How did woolly mammoths go extinct? One study has an answer

Scientists now believe some of the last woolly mammoths on Earth may have died out due to a sudden event, a departure from previous hypotheses about their extinction.

About 10,000 years ago, the last woolly mammoths on the planet became isolated on a small island in the Arctic, and until now, scientists believed they eventually went extinct because of in-breeding among the contained population.

But data released Friday shows "some other form of sudden event, such as a disease outbreak or dramatic change in environment" may have killed off the mammoths, scientists said in a report published in the journal Cell.

The study focused on Wrangel Island, a small island off the northeastern coast of Siberia, where a small population of mammoths survived for more than six millennia until they finally died off around 4,000 years ago.

Researchers analyzed the genomes of 21 Siberian woolly mammoths – 14 from the Wrangel Island population that dated between 9,200 and 4,300 years ago, and seven from their ancestors native to the mainland, said Love Dalén, the senior author of the study and a professor in evolutionary genomics at the Centre for Palaeogenetics in Stockholm.

The genomes came from almost 400 DNA samples taken from mammoth bones, teeth, and tusks, including some that the team collected on a 2017 trip to Wrangel Island, Dalén said. A dentist drill was used to extract powder from inside the tooth, which was then purified to collect a sample of DNA.

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Woolly mammoths isolated on island as ice age ended

The mammoths were driven onto the island after the massive ice sheets that covered much of North America and northern Europe during the ice age more than 12,000 years ago started to melt, inundating their habitat with water, according to Dalén.

The water also covered a land bridge that previously connected Wrangel Island to the mainland, Dalén said. It's likely that a mammoth happened to pass through Wrangel Island "right at the time when the sea level actually cut Wrangel off from the mainland, so they became stranded there," he added.

"Our analysis suggests that they were down to about eight reproducing individuals" when they first arrived on the island, Dalén said. "The population actually grew very rapidly, almost exponentially, up to 300 individuals within something like 20 generations."

Experts don't know exactly how the island's mammoths managed to grow and survive for so long. Dalén pointed to some theories about the island's plant diversity, and that the mammoths were safe from larger predators and humans.

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Scientists zero in on mammoths' final days on island

The reason behind the island mammoths' extinction is equally a mystery, Dalén said, leading researchers to dial in on the last 200 years before the island mammoths' disappearance. "Our youngest genome is 4,300 years old, and all the data we have suggested mammoths go extinct at 4,100," he said.

Scientists have some running theories, like a short "climatic crisis" or a brush fire that destroyed plant growth on the island.

"A small population like 200 individual mammoths would probably be very sensitive to just a small change in environment," Dalén said. Another possibility is a pathogen like influenza that came to the island carried by migratory birds, Dalén added.

"I don't know if we'll ever figure it out, but at least we have some ideas," he said.