ORIGINS

Did climate change or people see off Australia’s megafauna?

— What killed the giant wombats?

TOURISTS FLOCK TO Africa to see the big five: the elephant, buffalo, rhinoceros, lion and leopard. What they don’t realise is they’re catching a glimpse of an extinct world – that of the megafauna.

They remind us of a time not so long ago when the earth was populated by giant mammals on which our ancestors depended for survival.

Beyond Africa only a few remnants of this time still exist, such as the Asian elephant and Javan rhinoceros, the tiger and orang-utan, the jaguar in South America, and even that great Aussie icon, the red kangaroo. Except for big red, all of them are en route to extinction because of human hunting and land clearance. Is this just the final act in a drama that was set in motion long ago?

The age of the megafauna ended during the dying phases of the Ice Age, some 50,000 to 10,000 years ago as humans spread out across the planet from their African homeland. Almost 200 species disappeared across the globe, with half the world’s mammals weighing more than 44 kilograms vanishing in a near-instant of geological time.

Ancient Australia had the weirdest animals of all. Crazy creatures like the 200-kilogram kangaroo Procoptodon, the marsupial lion Thylacoleo with its flesh slicing premolars and retractable thumb claws, and the two-tonne wombat Diprotodon. It wasn’t just the mammals that disappeared, but giant lizards such as the four- to five-metre Megalania. After the dinosaurs, it was the world’s largest reptile. And then there was the over-sized duck, Genyornis, that stood more than two metres tall.

The reason these Ice Age jumbos went extinct makes for one of the greatest scientific mysteries of our time. It’s also led to a scientific squabble of mega proportions, with Australia as its principal setting. In one camp sit scientists who think human hunting is responsible. In the opposite one, those who blame climate change.

The key problem here is that we haven’t yet found the smoking gun! We don’t have evidence from anywhere in Australia of direct interaction between humans and the megafauna – no kill sites, no cut marks on bones or signs of cooking that would prove humans hunted and ate them. The one exception is the burned shell of Genyornis eggs, but we can’t be sure if the birds themselves were hunted.

The closest thing we have are stone tools and megafauna bones found alongside each other, like those from 46,000 year old deposits at Warratyi rock shelter in central Australia, described last year by Giles Hamm from La Trobe University and his team.

In this debate, at least there is one item of consensus: humans first made landfall on Australia’s northern coast around 55,000 years ago, based on dates from Madjedbebe rock shelter in Arnhem land. Then they dispersed along the east and west coasts of the continent.

The critical evidence to incriminate them as the agents of extinction relies on discovering how soon after they arrived in a particular area the megafauna went extinct.

The fierce scientific rivalry has delivered some remarkable detective work. Take as an example the Sherlockstyle report in Nature Communications this January, by Sander van der Kaars’ team at Monash University, in collaboration with Gifford Miller at Boulder University Colorado.

The scientists analysed the contents of a drill core of ocean sediment taken from 100 kilometres off the southwest coast of Australia. It preserved a 150,000-year record of what the winds were blowing off the South West forests, including wellpreserved spores of a fungus that thrived in the dung of giant herbivores.

Abundant from 150,000 years to 45,000 years ago, by 43,100 years ago the spores had dropped away.

In this region, there is also evidence of human occupation from 47,000 years ago. For the authors this collapse of dung fungus, a proxy for the collapse of the megafauna, less than 4000 years after humans arrived, is the finding that incriminates humans rather than climate change.

They argue that “imperceptible overkill”, most likely the hunting of juveniles, led to the extinction.

One argument in defence of humans is that some megafauna clearly died out well before we got here. These include several species of giant kangaroo, colossus koalas, jumbo turtles and monster grounddwelling crocodiles. But the fact that nature also causes extinctions doesn’t get humans us wholly the hook.

The fierce scientific rivalry has delivered some remarkable detective work.