About 30,000 years ago, mammoths, giant sloths, and other massive mammals roamed the earth. Twenty thousand years later they were all gone. Some researchers blame human hunting, but a new study claims that abrupt shifts in climate set in motion a downward spiral for many of these species, one that humans aggravated. The results, the authors say, are a warning to modern humans that, if not slowed, current warming could doom many more species.
Until recently, researchers relied primarily on fossils to assess the rise and fall of big mammals over the past 60,000 years. But a team led by Alan Cooper, a paleogeneticist at the University of Adelaide in Australia, added ancient DNA—isolated from the same fossils—to the mix. Based on how diverse a species’ DNA is at a given site, he and his colleagues can estimate how plentiful the animals were at a particular point in time. They compiled material from thousands of sites across North America and Eurasia, focusing on DNA from ancient mammal bones that had been analyzed by radio carbon methods to determine their ages.
The analysis showed that various species were disappearing at different times from different sites over the past 60,000 years. Sometimes they were replaced by new populations moving in as the climate cooled; other times they were not—and the permanent loss may have represented one of the large mammals' extinction.
Cooper and colleagues also documented climate fluctuations by looking for tell-tale signs of temperature change recorded in ice cores from Greenland. To pin these down more accurately, they matched those dates against fluctuations recorded in marine sediments in Venezuela. From the two records, they built a timeline of so-called interstadials—periods when climate suddenly warmed by as much as 16°C, sometimes over decades, and then cooled down again just as quickly. Several such fluctuations occurred before Earth entered a sustained period of cold from 27,000 to 19,000 years ago. And a few followed this last glacial maximum.
When the ice age was at its peak, mammoths, sloths, and other big mammals held their ground, indicating the cold was not causing extinctions as some researchers had assumed, the team reports online today in Science. But many of these species did decline and even disappeared from certain locations when the climate warmed rapidly—particularly 34,000 years ago and again between 28,000 and 30,000 years ago, the group concludes.
Cooper’s approach impresses David Steadman, a paleontologist at the Florida Museum of Natural History in Gainesville. “People haven’t ever correlated the radiocarbon data on extinct animals to the paleoclimate data in much as detail as these [researchers],” he points out. But he is cautious about the work’s conclusions. “They read too much into the completeness of the fossil record” by assuming that when fossils disappear from a site, the species has gone extinct. And he doesn’t buy the argument that climate change made such a big difference, as much earlier climate warming events didn’t drive many of these same species to extinction. To him, humans were the deciding factor because so many extinctions happened once humans came on the scene.
Cooper counters that in at least some cases, an animal’s disappearance does not depend on humans. His study shows that in North America, the giant shot-faced bear was already gone before humans reached the New World about 13,000 years ago. In Eurasia, many big animals persisted after modern humans arrived 44,000 years ago, surviving up to 30,000 years more and disappearing only when the climate suddenly warmed. The climate shifts, the researchers suggest, altered the environment, causing populations to die off or move away. Animals first hit by warming might have been particularly vulnerable to hunting or to humans interfering with their recolonization of an area, Cooper says.
“There are still people who are putting all the blame on humans, and some blame climate,” says Adrian Lister, an evolutionary paleontologist at the Natural History Museum in London who was not involved with the work. “But a growing number see it as it a synergistic effect, a powerful combination of those two factors happening at once.”