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The extinction of woolly mammoths was caused by a combination of climate change and human hunting, according to a study that simulated the processes that drove their demise.

The work suggests that these mammoths wouldn鈥檛 have died out when they did if it weren鈥檛 for humans.

鈥淚n the absence of humans, we would expect woolly mammoths would have persisted for an extra 4000 years in some areas,鈥� says Damien Fordham at the University of Adelaide in South Australia.

The model also predicts that the mammoths survived longer than currently thought in several remote locations in northern Eurasia, which suggests there may be plenty more remains to be discovered. 鈥淧eople should be sending out expeditions to find fossil material,鈥� says Fordham.

Woolly mammoths (Mammuthus primigenius) were the last surviving species of mammoth. Having roamed Eurasia for several hundred thousand years, their habitat gradually shrank until the last population was confined to Wrangel Island聽in the Arctic Ocean, where they died out about 4000 years ago.

Why mammoths went extinct is a long-standing mystery. One idea is that modern humans hunted them too heavily, but not everyone accepts this.聽It has also been argued that the warming climate played a key role by shrinking their tundra habitat. Similar arguments rage about the extinctions of other large 鈥渕egafauna鈥�.

鈥淵ou will find some people who adamantly argue that it鈥檚 only climate or only humans,鈥� says Fordham. But he says the debate has shifted towards it being a combination of the two. The question is exactly how it played out.

To find out, Fordham鈥檚 team simulated the Eurasian woolly mammoth population鈥檚 late history 鈥� from 21,000 years ago until their extinction. The model included shifts in the climate, which was warming up after the coldest part of the last glacial period, and which affected vegetation by forcing the mammoths鈥� tundra home northwards. It also incorporated modern humans moving into Eurasia and hunting the mammoths.

The team ran the simulation over 90,000 times with a range of variables 鈥� different intensities of human hunting at different times, for instance 鈥� to find the one that best matched the known mammoth fossil record and estimates of their changing population size from ancient DNA.

鈥淚t was generally thought, for mammoths, if humans had a large impact, it was near the end, once their range had contracted,鈥� says Fordham.

The idea was that the warming climate forced them ever further north into smaller patches of tundra, he says, and then humans dealt the final blow when they reached those areas.

鈥淲e鈥檝e been able to show that humans had a much longer role on that extinction pathway,鈥� says Fordham.

In simulations where the humans didn鈥檛 hunt mammoths, their population remained larger for longer. On the other hand, the impact of human hunting was more severe than it might have been because the mammoths鈥� habitat was shrinking and fragmenting, so local populations were small and vulnerable.

鈥淲hat I found really convincing about this study was their counterfactual approach, trying to model if humans had never shown up,鈥� says Alexis Mychajliw of Middlebury College in Vermont. 鈥淚t took a very quantitative approach, and it was very explicit about the different sources of uncertainty.鈥�

The researchers also found that they could only simulate the correct time and place of extinction if they assumed that woolly mammoths clung on in a few spots on mainland Eurasia: several in northern Siberia and one in the north of Finland and Sweden.

鈥淭hese refugia are in places where there hasn鈥檛 been lots and lots of effort put into trying to find fossils,鈥� says Fordham. 鈥淲hat we鈥檙e hoping is our research will [encourage] others in the field to go looking at these places, because we believe they could be these sites of long-term persistence.鈥�

There may be other reasons to look in those places, says Mychajliw. 鈥淢aybe those pockets are really special for other species that we haven鈥檛 examined yet.鈥�

Reference: bioRxiv,