Early Humans’ Fire Use Linked to Extinctions

For a century, experts have puzzled over what wiped out dozens of the strangest species ever to walk the Earth--from carnivorous kangaroos, giant lizards and horned tortoises the size of automobiles to burly ground sloths and shovel-nosed mammoths--at the dawn of modern time.

Now University of Colorado scientists studying ancient eggshell remnants in Australia think they have discovered the answer: people careless with fire.

The researchers said Thursday that they analyzed hundreds of the prehistoric eggs to pinpoint when the bird species that laid them abruptly died off, along with about 60 other major species there. They concluded that the first humans to land in Australia, 50,000 years ago, most likely set fires that inadvertently scorched the isolated continent clean of the shrubs and brush on which the creatures depended for survival.

Every species in Australia larger than a human being was wiped out. Many smaller reptiles, birds and mammals also died out suddenly.

The catastrophe was a harbinger of widespread extinctions that also occurred in North America about 11,000 years ago, shortly after humans first arrived. Three-quarters of the animals weighing more than 100 pounds, such as the giant sloths and mammoths, inexplicably vanished forever.

The new research on the Australian extinction “contributes something we desperately need to know, which is when did it happen, what was the environment like when it happened and, above all, what might have caused it?” said paleo-ecologist Paul Martin at the University of Arizona.

The new study illustrates the unusually destructive effects that even the simplest and most benign human activities can have on the world around them. From the beginning, it seems, humanity has been at odds with nature. No one is certain when people mastered fire. The earliest known campfires appear to be about 500,000 years old.

Like detectives deadlocked over which homicide suspect to handcuff, experts have long argued over the reason for “megafaunal” extinctions in Australia and the Americas.

Some scientists indict climate as the culprit. They theorize that as glaciers ebbed and flowed, the dramatic temperature changes could have killed many creatures. But those who favor climate are hard-pressed to say why species that easily survived earlier ice ages should have suddenly succumbed to the cold.

Other researchers believe many large species were simply hunted into extinction. Surely it is no coincidence, they say, that these animals in each locale disappeared at the same time that early humans arrived on the scene.

The earliest reliable evidence of humans in Australia dates from about 55,000 years ago; the most commonly accepted date for the arrival of people in North America is about 12,500 years ago, although there is some evidence of earlier settlement.

But those who think hunters are at fault have been at a loss to explain why so many unappetizing species disappeared.

Historically, no matter which theory has been argued, an explanation suitable for one time and place may not hold true for another. The climate was cooling off in Australia when the animals died, but it was warming slightly in the Americas when species disappeared. And no one seems sure why so many species should die suddenly in Australia and the Americas, but much more gradually in Europe, Asia and Africa.

The new findings “have broken the current deadlock,” said Harvard University biologist Timothy F. Flannery.

“I do think it should rekindle the debate,” said geoscientist Gifford H. Miller, who led the University of Colorado research team. “If you can establish in Australia that it is a human cause, and not climate, responsible for the extinctions, then it is reasonable to say that it may be humans in the Americas.”

Based on an exhaustive chemical analysis of the ostrich-like eggs, the researchers for the first time could date the Australian extinction precisely, tying it to the advent of human beings there. The eggs were laid by an extinct 200-pound bird called genyornis. To eliminate any error, scientists used four different dating techniques.

Based on those results--along with a comparison of the creature’s feeding habits to those of animals that survived--they suggest that systematic burning by the earliest human settlers may have been at fault. They speculate that humans disrupted the natural fire cycle by burning the landscape periodically to clear land, to cook or to keep warm. This, in turn, may have wiped out the trees and shrubs necessary for browsing animals like the extinct bird and the creatures that preyed on them.

“It seems entirely plausible,” said Stephen J. Pyne, an environmental historian at Arizona State University who has written several authoritative books on how fire has shaped the ecology of Australia and other continents. Conditions, at least in ancient Australia, would have been perfect for such devastating conflagrations, he said.

“People carry fire wherever they go, so fire is going to be central to the introduction of humans in Australia. And extinction is part of that fire story,” Pyne said. “People can supply the spark that can produce rather dramatic changes.”

Nonetheless, Pyne said, it is unlikely that fire played the same role in North America or Europe, because rainfall and vegetation were quite different.

Several scientists insist that climate-induced changes in the environment still played a major part in the extinctions.

As climate fluctuated from cold to hot and wet to dry and back again, it disrupted living conditions repeatedly. That may have favored the survival of only the most adaptable creatures, said Richard Potts, director of the human origins program at the Smithsonian Institution, who studies the relationship between the environment and evolution.

The extinctions may have been caused by the interplay between changing climate and increasingly aggressive human hunters, Potts suggested.

“I would have to say it is probably the two combined,” Potts said. “That humans and climate can have such a similar impact on the extinction of large animals is the reason why this controversy has existed without much resolution for so many years.”

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Debating Extinction

Fires set by primitive humans may have caused the extinction of dozens of unusual species that once roamed Australia and the Americas, new research suggests. Many researchers, however, suspect that rapid changes in climate were responsible. The chart below shows the timing of climate change, human settlement, and the disappearance of major species.

Years ago: 10,000-12,500

Species changes: Extinction of mammoth and other large species in North America

Human Activity: Humans settle the Americas

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Years ago: 50,000-55,000

Species changes: Disappearance of most large species in Australia

Human Activity: Humans settle Australia

*

Years ago: 400,000-700,000

Species changes: Species of elephant, baboon, pig and zebra disappear in Africa

Human Activity: Modern humans evolve in Africa

*

Years ago: 1,000,000-

Species changes: Disappearance of many species

Note: Time column is not to scale