It is the scientific theory that prehistoric people moving for the first time into new geographical areas during their spread around the world invariably hunted large animals into extinction. New work by American and Australian researchers is adding weight to the theory, while undercutting the notion that climate change and not human influence was the cause.
Scientists say the lessons of the past should not be lost on the people of today, considering the number of species already extinguished by humanity or pushed to the brink.
John Alroy, an evolutionary biologist at the University of California at Santa Barbara, used a complicated computer model to simulate what may have happened when humans first entered North America some 13,400 years ago over an Ice Age land bridge from Asia. Regardless of the variables he plugged in, the presence of human hunters triggered mass extinctions.
Alroy is a believer in the "blitzkrieg" or "overkill" theory proposed 34 years ago by University of Arizona scientist Paul Martin. "This is a clear case where you have a major, continental-scale environmental catastrophe by people who had no intention of causing a catastrophe and had no idea what they were doing. It's an example of how people can have major impacts in total innocence and with no ill will," Alroy said.
Martin suggested that while people may be realistic about the misdeeds of modern man, they may be hesitant to attribute bad deeds -- even unintentional -- to our distant forefathers. "We haven't wanted to extend that into our ancestry and prehistoric time," he said, "but we're the same species. And if we have powers, we tend to exercise them."
Many scientists believe modern humans emerged from Africa in the past 100,000 years and migrated into Europe and Asia, pushing much more recently into such remote places as North America, Australia and Madagascar.
In all those places, the arrival of humans coincided with catastrophic mass extinction of animals.
In North America, dozens of species disappeared 12,000 to 13,000 years ago, after the arrival of humans, including mammoths and mastodons (both relatives of modern elephants), giant ground sloths, tapirs, a large camel, llamas, a large-horned bison, prong-horned antelopes, oxen, a type of mountain goat, a giant armadillo and the glyptodonts, large mammals covered with solid armor. Large predators such as the saber-toothed cats, dire wolves and some bears also died off.
Alroy's computer model indicated the extinction took place in about 1,200 years -- the equivalent of a blink of the eye.
Human beings arrived in Australia somewhere around 60,000 to 50,000 years ago, crossing the ocean after spreading across Asia and through the Indonesian archipelago. Because traditional carbon dating methods had failed to provide good data on when that continent's mass extinction took place, it had been impossible to determine what role -- if any -- was played by the ancestors of today's aborigines.
But University of Melbourne geochronologist Richard Roberts and colleagues used advanced new techniques to get the answer. They found that the mass extinction occurred around 46,400 years ago, give or take 3,000 years.
Roberts said the fact that the extinction unfolded shortly after people first landed on the shores of Australia pointed to the culprit. "We can say it's definitely humans," he said.
He said climate change did not seem to be a factor, noting the last Ice Age was 25,000 years after the extinction. "We don't know for sure whether it was like a 'blitzkrieg' model of human interruption, which is what Alroy has for the North American situation ... or whether there was a much longer period of overlap, let's say 10,000 years, between people arriving and the megafauna (large animals) going extinct."
Mammalogist Timothy Flannery of the South Australia Museum in Adelaide, who worked with Roberts, believes the findings support the "overkill" theory. But Roberts proposes a variation on the theme, saying it is possible the extinctions took place over a longer period of time and were not the result only of hunting but also of environmental chaos wrought by humans, such as burning the landscape to facilitate hunting or travel.
The studies by Roberts and Alroy appeared this month in the journal Science.
By nature of its isolation from other continents, exotic animals historically have populated Australia, whose mammals were marsupials (sheltering and nursing their babies in a pouch) rather than the placental mammals common elsewhere.
Marsupials now lost forever included giant kangaroos, the ferocious Thylacoleo -- the size of a leopard but known as the marsupial lion -- and the herbivore Diprotodon, bigger than a cow but which looked like a wombat. The giant flightless bird Genyornis and a 26-foot lizard also disappeared.
The idea that climate change triggered the extinctions is undermined by the fact that they were not simultaneous, Roberts said. "If it had been a global climate change phenomenon, everyone would have gone extinct in all of those different places at the same time. The fact that they didn't really points the finger very, very strongly at human beings, as the new kid on the block, causing all the trouble."
Another theory is championed by Ross MacPhee, a biologist with the American Museum of Natural History in New York, who argues that as human beings entered virgin territory, they introduced diseases that knocked out the large animals.
He suggested domesticated animals such as dogs that may have accompanied people on their worldwide migration may have introduced diseases that devastated other animals whose immune systems were unable to cope. Looking to human history, it is the equivalent of Europeans bringing the smallpox virus to the New World in the 16th century, killing millions of Indians.
"Humans are implicated in these losses," MacPhee said. "I don't take the step and say that it was due to overkill because I don't think that overkill is a believable mechanism. But clearly when humans come for the first time, you tend to get these catastrophic losses."
He said he would have liked to see Alroy run a computer simulation of the impact of disease transported by migrating human populations, as opposed to strictly a hunting scenario.
Martin said the debate still rages: "The timing of human arrivals seems to coincide with the disappearance of these animals. So exactly what happened? The field evidence is virtually nonexistent. Exactly what people did and what the circumstances were, we don't have a handle on."
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File Date: 6.15.01