Humans who colonized Australia did not reach Tasmania until thousands of years later — granting the island's giant kangaroos a brief respite before they joined their Australian brethren in oblivion.
Many of the world's biggest animals (the megafauna) became extinct in the latter part of the last ice age, which ended around 14,000 years ago. That realization triggered two long-standing debates1. First, were the extinctions caused by humans, climatic or vegetational changes, fires, extraterrestrial impacts, or some combination of those agents? Second, if humans were responsible, did they cause the extinctions directly through hunting, or indirectly by some consequences of human settlement? Turney et al.2 now contribute to answering both of these questions. Writing in Proceedings of the National Academy of Sciences, they report improved dating of the demise of giant kangaroos and other giant marsupials, and of associated changes (or lack of them), on the island of Tasmania, off Australia.
Megafaunal extinction proceeded to different extents and at different times on different continents. If one defines megafauna as species with an adult body mass of more than 44 kilograms, then most megafaunal species living in the latter part of the last ice age have survived until today in Africa and Eurasia. But most or all of them became extinct in Australia about 46,000 years ago, in North America about 13,000 years ago, in South America about 13,000 years ago and in New Zealand about 700 years ago. Those four dates are close to the respective dates of human entry into each of these land masses, suggesting that humans were somehow responsible.
The first humans to colonize these four regions were anatomically and behaviourally modern humans with sophisticated hunting skills, who would have encountered animals with no previous experience of humans. Those tame animals would have paid the price for their lack of fear of these sudden invaders — in much the same way, during recent centuries, that animals of Antarctica and the Galapagos initially had no fear of humans. In contrast, the animals of Africa and Eurasia were saved by their gradual acquisition of fear of humans over millions of years, as human hunting ability slowly developed from modest ancestral skills such as those of chimpanzees. Although the coincidences in time point the finger at humans as the cause of megafaunal extinctions, a complication is that extinctions in North and South America coincided approximately not only with human arrival there, but also with a peak of global warming and glacial retreat at the end of the last ice age — as well as (in North America) with a sediment layer suggestive of an extraterrestrial impact3.
Arguments against a direct human role in the extinction of Australia's megafauna stem from reports of vegetational changes and increased fires on the Australian mainland around the same time4,5, and especially from the evidence for Tasmania available until now6,7. Today, Tasmania is an island that lies 240 kilometres southeast of the Australian mainland. But the seas between Australia and Tasmania are so shallow that Tasmania was intermittently connected to Australia during glacial episodes, when sea level dropped because much of the world's water was locked up in ice. By the time humans colonized Australia around 50,000 years ago, Tasmania was an island, and it did not become reconnected to Australia and accessible to human settlement until 43,000 years ago. Hence, claims that megafaunal species were already extinct in Tasmania before humans arrived, if supported, would be fatal to the hypothesis that humans eliminated the megafauna in Tasmania, and, by extension, in Australia.
Turney et al.2 have now compared the Tasmanian chronologies of megafaunal extinction and human arrival, and lake-core evidence of climate and vegetational changes and of fire charcoal, thereby providing a record unique in the Australian region for correlating all of those indicators within a small area. The authors reanalysed museum specimens of fossils from four previously reported sites, as well as assessing finds from a newly excavated site at Mount Cripps in the northwest of the island.
The overall Tasmanian record extends from the present back to at least 127,000 years ago, during the last interglacial period. It turns out that, as expected, there were drastic climatic and vegetational changes (cool temperate rainforest yielding to grassland, herbs and heath) around 110,000 years ago, at the transition from the last interglacial to the last glacial period. Those changes were reversed after the transition from glacial back to interglacial conditions around 15,000 years ago. But the switch to glacial climates and vegetation 110,000 years ago had no effect on the megafauna's composition. All seven species recorded on Tasmania in the last interglacial, more than 127,000 years ago, were still present in the most extensive glacial fossil deposit, Scotchtown Cave, dated at 56,000 years ago — strong evidence against the cause of the extinctions being climatic and vegetational changes before humans arrived. The seven now-vanished giants consisted of two 500-kilogram marsupial equivalents of a rhinoceros and a ground sloth; three antelope-like kangaroos weighing 100–150 kilograms and standing 2 metres tall; a leopard-like marsupial predator; and a giant echidna (not a marsupial but a monotreme).
The site at Mount Cripps yielded three specimens of the giant kangaroo, Protemnodon anak (Fig. 1), dated at between 42,900 and 40,900 years ago, an age statistically identical to the age of the oldest archaeological horizon (excavation level) in Tasmania, as dated by carbon-14, that contains evidence of human occupation. This discovery supports two conclusions. First, despite Tasmania's proximity and vegetational similarity to southeastern Australia, megafauna survived later in Tasmania than on the Australian mainland, from which they had disappeared about 46,000 years ago. Second, in Tasmania the megafauna overlapped in time with humans, who probably arrived when the land bridge was reestablished 43,000 years ago, as indicated by undated human cultural artefacts lying below the level of the carbon-14-dated human horizon. Thus, megafauna that had survived in Tasmania for at least 84,000 years (from 127,000 to 43,000 years ago), through big changes in vegetation and climate, disappeared within a few millennia of human arrival.
The results of Turney et al.2 also point to the cause of that event. Unlike the Australian mainland, Tasmania shows no signs of changes in vegetation or in fire charcoal around the time of human arrival. Those changes on the mainland have been interpreted4,5 to mean that humans eliminated the mainland megafauna indirectly, by lighting fires that altered the character of the vegetation. The absence of such evidence in Tasmania suggests that humans there instead eliminated megafauna directly by hunting. Even for the mainland, the role of fire-induced vegetational changes as a cause of megafaunal extinction is debated: vegetational changes might have been the result, rather than the cause, of the elimination of large browsing and grazing mammals.
Obviously, our understanding of the chronology and cause of the Tasmanian megafaunal extinction is still far from complete. As sceptics will point out, Turney et al. have demonstrated the survival of one species of giant kangaroo in Tasmania until humans arrived. What about the other six species of megafauna?
I, too, hope for more complete evidence. Proof to convince any sceptic would be the existence of many Tasmanian sites containing human remains dated at 42,800 years ago, all lying above deposits lacking human material and thus indicating human arrival at that date, plus remains of all seven megafaunal species at that same date, followed by the last remains of those species at seven different dates ranging from 42,700 to 41,900 years ago. That's asking a lot. But Turney and colleagues' study may motivate Tasmanian palaeontologists and archaeologists to seek such overwhelming evidence, or else to refute that scenario.
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Journal of Archaeological Science (2010)