Throughout the Ice Age that characterised our planet for much of the last two million years or so mainland Australia, Tasmania and New Guinea formed a single landmass — Sahul.
It was a strange and often hostile place – at times much drier and as much as nine degrees cooler than now - with a sometimes vast arid core that expanded to encompass 70% or more of the continent. And it was dominated by giants.
This “megafauna” included the largest marsupial that ever lived, Diprotodon, the size of a large rhinoceros; huge, short-faced kangaroos that exceeded 200kg in body mass; and massively-built terrestrial birds, around the height of an emu – but twice as heavy. They were preyed upon by a venomous goanna that may have been as big as a large saltwater crocodile, and a bizarre-but-deadly marsupial “lion” with incredibly powerful jaws and bolt-cutting teeth.
Not all were gigantic in any strict sense – some were simply much larger relatives of existing species; for example, there was an echidna the size of a large dog. Others were much larger “versions” of species still alive today, such as the giant grey kangaroo. All up, around 90 of these large to gigantic species and subspecies existed.
Now they are gone; only a few big kangaroos still survive.
Explaining these extinctions has locked scientists in heated debate since the 19th Century. While arguments have changed, the identity of the proposed “culprits” have not. Was it climate, or was it humans?
Historically, there have been times when some researchers have claimed victory and the ascendancy of one interpretation or another, but such claims have typically been short-lived. The data have been scarce: there are too few reliable dates on either humans and their artefacts, or extinct megafauna, and a very limited understanding of environmental change over the vast tracts of time in question.
In recent years this has begun to change. Humans arrived sometime around 50-45 thousand years ago, but it is increasingly clear that many or most of the megafauna had disappeared before humans arrived. Times of peak cold are known as glacial maxima (times of peak cold and aridity), but of the 90 or so extinct species of megafauna around 50 are not known from fossil deposits younger than the Penultimate Glacial Maximum (approximately 130 thousand years ago). Other species disappeared about 50 thousand years later, but still long before the arrival of the first Aborigines.
At most 14 and as few as eight species of now extinct megafauna clearly overlapped in time with humans. At localised levels, too, there is mounting evidence from specific sites that a staggered, stepwise extinction was well established long before humans made an appearance. There has never been any direct evidence of humans preying on any now extinct megafauna anywhere in Sahul – or even evidence of a tool-kit typical of big-game hunting hunter-gatherers.
Across geological time the vast majority of species that have ever lived have gone extinct, and the vast majority of these in the complete absence of humans. Climate or climate-related influences are undoubtedly to blame in almost every instance.
So how did human-driven explanations gain support in Sahul?
Underpinning all arguments for a human-driven process were two key assumptions. The first is that the megafauna were present when Aboriginals arrived; the second is that all previous glacial maxima – the last peaked between 28-19 thousand years ago - were much of a muchness, or at least that there was nothing remarkable or extreme about the last two or three. The reasoning was that because we “knew” that the megafauna were here and that there was nothing particularly unusual about the last few glacial cycles, the only feasible cause was the arrival and subsequent activity of people.
As we have seen, it is now clear that the first of these assumptions was poorly-founded at best. The evidence suggests that few of the megafauna were here when humans arrived.
Just as importantly, it is also now clear that the second assumption was likewise incorrect. In fact many palaeoclimatologists have long been of the opinion that Sahul was subject to a protracted, stepwise deterioration in climate over the last 300-400 thousand years. The long term trend is an increasingly arid and erratic climate.
In recent years the evidence for the protracted, stepwise aridification of Sahul has firmed, backed by new and mounting data from Antarctic ice cores and analyses of ancient central Australian lake levels. The 800-thousand-year Antarctic ice core record in particular has provided unprecedented resolution on the Southern Hemisphere story - and it has revealed a distinct change from 450 thousand years ago if not earlier.
From this time on things started to become more extreme. Moreover, the ice core record shows marked drying, beginning at around 50-45 thousand years ago – the time humans arrived. This is consistent with evidence for the decline of once vast inland mega-lakes. Other recent studies have suggested that climatic deterioration may have taken place to varying degrees across the planet – beginning as early as 700 thousand years ago.
Still further cracks have emerged in arguments for a human role. Spikes in fire activity deduced from charcoal analyses had been assumed by some to indicate increased burning by humans, laying a basis for the argument that human-driven environmental change drove the megafaunas’ demise. But more recent work shows that increased burning characterised Sahul long before people arrived.
The loss of a giant flightless bird from south-central Australia at around 50 thousand years ago had been attributed by some to human activity, but it is now clear that its disappearance clearly coincided with escalating climatic variability.
Many questions remain. Humanity’s role in the demise of now-extinct species that were still present when people arrived cannot be entirely discounted, but this remains to be demonstrated. However, it is increasingly clear that the disappearance of megafauna from Sahul took place over tens if not hundreds of millennia under the influence of an inexorable, albeit erratic, climatic ratchet, and that the first Aboriginals made footfall at a time when conditions were already rapidly deteriorating.