Doom and gloom this week for those who believe in the essential goodness of the human race, with two papers in the journal Science that implicate humans in mass extinctions of mammals in North America and Australia.
In the 4 billion-year history of life on Earth there have been five true mass extinctions, each caused by external phenomena such as the meteor which snuffed out the dinosaurs. A sixth, which is occurring now, is being caused by humans.
However, the papers in today’s Science say such human-driven extinctions are not just due to industrialization, deforestation and global warming, since other extinctions also occurred tens of thousands of years ago as human populations spread across the planet. What does it mean? That people just ain’t no good?
The first paper reports on computer simulations by John Alroy of the National Center for Ecological Analysis and Synthesis at the University of California, Santa Barbara. He was trying to find out why some 30 species of large, plant-eating mammals became extinct in North America at the end of the Pleistocene Epoch around 12,000-13,000 years ago. Whatever the cause, it accounted for the loss of the woolly mammoth, saber-toothed tiger, several sloth species and the short-faced bear.
Scientists have long debated whether these mammals died out as a result of some environmental pressure, such as climate change, or whether hunting by humans drove them to extinction. Alroy’s sophisticated mathematical model is the first to fully support the “human overkill” explanation.
Humans are first known in large numbers in the Americas from Clovis culture relics dating back 13,400 years. Alroy’s model is cautious, assuming slow human population growth rates from then on, random hunting and low “hunting effort.” Nevertheless, his computer model’s predictions for extinction and survival closely match what actually happened at the end of the Pleistocene. There is no need to invoke climate change or environmental changes that were knock-on effects of human activity. The extinctions were most probably caused by “total overkill” by human hunters, Alroy says.
In today’s second extinction paper, Richard Roberts, of the School of Earth Sciences at Melbourne University, and colleagues from France, the United States and Japan provide evidence that “megafauna” mass extinctions occurred in Australia during the Quaternary Period, about 45,000 years ago.
Megafauna are those animals weighing more than 45 kg, and by the late Quaternary many of them were extinct, including giant kangaroos, the marsupial lion and herbivorous, cow-size marsupial rodents called diprotodons.
Scientific debate about this particular mass extinction has also focused on whether it was caused by environmental change — particularly a drought period around 20,000 years ago — or human impact. The answer hinges on determining, reliably, the times at which the megafauna became extinct.
To do this, Roberts’ team used sensitive thorium and uranium dating as well as optical dating — a technique that measures the time elapsed since particles in sediment were last exposed to sunlight. They found that the megafauna died and were buried in sediment about 45,000 years ago, thus ruling out the drought hypothesis, as the arid period had yet to come. However, the extinction of the megafauna came about 10,000 years after humans arrived on the continent.
Similar extinctions occurred in the Americas, Madagascar and New Zealand — each preceded by human colonization. Surely more than coincidence? “Without the arrival of humans in Australia I think the megafauna would be alive today,” says Roberts.
Alroy, in his paper, goes further. “The overkill model thus serves as a parable of resource exploitation,” he writes, “providing a clear mechanism for a geologically instantaneous ecological catastrophe that was too gradual to be perceived by the people who unleashed it.”
Whether we can learn from this parable is another question. E.O. Wilson, the Harvard evolutionary biologist, said that humans are “an environmental abnormality.” A career spent documenting the destruction of the natural world by humans led him to observe: “It is possible that intelligence in the wrong kind of species was foreordained to be a fatal combination for the biosphere. Perhaps a law of evolution is that intelligence usually extinguishes itself.”
Neither is he alone in his views. In his book “The Sixth Extinction,” the anthropologist Richard Leakey said: “Homo sapiens are in the throes of causing a major biological crisis, the sixth such event to have occurred in the past half a billion years. And we, Homo sapiens, may also be among the living dead.”
Alroy agrees that a major mass extinction is currently under way, but he emphasizes that there are things we can learn — and learn we need to. Sixty percent of Americans in a Louis Harris poll had little or no familiarity with the concept of biological diversity. Only half ranked species loss as a major threat.
“This lack of knowledge relates to problems in our educational system,” says Alroy, “which systematically downplays ecological and evolutionary science for political reasons.”
So are people just no good?
“Environmental destruction does not require active ill will,” says Alroy. “Clovis hunters had no idea what they were doing, and probably saw animals as sacred beings, just as many Native Americans do today. We need to make a consistent distinction between good intentions and good deeds; good intentions are just not enough.
“Speaking charitably, our current president may well be an example of this phenomenon.”