The Last Stand of the Woolly Mammoths

For millions of years, mammoths roamed Europe, Asia, and North America. About 15,000 years ago, the giant animals began to disappear from their vast range until they survived on just a few islands.

Eventually they disappeared from those refuges as well, with one exception: Wrangel Island, a landmass the size of Delaware more than 80 miles north of the coast of Siberia. Mammoths survived there for thousands of years — they were still alive when the Great Pyramids were built in Egypt.

When the mammoths disappeared from Wrangel Island 4,000 years ago, mammoths became extinct for good.

For two decades, Love Dalén, a geneticist at Stockholm University, and his colleagues have been extracting bits of DNA from fossils on Wrangel Island. In recent years they have collected entire mammoth genomes. Thursday published a reconstruction of the genetic history of these enigmatic animals.

The scientists concluded that the island’s population was founded about 10,000 years ago by a small herd of fewer than 10 animals. The colony survived for 6,000 years, but the mammoths suffered from a number of genetic disorders.

Oliver Ryder, director of conservation genetics at the San Diego Zoo Wildlife Alliance, said the study has important lessons for trying to save species from extinction today. It shows that inbreeding can cause long-term harm.

“The mammoth study allows us to study that process over thousands of years,” said Dr. Ryder, who was not involved in the new study. “We don’t have data like this for the species we’re trying to save.”

Dr. Dalén and his colleagues examined the genomes of 14 mammoths that lived on Wrangel Island from 9,210 to 4,333 years ago. The researchers compared the DNA of the Wrangel Island mammoths with seven genomes of mammoths that lived on the Siberian mainland until 12,158 years ago.

Each animal’s genome contains a vast amount of information about the population to which it belonged. In large populations there is a lot of genetic diversity. As a result, an animal will inherit different versions of many of its genes from its parents. In a small population, animals will be inbred and inherit identical copies of many genes.

The oldest fossils from Wrangel Island contain identical versions of many genes. Dr. Dalen and his colleagues concluded that the island was founded by a remarkably small population of mammoths.

Before about 10,000 years ago, Wrangel Island was a mountainous area on the Siberian mainland. Few mammoths spent time there, preferring lower areas where vegetation was more abundant.

But at the end of the Ice Age, melting glaciers flooded the northern edge of Siberia. “There was a small herd of mammoths that happened to be on Wrangel Island when it was cut off from the mainland,” said Dr. Valleys.

The mainland mammoths faced significant challenges to their survival. Humans hunted them, while the changing climate wiped out much of their grassland habitat, turning it into tundra.

But the few mammoths stranded on Wrangel Island enjoyed a huge windfall. The island was free of humans and other predators, and they faced no competition from other grazing mammals. Furthermore, Wrangel Island’s climate turned it into an ecological time capsule, where the mammoths could still enjoy a diversity of Ice Age plants.

“Wrangel Island was a golden place to live,” said Dr. Dalén.

He and his colleagues found that the population on Wrangel Island grew from fewer than 10 mammoths to about 200. That was probably the maximum number of mammoths the island’s plant life could support.

But life was far from perfect for the Wrangel mammoths. The few animals that founded the island had very little genetic diversity, and Dr. Dalén and his colleagues found that levels remained low for the next 6,000 years.

“They carried with them the inbreeding that they got in the early days,” he said.

As a result, the mammoths probably suffered from high levels of hereditary disease. Dr. Dalén suspects that these diseased mammoths were able to survive for hundreds of generations because they had no predators or competitors. The Wrangel Island herd would probably have quickly disappeared on the mainland.

The new study does not reveal exactly how the Wrangel mammoths met their end. There is no evidence that man is to blame; the earliest known visitors to Wrangel Island appear to have set up a summer hunting camp 400 years after the mammoths became extinct.

For now, Dr. Dalén only speculate about the true cause of the mammoth’s extinction. The war in Ukraine has made it impossible for him and his colleagues to travel to Russia for further research.

It is possible that a tundra fire killed the Wrangel mammoths, or that an Arctic volcano erupted. Dr. Dalén can even imagine that a migrating bird brought a flu virus to Wrangel Island, which then jumped to the mammoths and wiped them out.

“We still have a number of possible explanations, and we still haven’t been able to narrow them down,” he said.

Dr. Dalén thinks the new study is bad news for conservation biologists trying to save species that are close to extinction. Even if they bring a species back to a larger population, it can still be burdened with low levels of genetic diversity.

Dr. Dalén said it could be essential to increase the genetic diversity of recovering populations. Conservation biologists have explored how to do this: by moving individual animals between populations so they can reproduce, for example.

Cloning may be another way to help species recover. Dr. Ryder and his colleagues have frozen cells from endangered animals to preserve some of their genetic diversity. In 2021, researchers managed to produce a clone of a black-footed ferret from a population that had gone extinct in the 1980s.

Without these interventions, an endangered species may struggle to escape a legacy of inbreeding, even after hundreds of generations. “It could still have these time bombs in its genome that don’t bode well for the long term,” said Dr. Ryder.

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