Humans not to blame for all Ice Age mammals' demise

The extinction of the woolly rhino and the Eurasian muskox of the last Ice Age can't be blamed on humans, a new study has found. But humans likely played a role in wiping out wild horses and ancient bison.
Mammoth, horse, bison and musk ox are large mammals that roamed both North America and Asia for hundreds of thousands of years. But many went extinct during the most recent period of global warming. (George Teichmann/Yukon Government)

The extinction of the woolly rhino and the Eurasian muskox of the last Ice Age can't be blamed on humans, a new study has found.

But humans likely played a role in the demise of other large mammals such as wild horses and ancient bison, says an international team of scientists in a study published Wednesday in Nature.

Duane Froese, a University of Alberta climate scientist who co-authored the study, said the finding that different species were uniquely affected by different factors around the time of their extinction ends a long-running scientific debate.

"We know that these large mammals survived for many hundreds of thousands of years and through periods of warming," he said. "The question has been why at this final warming interval…were there so many extinctions?... What this paper does is it sort of ends the debate that there's a single cause."

Wild horses went extinct in both Europe and North America. This wild horse skull was excavated in the Klondike region of the Yukon. (D.G. Froese/University of Alberta)
Eurasia lost 36 per cent of its large mammal species and North America lost about 72 per cent during the last period of warming starting around 50,000 years ago.

The research team, led by Eske Willerslev at the University of Copenhagen in Denmark, carefully examined the distribution of humans and six different Ice Age mammal species – woolly rhinoceros, woolly mammoth, wild horse, reindeer or caribou, bison and muskox — over the past 50,000 years. They also looked at the animals' DNA to estimate changes in genetic diversity over time. Lower diversity often means that the species is going through or has gone through a sudden decrease in its population.

Froese and the other Canadian collaborators provided fossils from Yukon's provincial collection and the Canadian Museum of Nature to use in the study, as well as data about how the environment in North America changed over the past 50 millenia. The study found that the range of humans mostly did not overlap with the range of the woolly rhinoceros and the Eurasian muskox at the time of their decline and extinction, suggesting that climate change was entirely to blame.

The muskox and bison, which also went extinct in Eurasia, have survived in North America to the present day.

Froese said humans existed in North America in much smaller numbers than they did in Eurasia at the time of the extinctions.

"That may have helped at least some of the survival," he said.

Horses went extinct in both North America and Eurasia where their ranges overlapped heavily with those of humans, and their remains, along with those of bison, were found frequently at human archeological sites, the study found. That suggested that humans played a large role in their extinction.

Froese said in addition to hunting large mammals, humans may also have had an impact by cutting some populations off from parts of their habitat.

The study said it was not clear how big a role humans played in the extinction of woolly mammoths.

As for reindeer, they were hunted by humans and found their range restricted by climate change, but are now the most abundant of the mammals in the study. The researcher suggested it is likely because they are very mobile and breed quickly. In all cases studied where the animals went extinct, it appeared that climate played a role. They were all animals adapted to cold, dry conditions, and many, such as the woolly rhino and wild horse, needed large areas of grassland and steppe to survive, Froese said.

He added that other mammal species - such as elk and moose - thrived and exploded in population in the presence of humans and climatic changes, and are better adapted to current-day environmental conditions.