the fossilized remains indicates that this occurred between about 40 and 31 ka. Roberts et al. (2001) subsequently obtained OSL dates between 30 and 27 ka on sand grains from the megafaunal fossil beds, but the apparent age of the site was disregarded on the premise that sediment mixing had occurred, and that the remains must have been reworked. Field and Fullager (2001) refute this interpretation. In their opinion, megafauna had disappeared from the Cuddie Springs record around 28 ka, well after the arrival of humans in Australia, and that although archeological evidence is not suggestive of specialized megafaunal hunting strategies, interpretations of the fossil records remain inconclusive. Despite analysis suggesting that the deposit remains intact (Field, 2002), some argument exists for the disruption of the sediment sequence, claiming that megafaunal fossils in archaeological levels are from underlying buried horizons (Roberts, 2001). By demonstrating that relevant strata possess distinct geochemical signatures, rare-earth element analysis refutes the possibility of contamination, further validating the 36-30ka ages for megafauna (Trueman, 2005 cited in Wroe, 2006). In opposition to anthropogenic causation theory, Wroe et al. (2004) argued that no megafaunal “kill” sites have been found in Australia, and that it is probable that early human populations were too small to bring about rapid extinctions of large mammals. They also argued that more recent evidence from the Cuddie Springs site indicates that extirpation of the Genyornis species did not occur until 40ka. Johnson (2005) takes a middle ground between the two extreme views arguing that, despite evidence for younger ages of extinctions in various parts of Australia, “the evidence supports overkill, not climate change, as the cause of the extinctions.” For obvious reasons, he rejects the ‘blitzkrieg’ model of extinction, but supports anthropogenic causation, albeit on a slower timescale.
When Martin advanced the notion of ‘Blitzkreig’ as the primary cause of Late Pleistocene megafaunal extinctions, he rested it on several assumptions that have since been denounced. He posited that (1) the large Pleistocene mammals were decimated; (2) small mammals (except on islands) remained relatively untouched during the Late Pleistocene; (3) increased chances of survival for large mammals in Africa; (4) rapid onset of extinction phases; (5) temporal disparity between continents; (6) there was no replacement of extinct species by new taxa; (7) extinctions followed in the wake of human colonization; and (8) the archeology of extinction is obscure (Elias & Schreve, 2007). Martin argued that large mammals were the preferred prey of Paleolithic hunters who specialized in big game hunting (Elias & Schreve, 2007) and hypothesized that megafauna species living on continents previously uninhabited by people would have had little or no fear of humans, making them easier to kill (Wroe, 2004). For the continents such as Australia, Martin’s conception was of a ‘blitzkrieg’ styled momentum where multitudes of human hunters dispersed throughout the continent, killing large mammals and resulting in megafaunal extinction. Such devastation, he claimed, didn’t to occur in Eurasia and Africa since humans had been hunting there for hundreds of thousands of years, resulting in megafaunal accustomization to human predation (Martin, 1984).
Arguments that megafaunal extinctions in Australia were anthropogenicly mediated have according to Wroe (2006) focused on establishing “terminal appearance ages”. He claims that, this approach has been sustained by three foundation principles:
(1) if megafaunal disappearance occurred prior to significant climate change, but after human colonization, then
it can be inferred that extinctions were anthropogenicly mediated;
(2) climate change within the last glacial cycle was comparatively insignificant to previous cycles; and
(3) all or most Pleistocene megafauna were present during the initial colonization of the continent.
Implicit in these arguments for anthropogenic mediation is that pre-LGM (Last Glacial Maximum) changes were insignificant. This implies that pre-LGM were no more extreme than those of preglacial maxima in previous cycles, which proponents of anthropogenic causation assert, failed to cause major extinction (Burney, 2005). However, as Wroe (2006) points out, if pre-LGM climatic deterioration occurred within a uniquely dry cycle this interpretation is not supported. In Australia, regional evidence suggests that a uniquely altered climate produced stepwise, progressively a more arid and variable conditions from around 400-300ka (Kershaw, 2003). From this Kershaw (2003) concluded that all Australian records indicate some orientation toward aridification overlaying glacial cyclicity and suggested that the process was tectonically forced. In support of this, evidence for a general increase in sea surface temperature over the last 400ka as inferred from oxygen isotope data taken from ODP80, is cited (Peerderman, 1993 cited in Wroe, 2006). Kershaw (2003) contends that this event propelled increasing ENSO (El Nino Southern Oscillation) activities after around 400ka. Other explanatory notions have been advanced to explain this process, but regardless of commonalities, argument exists that Australia was subject to increasingly erratic and arid conditions from 400-30ka. Human activities, Wroe (2006) contends, cannot explain these long-term trends.
