Did a fast-warming climate wipe out the arid grassland and food sources that nurtured the mythic megafauna like woolly mammoths and woolly rhinoceros over the previous 2 million years? Or did roving bands of savvy, spear-chucking humans -- just then fanning out across the steppe as they colonized Far Eastern Siberia, Alaska and the rest of North America -- provide the ecological push that tipped many of these creatures to extinction?
And if so, how come the caribou thrived by the millions?
Explaining what eliminated Ice Age mammals has been one of the most controversial issues among paleontologists for decades. But a new exhaustive study that compared genetic material from thousands of fossil remains to climatic and human factors over the past 50,000 years has concluded that each of these storybook species survived -- or disappeared -- for their own unique reasons.
In particular, the fates of six iconic Ice age herbivores — woolly rhinoceros, woolly mammoth, wild horse, reindeer, bison and musk ox — can't be blamed on a single cause such as climate change or overhunting.
"We find that neither the effects of climate nor human occupation alone can explain the megafauna extinctions," wrote a team of 55 scientists from 11 countries in a study published this week in the prestigious journal Nature.
"Rather, our results demonstrate that changes in megafauna abundance are idiosyncratic, with each species (and even continental populations within species) responding differently to the effects of climate change, habitat redistribution and human encroachment."
The study portrays an ecologically complex picture that spans two continents and about 50 millennia, the same period that saw the rise of modern humans. Despite the popular notion that prehistoric hunters overran peacefully grazing herds all over the Far North, people simply didn't target -- or rarely even saw -- several of the species. Over the course of tens of thousands of years, climate change alone appears to be the major cause of the extinction of the woolly rhinoceros and the Eurasian musk ox. (Musk ox survived in North America and Greenland, and were reintroduced into Alaska in the 1930s.)
Yet the unfortunate alchemy of climate shifts with human interaction wiped out most of the world's wild horses and steppe bison. At the same time, reindeer — known as caribou in North America and Alaska — remained in healthy numbers throughout the Arctic despite enduring the same influences.
In what may be the most poignant conclusion from the study for Alaskans, the scientists say they still don't know exactly why the Far North lost the woolly mammoth, the official fossil of the most northern U.S. state.
"Our findings put a final end to the single-cause theories of the Ice Age extinctions, and suggests that care should be taken in making generalizations not just regarding past and present species extinctions but also those of the future," said Prof. Eske Willerslev, whose Centre for GeoGenetics at the Natural History Museum led the study, in this story.
"The impacts of climate change and human encroachment on species extinctions really depends on which species we're looking at."
"We do find that climate change has been intrinsically linked with major megafauna population size changes over the past 50,000 years, supporting the view that populations of many species will decline in the future owing to climate change and habitat loss," added lead author Eline Lorenzen, from the University of Copenhagen, in the same story.
The stubborn mystery of Ice Age extinctions
Scientists have been arguing for decades over what triggered the extinction of 72 percent of the large-bodied mammals from North America (and 36 percent of similar species from Eurasia) as the last glacial age thawed and humans migrated into North America. A progression of dueling studies have variously blamed humans, climate change, a killer shower of comets — or some combination of many factors.
"The debate surrounding the potential causes of these extinctions has focused primarily on the relative roles of climate and humans," the authors explained in the paper.
"In general, the proportion of species that went extinct was greatest on continents that experienced the most dramatic climatic changes, implying a major role of climate in species loss. However, the continental pattern of megafaunal extinctions in North America … approximately coincides with the first appearance of humans, suggesting a potential anthropogenic contribution to species extinctions."
To "disentangle" the factors, the team marshaled an astonishing amount of genetic analysis — 846 radiocarbon-dated DNA samples, 1,439 bits of remains dated directly and 6,291 remains associated with prehistoric people and archaeological sites in Eurasia. The researchers then reconstructed the demographic histories of the six species, along with their distributions and connections to people at four key moments in time — about 42,000 years ago, 30,000 years ago, 21,000 years ago and about 6,000 years ago.
At the core of the investigation was a mystery. Why did these six species last through multiple glacial climate cycles — 2 million years of wild swings from frigid ice age to warm, brushy interglacials — only to succumb during the most recent thaw?
"Although these cold-adapted animals certainly fared better during the colder, glacial periods, they still managed to find places where the climate was just right — refugia — so that they could survive during the warmer, interglacial periods," explained co-author Beth Shapiro, the Shaffer Associate Professor of Biology at Penn State University and a leading expert in the study of prehistoric DNA, in a story about the research.
