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.

Take as an example the Sherlock-style
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 well-preserved
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 ground-dwelling
crocodiles. But the fact that
nature also causes extinctions doesn’t get us humans wholly off the hook.