The latest volley in a long-running debate over why woolly mammoths, giant sloths, mastodons and cave lions died out worldwide suggests that humans are to blame.
A new global look at the extinctions of large mammals over the past 130,000 years finds that the loss of species correlates more closely with the arrival of humans than with changes in climate, which some studies have cited as a possible culprit.
Nonetheless, the paper is unlikely to settle the debate over what really caused the Quaternary extinction, a die-off of large mammals worldwide at the end of the Pleistocene epoch about 12,000 years ago. It is, however, one of the first fine-grained, yet global, look at how and when species died. [6 Extinct Animals That Could Be Brought Back to Life]
"The evidence really strongly suggests that people were the defining factor," said study leader Chris Sandom, co-founder of the consulting firm Wild Business Ltd., who completed the work as a postdoctoral researcher at Aarhus University in Denmark.
A mysterious extinction
Scientists have proposed multiple explanations for why mammoths and giant sloths no longer roam the planet, as they once did. (They once inhabited every continent but Antarctica.) Did climate change drive these animals to extinction? Did an impact by an asteroid or comet kill them off, as with the dinosaurs? Could disease have spread like wildfire through the population? Or were human hunters to blame?
Sandom and his colleagues focused on the two possibilities they gauged as most likely: climate and humans. (The notion that there was an extinction-causing asteroid impact during this time is very controversial, and direct evidence for a pandemic is lacking.)
Many archaeologists and paleontologists argue that human hunting makes little sense as a cause for the extinction, because few "kill sites," where large fauna were butchered, exist in the archaeological record. Other researchers argue that there are plenty of kill sites to suggest that ancient humans were hunting large mammals in significant numbers. For example, in a 2008 study published in the journal Quaternary International, scientists argue the 14 definite mammoth and mastodon kill sites found dating back to the North American hunter-gatherer Clovis culture around 13,000 years ago do, in fact, represent a large record when compared with other places where large-animal hunting is known to have occurred.
Humans versus climate
Sandom and his team gathered records on individual species known to have gone extinct between 132,000 years ago (at the beginning of the last interglacial period) and 1,000 years ago. They focused their analysis not on the continent level, as many studies have, but country-by-country or even state-by-state, in large nations like the United States. All told, the researchers analyzed 177 extinct mammals that had weighed more than 22 lbs. (10 kilograms).
The researchers then compared the timing of the extinctions with changes in climate and precipitation, and with human migration.
"What we found was, in sub-Saharan Africa, you've got the least extinction," Sandom told Live Science. "In Eurasia, you've got the next-least." [Wipe Out: History's Most Mysterious Extinctions]
This fits the human-hunting hypothesis well, he said. Large animals in sub-Saharan Africa would have had millions of years to co-evolve with humans as they learned to use tools. When early humans moved into Europe and Asia with their primitive hunting methods and weapons, they would have had access to a new population of animals unaccustomed to their clever ways.
In Australia and the Americas, where humans arrived comparatively late, the extinctions were the most extreme, Sandom said.
"You've got this very advanced hunter arriving in the system," he said, not unlike the invasive species that cause native extinctions today. The researchers did not find a strong overall relationship between extinctions and climate, except in Eurasia, Sandom said. Climate there might have interacted with human arrival in a complicated way, with temperatures determining where people migrated, he added.
Overall, humans' arrival was responsible for 64 percent of the variation in extinction rates around the globe, while temperature changes explained 20 percent of the variation, mostly in Eurasia.
Climate change can stress animals, Sandom said, but climate variations do not always spell doom for species — animals may simply alter or restrict their range in order to find a habitat that sustains them. Humanity may have disrupted this adaptive process for large mammals, he said.
"That was the final straw," Sandom said. "They couldn't handle the new predator turning up."
The findings were published today (June 3) in the journal Proceedings of the Royal Society B. Other research on single species also has turned up tantalizing clues about the possible cause of the extinction. For example, a 2012 study suggested that mammoths died out for a number of reasons, including both climate change and human hunting, and a study published this year in the journal Evolution found that the last few mammoth survivors were under great stress, perhaps from disease or inbreeding. Moreover, starvation was not what killed off the saber-toothed cats, according to a 2012 study detailed in the journal PLOS ONE.
Editor's note: Due to an editing error, this story originally implied that the asteroid that killed the dinosaurs is the same one theorized to have caused the Late Quaternary extinctions. This story was updated at 4:22 p.m. on June 4 to correct the phrasing.