National Geographic : 2010 Oct
faced a technological predator. But the extinction spasm wasn't comprehensive. North America kept its deer, pronghorn, black bears, and a small type of bison; brown bears and newly arrived elk and moose expanded their ranges. South America retained jaguars and llamas. In Australia the largest indigenous land ani- mals are red kangaroos. What happened to Australia's large animals is one of the planet's most ba ing paleontologi- cal 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 re to hunt, which led to deforestation and a dra- matic 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 a er the invasion of a tool- wielding, highly intelligent predator. In 1994 Flannery published a book called e Future Eaters, in which he advanced the antipo- dal version of Paul Martin's blitzkrieg hypoth- esis. 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. e 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 ylacoleo bones next to the charcoal of a human camp re. Such kill sites have been found in the Americas. But there's no archaeo- logical 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 rst base. It would be laughed out of court." Another challenge to the Flannery model of Australian mega- fauna 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 di erent: By de nition there can be no survivors. megafauna pivots to a great degree on the techniques for dating old bones and the sediments in which they are buried. It's all about timing. If scientists can show that the megafauna died out fairly quickly and that this extinction event happened within a few hundred years, or even a couple thousand years, of the arrival of humans, that's a strong case---even if a purely circumstantial one---that the one thing was the direct result of the other. Flannery con- tends that islands hold another clue to the mys- tery. Some species of megafauna survived on Tasmania until 40,000 HOW COULD PEOPLE ARMED WITH ONLY SPEARS AND FIRE HAVE ERADICATED SO MANY SPECIES? BY DEFINITION, IN EXTINCTION THERE CAN BE NO SURVIVORS. • (Continued on page 105) Joel Achenbach is reporting on the Gulf oil spill for the Washington Post. Amy Toensing covered the drought in Australia's Murray-Darling River Basin in April 2009. Dutch twin brothers and artists, Adrie and Alfons Kennis specialize in paintings and models of extinct animals and humans.