For millions of years, mammoths roamed Europe, Asia and North America. About 15,000 years ago, the giant animals began disappearing from their vast range, until they survived on only a few islands.
Eventually they disappeared from those shelters, too, 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 on Wrangel Island disappeared 4,000 years ago, the mammoths became extinct for good.
For twenty years, 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. On Thursday they 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 host of genetic disorders.
Oliver Ryder, director of conservation genetics at the San Diego Zoo Wildlife Alliance, said the study holds important lessons for trying to save species from extinction today. It shows that inbreeding can cause long-term damage.
“The massive study makes it possible to investigate that process over thousands of years,” said Dr. Ryder, who was not involved in the new study. “We do not have such data for the species we are now trying to save.”
Dr. Dalén and his colleagues examined the genomes of 14 mammoths that lived on Wrangel Island from 9,210 years 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.
The genome of each animal contains a huge 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 mainland Siberia. Few mammoths spent time there, preferring lower areas where plants grew more abundantly.
But at the end of the Ice Age, melting glaciers submerged 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. People hunted them as 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. Valleys.
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 likely suffered from a high rate of hereditary diseases. Dr. Dalén suspects that these sick mammoths were able to survive for hundreds of generations because they had no predators or competitors. The Wrangel Island herd would likely have disappeared onto the mainland quickly.
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.
Dr. Dalén can now 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 to conduct 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 does not bode well for conservation biologists trying to save species that are close to extinction. Even if they return a species to a larger population, it may remain burdened with a low level 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: moving individual animals between populations so they can interbreed, for example.
Cloning could be another way to promote species recovery. Dr. Ryder and his colleagues 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 went extinct in the 1980s.
Without these interventions, an endangered species may struggle to escape the legacy of inbreeding, even after hundreds of generations. “It may still have time bombs in its genome that don’t bode well for the long term,” Dr. Ryder said.