What Killed Ice Age Beasts?

Scientists have argued for decades about whether changing climate, human hunting, or other factors alone or in combination led to the extinctions of large mammals such as mammoths at the end of the last ice age. Now, a new analysis suggests that different species ultimately succumbed in different ways and that there apparently is no single explanation for the massive die-offs.

Beginning around 50,000 years ago, in the depths of Earth's most recent ice age, Eurasia and North America lost substantial numbers of large-bodied mammal species, which scientists often refer to as megafauna. These creatures include some of the most well-known extinct animals to have strolled the ancient landscape: saber-toothed cats, ground sloths, and woolly mammoths. It has been difficult to ascribe blame for the die-offs, especially in North America, because humans arrived and began settling that continent at approximately the same time that the warming that melted the Northern Hemisphere's high-latitude ice sheets really kicked into gear.

Now, Eske Willerslev, an evolutionary biologist at the University of Copenhagen, and his colleagues have taken a detailed look at how the populations of six different ice age species fluctuated as the ice age ended. These half-dozen species—three that died out and three that ultimately survived—included woolly mammoths, woolly rhinos, steppe bison, wild horses, musk oxen, and reindeer (also known as caribou). The researchers used climate data to estimate the amount and location of suitable habitat for each species and archaeological data to estimate where those areas overlapped with human occupation. They then estimated the genetic diversity of each species by analyzing samples taken from fossils. Lower amounts of genetic diversity denoted small populations of animals that lived in relatively limited areas of fragmented habitat and were therefore susceptible to extinction, the researchers contend.

Results of the analysis, which appear online today in Nature, "were quite surprising," Willerslev says. "The extinctions seemed to be a random process."

For example, populations of woolly rhinos, which lived only in Eurasia, apparently became fragmented and dwindled across their entire range at the same time, with the creatures disappearing from the fossil record about 14,000 years ago. But because the estimated worldwide population of the rhinos had actually increased five- to 10-fold during the 10,000 years after humans first arrived in regions where the creatures lived, hunting probably didn't play a role in the extinction, the researchers say. Similarly, there's little evidence that early humans decimated musk oxen, which died out in Eurasia about 6000 years ago.

But for steppe bison, a gradual decline in genetic diversity that began about 35,000 years ago—well before humans had any contact with the species—accelerated about 16,000 years ago, coincident with the earliest known expansion of humans in the Americas, the researchers say. This pattern hints that climate change began driving the extinction but that humans played a big role in the ultimate demise of the steppe bison, they note.

The team wasn't able to determine the cause or causes of extinction for the woolly mammoth, whose habitat in Eurasia overlapped with humans throughout the late stages of the last ice age. Although the proportion of archaeological sites in southern Siberia that included mammoth remains gradually declined as the ice age wound to a close, there could be several reasons for that trend, the researchers say. The mammoths could have been moving northward due to climate change, becoming more scarce due to the long-term effects of human predation, or simply moving down the list of hunters' favorite prey species.

"Overall, the team has done a really nice job of putting together three disparate types of data," says Anthony Barnosky, a paleontologist at the University of California, Berkeley. It makes a lot of sense that each species responded differently to environmental pressures, he notes, because each creature was adapted to a particular ecological niche.

But other factors besides climate change and human influence likely played a role in the die-offs, says Ross MacPhee, a vertebrate paleontologist at the American Museum of Natural History in New York City. After all, each of these species, including those that went extinct at the end of the last ice age, had experienced similar if not larger climate fluctuations during previous ice ages. "Why, in this case, did they go all the way to extinction?" he asks. "It's an intellectually interesting problem."