"Then, after the peak of the last ice age around 20,000 years ago, their luck started to run out. The question is, what changed? Why were these mammals no longer able to find safe refugia where they could survive a warm climate?"
The findings weren't simple. One angle tracked the genetic diversity among the species as time went on. Another tried to analyze distribution — whether populations were isolated or in refugia, and to what extent they overlapped with humans. (The scientists added another 1,557 prehistoric fossil remains to the samples to help with the analysis.)
Over the 50,000-year period, the species sometimes fluctuated widely in population, with ranges extending and contracting in response to climate shits. Still, they survived. Then, suddenly, about 14,000 years ago, many of them died out completely or disappeared from regions.
"The take-home message is that during the most recent warming event, when the last ice age faded into the warm interval we have today, something kept these animals from doing what they had always done, from finding alternative refugia — less-than-ideal, but good-enough chunks of land on which to keep their populations at a critical mass," Shapiro explained. "That 'something' was probably us — humans."
The species and their fates
Woolly rhinoceros. This hairy, horned creature didn't lose genetic diversity as time passed and remained widely distributed across the frigid steppes of Eurasia until it suddenly disappeared about 14,400 years ago. Its population increased for thousands of years after humans arrived on the scene, and the scientists say they found little evidence that humans hunted it much — fewer than 11 percent of Siberian archaeological sites had rhino remains. Climate change appears to be the major factor in its demise.
Eurasian musk ox. "We find no evidence that Palaeolithic humans greatly impacted musk ox populations, in agreement with previous conclusions that humans were not responsible for the extinction of musk ox in Eurasia," the authors wrote. "Musk ox remains are found in only 1 percent of European archaeological sites and 6 percent of Siberian sites, and do not overlap noticeably in range with Palaeolithic humans in either Europe or Siberia."
But musk ox range did shrink by 60 percent as the glacial period started to end about 19,000 years ago, and the animals — which don't do well with summer temperatures averaging above about 50 degrees Fahrenheit — became more isolated genetically. Climate change appears to be main driver of extinction in Eurasia.
Wild horses. This ice age mammal roamed across 3.5-million square miles of steppe for thousands of years, suggesting a huge population that wasn't hampered by the changing climate. But the species began to lose genetic diversity as time went on. People ate them regularly — horse remains were found in 58 percent of European and 66 percent of Siberian sites. A combination of climate change and human factors gets the blame.
Steppe bison. Climate change about 35,000 years ago probably limited their range, leading to a decline in both population and genetic diversity. The genetic diversity plunged steeply about 16,000 years ago, about the time humans arrived on the scene. Prehistoric hunters targeted bison more often than any other large prey; bison remains were found in 77 percent of Siberian archaeological sites. A combination of climate change and human factors get the blame.
Reindeer (caribou). Everything worked against the "wandering deer of the north," but never got them down. Like the wild horse, reindeer roamed across the northern steppes of both continents, overlapping with humans for thousands of years. They were favored prey, too, found in 67 percent of Siberian archaeological sites and in North America. Climate change shrank reindeer range even more dramatically than that of the horse and bison. And yet they continued to thrive — largely due to high birth rate and their innate ecological flexibility.
"Despite the apparently detrimental influences of both humans and climate change, wild and domestic reindeer currently number in the millions across the Holarctic," the authors wrote. "Although individual populations are affected by changing climate, the species is not currently under threat of extinction."
Woolly mammoth. The beloved Ice Age icon roamed the Beringinian steppe until about 9,500 years ago. The remains of this hairy, lumbering elephant can be found throughout the Far North, with the permafrost of Interior Alaska littered with bits of mammoth flesh, bone and hide. Some evidence suggests a dwarf version of the critters survived on St. Paul Island until 8,000 years ago and on Russia's Wrangel Island until maybe 4,000 years ago.
Explaining their demise is difficult. The scientists found that woolly mammoths remained genetically diverse, even as their range shifted farther north over the final eons of the Ice Age. The animals overlapped with people for thousands of years and, like the woolly rhino, experienced a five-to-tenfold surge in population size after humans first moved into the their territory between 34,000 and 19,000 years ago.
"This result directly contradicts models of population collapse from human overkill (blitzkrieg) or infectious diseases following the first human contact (hyperdisease)," the authors wrote. Even so, archaeological evidence suggests the animals were regularly hunted down, killed and eaten. Taken as a whole, the evidence simply doesn't explain what happened.
"The data from woolly mammoth are inconclusive about the causes of extinction," the authors conclude.