Contra Costa Times Posted on Mon, Oct. 11, 2004
By Betsy Mason
Saber-toothed cats, woolly mammoths and giant ground sloths roamed across North America until 10,000 years ago, when nearly every large mammal on the continent disappeared. For decades, scientists have been trying to uncover why.
Now a University of California research team says the answer could be a harbinger of things to come.
The scientific community has been ripped in two over this issue and tensions run high between those who would blame humans' arrival, thought to be just a thousand years or so before the extinctions, and those who point to climatic upheaval at the end of the last ice age.
In recent years, evidence against prehistoric humanity has been piling up. But now a team from UC Berkeley and UC Santa Cruz have done the scientific equivalent of a bipartisan look at the evidence and weighed in on the intense debate in the pages of the journal Science last week.
Their verdict is that neither suspect is innocent.
"It's really a combination of humans and climate change," said UC Berkeley paleobiologist Anthony Barnosky. "It's this one-two punch that's the killer, so to speak."
Today, with a growing population and global warming, both elements of that combination are ramping up at unprecedented rates. It's a situation that could spell disaster for the planet's current crop of large mammals, Barnosky said.
The oldest solid evidence of human presence in central North America dates to somewhere between 11,000 and 11,500 years ago. This first group, known as the Clovis civilization, was full of hunters who left stone spearheads scattered across the landscape. In a few cases, the spearheads have been found at "kill sites" along with mammoth and mastodon bones.
Around 1,500 years later, mammoths, mastodons and more than 30 other large North American mammals including camels, rhinos, saber-toothed cats and giant ground sloths, were all extinct. The suspicious timing, along with the kill sites, led many scientists to the idea that humans either hunted these animals into oblivion one by one in a prehistoric blitzkrieg, or at least killed enough to trigger a population collapse.
But clouding the picture is the fact that the last ice age was ending at the same time. Glaciers that once covered most of the continent were receding and weather systems and vegetation patterns were undergoing drastic reorganization. Some scientists think the stress of this upheaval was the more likely culprit in the extinctions.
The most likely scenario is that the combination was to blame, said UC Santa Cruz paleobiologist Paul Koch. "Humans were setting this catastrophe in motion, but at the same time there was a strong climate signal."
The picture is somewhat less muddled in other parts of the world that also suffered mass extinctions. In Australia, people arrived around 50,000 years ago and most of the largest animals were gone 10,000 years later, including giant carnivorous kangaroos, horned tortoises the size of Volkswagen Beetles, and six-foot-tall birds. All the while, climate was relatively stable.
Disagreement persists about whether the Australian extinctions were caused directly by hunting or by people burning the landscape to ease travel and hunting, but most agree that somehow people are to blame.
In Alaska and the Yukon, however, several large animals such as short-faced bears, two species of horses and the local population of mammoths all disappeared before there is any evidence of human invasion. The same is true for some species of deer and elk in Ireland. In both cases, climate change coincides with the extinctions.
And in Europe, there is evidence of human hunting for at least 400,000 years with no extinctions. But around 50,000 years ago, when the climate was cooling rapidly, and 20,000 years ago, when it started heating back up, there are spikes of extinctions.
Similarly, in Africa many large mammals have managed to escape extinction over hundreds of thousands of years of commingling with humans.
All of this evidence from around the globe points to a lethal combination, Barnosky said. "Humans were very much a driving force, but where you really had the most pronounced extinctions is where you had climate change and humans coming together at the same time."
Central North America remains the most controversial, as well as the most studied, case of recent mass extinctions, and each new piece of evidence fuels the debate.
Looking even further back in the history of mammals before humans had evolved, over the last 65 million years extinctions don't match up with major climate changes, said evolutionary biologist John Alroy of UC Santa Barbara.
"There are tons and tons of climate changes in the fossil record of mammals, but these recent mass extinctions are unprecedented," Alroy said. "I think once you got humans onto the (North American) continent, climate became totally irrelevant."
Still, others think humans merely swept into North America for the coup de grâce, speeding the inevitable end of a process brought on by climate change.
"Without environmental change, people weren't able to bring them down elsewhere like Africa and Europe," said paleontologist Holmes Semken of the University of Iowa. "So there's got to be more than just people involved."
If Barnosky and Koch are right about human population growth and rapid climate change being a killer combo in the past, they think this could be an important lesson for the future: With global warming potentially heating up the climate at an unprecedented rate, and ever increasing pressure from spiraling population growth, today's large mammals may soon go the way of the mammoth.