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Feature Story
Mammoth Mystery
Page 3

Robinson's data on dung spores may reset the clock for mass extinctions in New York. But convincing his colleagues that humans were walking the banks of Lake Fairchild 12,500 years ago will be a harder sell. There's scant archaeological evidence to help date the arrival of the first people in the Northeast, and what little has been found is cryptic. Clovis sites discovered in New Hampshire and Maine are 11,000 years old, and until the 1990s, most archaeologists believed that the Clovis people were the first to reach North America. But in recent years, a few sites have yielded human artifacts thousands of years older than those left by the Clovis culture. Researchers working at a small number of sites in the eastern United States claim to have found evidence of a human presence as much as 18,000 years ago, though others dispute whether the objects in question were made by humans at all. Ancient spear points from Dutchess Quarry, just up the Hudson Valley from Robinson's study sites, are not formed in the same style as classic Clovis points, but researchers have not been able to date them.

Robinson thinks microfossils can fill in the gaps. If he is interpreting the shifts in microscopic charcoal concentrations correctly, humans were having a dramatic effect on the ecology of the valley more than a thousand years before the dates of the oldest known Clovis artifacts.

Some paleontologists think Robinson is reading too much into the patterns he's found in ancient bits of charred grass. After all, fire was a part of North America's ecology long before human hands held a flint. But Robinson and his mentor at Fordham, David Burney, have seen the same pattern -- the sudden disappearance of dung spores followed by a major spike in microcharcoal -- in such far-flung spots as Madagascar and Hawaii. Their argument -- that humans left a signature written in ash as they settled new places and killed off the local megafauna -- relies on evidence from islands. And even the most dedicated believers in climate as the culprit in the death of North America's monster mammals concede that prehistoric people wiped out island creatures around the world.

The island blitzkriegs took place in relatively recent times. For instance, Madagascar, off the coast of East Africa, was populated with giant lemurs, pygmy hippos, giant tortoises, and flightless elephant birds until about 1,700 years ago, when sailors from Malaysia settled the island. Madagascar has always been a place of fire. Burney, who has studied the country's prehistory for decades, has used preserved pollen and microscopic charcoal to reveal the presettlement landscape: a fire-adapted mosaic of grassland and forest.

The timing of human arrival in Madagascar can be established by radiocarbon dating of the human-modified leg bones of extinct hippos, and by the pollen of plants introduced to the island by people. Dung spores, the marker for big herbivores, vanish when these signs of human habitation appear. Within a few hundred years the amount of micro-charcoal in the sediments jumps tenfold, and it stays high for several centuries. That's the same sequence Robinson has found in the sediments of southeastern New York, although there it was 10,000 years older. These striking parallels with the data from Madagascar led Robinson to his hypothesis that people caused the micro-charcoal spike in New York.

Similar changes in micro-charcoal deposits in a lagoon in Puerto Rico have led Burney to suggest that people arrived there about 5,000 years ago, two millennia before the first archaeological signs of human habitation. That ties in neatly with recent evidence that giant sloths, armadillos, and tortoises died out in Puerto Rico and Cuba about 5,000 years ago. As Burney notes, the lack of known artifacts doesn't rule out earlier human arrival; surviving traces of early cultures are always scattered and hard to find. He argues that dramatic spikes in the accumulation of microscopic charcoal, which lofts high and spreads wide during landscape fires, can sometimes indicate human presence more reliably than cultural artifacts.

Robinson's and Burney's microfossil studies strengthen evidence of what some researchers call a "deadly syncopation" -- a mass extinction of animals that occurs as soon as that ultimate invasive species, Homo sapiens, reaches their habitat. As our ancestors spread out from their original home range in Africa and Eurasia, they found a new world of unwary prey. Darwin recorded the fate of such naive creatures: He wrote of watching a young boy, one of the first human visitors to the Galapagos, methodically slaughter hundreds of finches by simply whacking them with a stick as they approached him. Fifteen hundred years ago, Polynesian settlers, along with their imported dogs and pigs, wiped out Hawaii's large flightless ducks. Seven hundred and fifty years ago, the ancestors of modern Maoris reached New Zealand and killed off some 160,000 ostrich-like moas, driving an entire genus of birds to extinction in a matter of decades. This rapid destruction was possible in part because moas, like most other large creatures, were slow breeders. It was easy for people to kill them off faster than they could replace themselves or evolve a strategy to cope with their new predator.

The phenomenon of island blitzkriegs is well studied. But what exactly happened when the first people reached new continents? It's startling enough to realize that in New Zealand's rugged terrain, a group of 1,000 or so humans could find and exterminate every moa in less than a century. It's even harder to accept the idea that in the vast and varied expanses of Australia and the Americas, scattered tribes wielding only Stone Age weapons could wipe out dozens of species. The first humans to settle Australia arrived 50,000 to 60,000 years ago, and researchers Down Under are engaged in a vigorous argument over when that continent's megafauna -- including a nine-foot-tall kangaroo, a rhinoceros-like marsupial, and a wombat the size of a hippopotamus -- bit the dust.

In North America, too, the sequence of events has been difficult to pin down, again in part because of the nature of radiocarbon dating. For decades, most scientists assumed that the Clovis people could not have had a significant impact on thriving populations of mammoths, camels, giant sloths, and other mighty Pleistocene creatures. But Paul Martin began a revolution in scientific thought when, as a graduate student in the 1960s, he read through a list of extinct mammals compiled by the paleontologist George Gaylord Simpson. Martin made a simple observation: Where earlier periods of extinction had affected creatures great and small, the late Pleistocene die-off in the Americas hit big animals -- the sort that might interest human hunters -- hardest.

When Martin came out with his blitzkrieg theory of mass extinction, his fellow paleontologists regarded him as an interesting crackpot. Martin remained unfazed. "I like an argument that goes out on a limb and sees how long it can stay there," he says. More than three decades later, his view dominates the field.

Bad Chemistry: A Special Report
Forecast: Dry and Brutal

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OnEarth. Winter 2006
Copyright 2005 by the Natural Resources Defense Council