ORI­GINS

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

Cosmos - - Contents - DAR­REN CURNOE is an palaeoan­thro­pol­o­gist with an in­sa­tiable cu­rios­ity for un­der­stand­ing the kind of crea­ture we are and how we came to be this way.

— 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.

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