Four thousand years ago, on an island off the coast of what is now Siberia, the world’s last woolly mammoth took its final breath. Living on that island, isolated from other mammoths, could have led to fatal levels of inbreeding and catastrophic population drops, leading to extinction, scientists have said. A new study confirms that the woolly mammoth population on Wrangel Island was inbred but suggests they were not doomed to die. The mammoth population gradually lost harmful genetic mutations that would affect survival, indicating that some other random event — such as disease or environmental changes — sealed the mammoths’ fate.
“This paper does a remarkable job,” says Joshua Miller, a paleontologist at the University of Cincinnati who was not involved in the study. The research, Miller says, both offers valuable insight into the end of the Wrangel Island mammoths and suggests how genetics should be monitored in modern endangered animal conservation efforts.
Until around 10,000 years ago, the woolly mammoths lived on mainland Siberia, but rising global sea levels left the populations stranded on disparate islands, potentially limiting genetic mixing among the mammoths. Genetic variation is the general toolbox that animals have in order to adapt to changes in the environment. Previous research on mammoth extinction theorized that population isolation increased the level of inbreeding, decreased genetic variation and made the mammoths more vulnerable to harmful genetic mutations, diseases and death.
But Dalén and colleagues reject this idea — and have for more than a decade. Over the years, the researchers have collected woolly mammoth bone shards, tusks and teeth in Siberia, and from them extracted woolly mammoth genomes. In the new study, the team analyzed 21 genomes, including eight that had already been previously published. The genome data cover the last 50,000 years of woolly mammoths’ existence, including when the animals became isolated on Wrangel Island.
Using computer modeling software, the team compared the woolly mammoths’ genomes with the genomes of elephants, the closest modern-day relation, and humans to predict how harmful genetic mutations were to the mammoth and whether they were purged from the population over time. The analysis showed that though Wrangel Island’s mammoth population started with at most eight individuals, it jumped to about 200 to 300 individuals and stayed level until the mammoths went extinct.
The most harmful genetic mutations in the mammoth population also became less frequent over time, likely because animals with those mutations couldn’t or didn’t reproduce, the researchers say. Minor genetic mutations likely would not have caused the Wrangel Island mammoths to die out completely, Dalén says.
“It is really good evidence against the meltdown model, but it doesn’t completely exclude that model,” says Vincent Lynch, an evolutionary biologist at the University at Buffalo in New York. Though the island isolation and drop in genetic variation might not have been the final nail in the mammoth coffin, even the accumulation of minor genetic mutations could have made the woolly mammoths more vulnerable to other environmental changes like disease, climate shifts and the arrival of humans.
Due to challenges obtaining high-quality DNA, the team was not able to analyze the genetic condition of the Wrangel Island population during their final 300 years, roughly five generations. In the future, with rapidly improving sequencing technologies, the researchers are looking to complete their analysis of the Wrangel Island mammoths’ genetic trajectory. As scientists continue to study the woolly mammoth, the animal’s final moments remain a mystery.
“Maybe they were just unlucky,” Dalén says. If some disaster had not struck Wrangel Island, perhaps “we would have had mammoths walking around still today.”
