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.
Here's what's certain, Flannery says. Something dramatic happened to Australia's dominant land creatures—abruptly (how abruptly is a matter of debate)—somewhere around 46,000 years ago, strikingly soon after the invasion of a tool-wielding, highly intelligent predator.
In 1994 Flannery published a book called The Future Eaters, in which he advanced the antipodal version of Paul Martin's blitzkrieg hypothesis. He 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.
Flannery's book 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. It would be helpful if someone uncovered a Diprotodon skeleton with a spear point embedded in a rib—or perhaps a pile of Thylacoleo bones next to the charcoal of a human campfire. Such kill sites have been found in the Americas. But there's no archaeological analog in Australia. As one of Flannery's most prominent critics, Stephen Wroe of the University of New South Wales, puts it, "If this were a murder trial, it wouldn't get past first base. It would be laughed out of court."
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? Relatively few people, maybe numbering in just the thousands, would have had to kill a population of animals dispersed in a wide variety of habitats and refuges across an entire continent. Extinction is different: By definition there can be no survivors.