Around 20,000 years ago, as the Last Glacial Maximum—the world's last significant glacial period—was coming to a close, Earth began to change dramatically. The climate warmed, large ice sheets began to melt, and humans crept increasingly into northern latitudes. At the same time, large mammals, or megafauna, such as woolly mammoths, woolly rhinoceros, and cave lions, were disappearing. In the end, North America lost 72 percent of its megafauna, and Eurasia 36 percent.

Climate change and humans appear to have served as the driving factors behind megafaunal extinction at the end of the Late Quaternary period (which began around 50,000 years ago). According to a study published recently in the journal Nature, however, different species responded in different ways to climate and humans. For example, whereas climate change was a major factor in the downfall of the woolly rhinoceros and Eurasion musk ox, a combination of climate and human activity led to the demise of the Eurasian steppe bison. The findings are significant because they help dissipate some of the contention surrounding the specific roles of climate and humans in the context of this historical extinction episode. Perhaps more importantly, however, they highlight the challenges that lie ahead for scientists who are working to identify extant species that are at risk of extinction from climate change and human activity in the modern era.

To identify the specific contributions of climate and humans to megafaunal extinction, the scientists of the Nature study analyzed the demographic histories of six extinct or extant Late Quaternary megafauna herbivores of Eurasia and North America. The species included woolly mammoth (Mammuthus primigenius), woolly rhinoceros (Coelodonta antiquitatis), reindeer (Rangifer tarandus), musk ox (Ovibus moschatus), bison and Eurasion steppe bison (Bison bison and B. priscus), and wild and domestic horse (Equus ferus and E. caballus).

The scientists also determined the age of megafaunal remains using radiocarbon dating and investigated each species' genetic signature using mitochondrial DNA isolated from bone samples. From their analyses, they were able to determine past species distributions and the geographical overlap of each species with humans at the end of the Late Quaternary.

After the Last Glacial Maximum (LGM), the world's ice sheets began to retreat and climatic shifts forced habitat redistribution. For the woolly rhinoceros in Siberia, which had relatively little regional overlap with Paleolithic humans and was found in less than 11 percent of Siberian archaeological sites dating to the period after the LGM, climate likely was the predominant driver of extinction. A similar conclusion was reached for musk ox in Eurasia.

Wild horses, on the other hand, had a large geographical distribution, indicative of a large population, in Eurasia until about 6,000 years ago, which is inconsistent with climate-driven extinction during this time. Subsequent declines in the genetic diversity of the wild horse after the LGM, however, are coincident with human expansion in Europe and Asia, indicating that humans, likely through selective hunting, were responsible for the species' decline. Declines in bison and reindeer observed well after the LGM also reflect the impact of expanding human populations at that time.

The only species for which no conclusion could be reached regarding the cause of extinction was the woolly mammoth. Although archaeological evidence suggests geographical overlap between humans and woolly mammoth in Eurasia, Siberia, and North America, Siberian sites containing woolly mammoth remains decline markedly after the LGM, a phenomenon that could have been the result of human or climatic factors.

Evidence that shifts in habitat distribution were linked with species population size at the end of the Late Quaternary reinforces the significance of the relationship between habitat loss and the future of species alive today. Stemming the loss of habitat from climate change and human activities, particularly development and agriculture, is one of the greatest challenges now facing conservation.
____________
Kara Rogers is a freelance science writer and senior editor of biomedical sciences at Encyclopaedia Britannica, Inc. She is a member of the National Association of Science Writers and author of Science Up Front on the Britannica Blog. She holds a Ph.D. in Pharmacology/Toxicology, but enjoys reading and writing about all things science. You can follow her on Twitter at @karaerogers.