The last woolly mammoths lived on a small island for thousands of years after their tusked relatives went extinct on the mainland. The shaggy beasts had found their way to Wrangel Island, a 93-mile-long spit of land off the coast of Siberia, around 10,000 years ago—what remained of a species that had once spread across much of the Northern Hemisphere. But these mammoths didn’t make it either. Around 4,000 years ago, the very last woolly mammoths perished and forever relegated the species to extinction.

No one knows for sure why the Wrangel Island mammoths ultimately vanished. Recent genetic analyses, however, indicate that the severe contraction of the mammoth population around this time left the beasts genetically vulnerable in a rapidly-changing world. Mutations the mammoths accrued through inbreeding likely didn’t kill them, but were instead a contributing factor to an extinction that played out over thousands of years.

In the new study, University of Copenhagen geneticist Patrícia Pečnerová and her colleagues document the genetic changes that this group of woolly mammoths experienced prior to their demise. The new data, published last month in Cell, represent the final chapter in a story of extinction that had been unfolding long before the mammoths arrived at their last refuge.

“The final extinction of mammoths on Wrangel Island is just the last step of a very long chain of events that led up to the species’ demise,” says Pečnerová, who is also a National Geographic Explorer.

Humans or climate change?

The earliest woolly mammoths evolved about 800,000 years ago, spreading as far as prehistoric Spain to the west and the Great Lakes region of North America to the east during their heyday. The global expansion of the woolly mammoths can be attributed to the peculiar environment the giant herbivores preferred. Open, grassy plains called mammoth steppes broadened their range during colder parts of the Ice Age when glaciers covered more of the planet. Woolly mammoths were grazers that thrived in these habitats, in contrast to the elephantine mastodons that preferred forests and fared better in the warmer interglacial periods.

As the glaciers once again receded and the world began to warm around 11,700 years ago, woolly mammoth habitat once again began to shrink and retreat towards the poles. Ancient humans also hunted mammoths in some parts of the animals’ range. Plus, the animals took a long time to reproduce. These combined pressures caused most mammoth populations to collapse.

“While there is still a lot of discussion about the roles of climatic changes and human hunting,” Pečnerová says, “the current consensus is that both of these contributed to the extinction.” The extinction of the woolly mammoths is not synonymous with the death of the last Wrangel Island mammoth, but part of a process that played out over thousands of years as the world changed around them.

Mammoth bones, similar to this tusk and partial skull on Bolshoy Lyakhovsky Island off the Russian coast, can be a source of ancient DNA that researchers can extract and analyze.
PHOTOGRAPH BY EVEGENIA ARBUGAEVA, NAT GEO IMAGE COLLECTION

The last holdouts

The Wrangel Island mammoths were the last holdouts. The animals represent a special case: the extinction of the last members of a species that had persisted in a refuge, dealing with different pressures than their mainland forerunners. A warmer, wetter global climate had caused the mammoth steppe to shrink towards the pole and so the fact the mammoths made it to Wrangel Island in the first place is a sign of ancient climate change. Yet climate change by itself did not drive the mammoths to extinction, and there’s no indication that the last mammoths were hunted by humans. By looking at mammoth DNA extracted from bones and teeth, geneticists have tried to figure out why the last population succumbed. (Learn more about what scientists can glean from ancient mammoth tusks.)

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The researchers behind the new study explored the fate of the last mammoths through 21 woolly mammoth genomes from different slices of time, representing both the Wrangel Island population and earlier mainland mammoths. Previous research had hinted that the Wrangel Island mammoths were so isolated that harmful mutations quickly accumulated among the population. In this scenario, the mammoths built up so many mutations through inbreeding that they could no longer produce enough healthy offspring for the population to survive. When Pečnerová and coauthors looked at the mammoth genes, however, they found a more complicated path to extinction.

“It seems like the whole population was started by at most eight breeding individuals, but increased to a few hundred rather quickly,” Pečnerová says. The mammoths’ genetics reflect this bottleneck and expansion. “A population of slow-reproducing megaherbivores that survived for 6,000 years was started by fewer animals than you can count on both hands,” she notes, which is more of an extreme scenario than the researchers had expected.

The timeline for the last mammoth populations on Wrangel Island laid out in the new paper is “a fantastic chance to look at how genomes changed in this small, isolated population,” says University of North Carolina at Charlotte geneticist Rebekah Rogers, who was not involved in the study.

The mammoth genomes indicate that despite inbreeding, the island population soon stabilized and even began purging harmful mutations over time. “I was very surprised how stable the population seemed genetically,” Pečnerová says. The mammoths were able to survive for about 200 generations before they went extinct. Rather than dwindling, the animals were hanging on when they suddenly plummeted into extinction.

“The story that we have been telling ourselves, that I wrote in my PhD thesis and presented at conferences, was wrong,” Pečnerová says. The last woolly mammoths did not suffer a genetic meltdown. The herbivores did experience the effects of inbreeding, but this itself does not seem to have tipped them over the edge into extinction. (Read more: Why do some scientists want to bring woolly mammoths back with mammoth-elephant hybrids?)

If not inbreeding, then what?

The genetic data does not pinpoint what sort of fatal pressure ended the Wrangel Island mammoths but suggests the decline was quick. Humans didn’t arrive on Wrangel Island until four centuries after the last mammoths died. The mammoths could have died out because they couldn’t keep up with environmental changes or disease outbreaks, but such shifts are difficult to track through a limited fossil record.

Even though the last mammoth population stabilized, the authors of the new study can’t totally rule out the genetic consequences of inbreeding. Mutations still could have left the mammoths vulnerable to other pressures and, even if the population had held on longer, might have been too persistent for the mammoths to survive in such an isolated spot.

“Extinction events are much more complicated than just one factor,” Rogers says, noting that climate change, shifting food sources, droughts, storms, disease, or other factors could have all contributed. The detrimental mutations among the mammoths did not help them, she notes, “but any claim that mammoths disappeared just because of their genomes does not reflect the complete story.”

Rather than there being a single event or pressure that finished the woolly mammoth, their extinction came about through thousands of years of climate and environmental changes, human hunting, and ultimately the consequences of going through a genetic bottleneck. The hairy herbivores were holding on to slivers of habitat as Earth’s climate and environments shifted. The genetic consequences of previous population crashes on the mainland meant that the Wrangel Island population may have been dead mammoths walking, unable to hold out long enough for the mammoth steppe to return.