A further concept central to all predation-driven models, including most anthropogenic causation models, is the prominent role of prey ‘naïveté’ wherein species not previously exposed to humans lacked anti-predator responses, and were therefore easily killed (Martin, 1984). Although it should be noted that “no human-specific anti-predator responses have been identified in extant species that respond to humans as they do to other predators” Wroe et al. (2004) claim continental Pleistocene megafauna were prey to large endemic carnivores and that continental populations retained generalized anti-predator responses that were rapidly transferred to include new threats. Subsequently the ‘naïveté’ indicated through studies of remote island species is unlikely to have been characteristic of Australian megafauna. This concept opposes predator-centered models for continental species, particularly the efficy of ‘Blitzkreig’, which illustrates that extinction would only occur if instantaneous and compete losses occurred along expanding colonization fronts (Wroe, 2004). To accommodate this problem it was proposed that some species were slower to develop these mechanisms and therefore prone to extinction, although no empirical evidence can be identified to support this assertion (Burney, 2005).
In representations studying anthropogenic impact, the alternative to naïveté is considered to be hunting efficiency. These studies recognize the value in using specialized ‘big-game’ technologies in the successful hunting of large prey (Wroe, 2006). The existence of such developments are commonly cited to support anthropogenicly mediated extinction models, similarly, their absence is also considered to be a limitation (Martin, 1984). Wroe (2006) stresses that “the absence of specialized technologies does not demonstrate that a society did not hunt large animals”, however these limitations strongly influence predator-prey modeling. In Australia, stone spear points, spear throwers and specialized butchery tools widely associated with the hunting of large game do not appear until post-LGM (Flood, 1999 cited in Hiscock, 2008).
Even though the traditional debates of climate versus human agency have given way more recently to an increasing emphasis on combinations of the two, many of the more recent theoretical contributions have revised ideas based on traditional conceptions of climate and overkill. Many of these invoke interactions of potential factors, or some other mechanism generally involving humans, but less directly than over hunting, such as disease, biological intrusion, or habitat alteration. If any singular notion could be proffered as a sufficient explanation for a well-studied example, it would potentially clarify the more destructive aspects of human colonization, and would also indirectly cast doubt on climatic explanations. A fundamental theoretical problem, however, is that uniquity of each independent continent (Burney, 2005).
The irrefutable evidence required to prove anthropogenic causation pertaining to the extinction of megafauna during the Late Pleistocene, is unlikely to ever be found. Several reasons can be understood for this:
(1) both faunal and archeological evidence are fragmentary in nature.
(2) the dating of events will never be exact.
(3) early populations of all inhabited continents were mostly nomadic hunter-gatherers.
(4) scant evidence remains of human occupation and the little that does is often fragmentary
One model that appears congruent with the most proven facts may be a coup de grâce scenario, in which human predation, in varying degrees, contributes to the demise of the Pleistocene megafauna. Science has yet to determine the extent of anthropogenic influence on megafaunal extinction, but many models illustrate that humans most likely played a larger role in some regions than in others.
Despite alternative theoretical developments including those suggestive of climatic deterioration, it is impossible to exclude anthropogenic influences. Considering the evidence obtained through the excavations as Cuddy Springs it is difficult to conceive the possibility that humans had no contact with megafauna. This contact however is limited to the 13 percent of species that can be linked to a particular period and place. Although, humans undoubtedly drove recent prehistoric extinctions on remote islands and must take full responsibility for the current biodiversity crisis, it must also be remembered that the extinction of Pleistocene megafaunal species was not a singular event, but rather a process that spanned many thousands of years occurring in broad synchrony with both changing climatic conditions and human arrival. It was also time transgressive on the various continents, so it was not tied to a single, global climatic change and, while it may yet be demonstrated that human activities combined with climate change to press surviving populations to extinction it will likely remain impossible to convincingly qualify the respective influence of either factor in the near future.
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