With the dinosaurs long since extinct, and the Earth recovered from the events that killed them, a new breed of predator emerged to rule the Australian landscape.
Ferocious marsupial lions dominated, ripping pig-sized anteaters and 200 kilogram kangaroos limb from limb. Wombat-like creatures the size of rhinos ambushed their prey, huge flightless birds, tearing out their throats and swallowing everything down to the last feather.
Then humans arrived, and most of the giant animals vanished. Did the Ice Age finally catch up with them? Or did humans hunt our ancient megafauna to extinction?
What happened to Australia's large animals is one of the planet's most baffling paleontological mysteries. For years scientists blamed the extinctions on climate change.
Indeed, Australia has been drying out for a million years or more, and the megafauna were faced with a continent that became increasingly parched and denuded of vegetation.
Australian paleontologist Tim Flannery suggests that humans, who arrived on the continent around 50,000 years ago, used fire to hunt, which led to deforestation and a dramatic disruption of the hydrologic cycle.
A skeleton of a Marsupial Lion (Thylacoleo carnifex) at Naracoorte Caves National Park
Here's what's certain, Flannery says. Something dramatic happened to Australia's dominant land creatures – how abruptly is a matter of debate – somewhere around 46,000 years ago, strikingly soon after the invasion of the tool-wielding, highly intelligent predator known as man.
Flannery put forth an even broader and more ambitious thesis as well: that human beings, in general, are a new kind of animal on the planet, one prone to ruining ecosystems and destroying their own futures.
His theory proved highly controversial. Some viewed it as critical of the Aborigines, who pride themselves on living in harmony with nature. The more basic problem with Flannery's thesis is that there is no direct evidence that human beings killed any of the megafauna – not so much as a single animal.
Another challenge to the Flannery model of Australian megafauna extinction is more mechanistic: How could people armed with only spears and fire have eradicated so many species?
[Images: Adrie and Alfons Kennis]