When species after species of animal continue to go extinct today, it’s pretty clear who to blame. Humans are ushering in a sixth mass extinction event, largely through human-caused climate change but also through deforestation, overfishing, and other distinctly Homo sapiens activities. But how far back does our ability to obliterate stretch?

A debate has continued for decades now on exactly what was the primary cause of big die-offs of large mammals from the end of the last ice age: was it a changing climate (a naturally changing one, that is, unlike today), or the spread of early humans? A study by researchers at Aarhus University in Denmark claims to have cleared up the cloudy question, with computer modeling laying the blame squarely on humans and only very slightly on climate.

The study found a total of 177 large mammal species (weighing at least 10 kilograms, or 22 pounds) died off between 132,000 and 1,000 years ago, the period before human-caused extinctions became a tad more obvious. These included mammoths like that skeleton above, giant sloths, giant armadillos, giant kangaroos and wombats—put “giant” in front of a lot of mammals and you’ll find one that doesn’t exist anymore. They found certain extinction hot spots around the world, like South and North America and parts of Asia, while Africa was spared much of the die-off.

The modeling showed a weak correlation between changes in climate and extinctions; it had a correlation coefficient, known as R-squared, of 0.202, where closer to 1 means highly correlated and closer to zero means less strongly correlated. The same modeling with “human paleobiogeography” yielded a much higher R-squared, at 0.637. The one part of the world that did show a slightly stronger climate link to extinction was Eurasia, though humans clearly played a major role there as well.

“We consistently find very large rates of extinction in areas where there had been no contact between wildlife and primitive human races, and which were suddenly confronted by fully developed modern humans,” said Jens-Christian Svenning, the senior author on the study, in a press release. “In general, at least 30 percent of the large species of animals disappeared from all such areas.”

Though it may be tempting to say something like, “See, animals can adapt to a changing climate; as long as we don’t physically hunt them down and kill them ourselves they will be fine.” But that doesn’t quite follow from this work; today, humans have fine-tuned our kill-’em-off technique, drastically reshaping the climate even while we hunt to extinction and fragment and destroy habitats. Scientists 100,000 years from now would come to the same conclusion, that humans caused this era’s mass die-offs, though the primary tool would be the climate itself.

And the fact that the big mammals’ disappearances was our fault isn’t purely academic. “Our findings indicate that the current low diversity in large mammals in many continental areas is an anthropogenic phenomenon, not a natural one, with important implications for nature management,” the authors argue, citing plans to “re-wild” North America as an idea where this would come into play. If the world selected them out, fine; but if humans did it, on a much more rapid scale than evolution tends to get things done, it may be time to rethink how we’re trying to conserve all those endangered large mammals today. – Dave Levitan | June 10 2014

Source: Sandom C, Faurby S, Sandel B, Svenning, JC (2014). Global late Quaternary megafauna extinctions linked to humans, not climate change, Proceedings of the Royal Society B. DOI: 10.1098/rspb.2013.3254.

Image: shutterstock.com, Eduard Kyslynskyy