The woolly mammoths of Wrangel Island, descended from a very small original population, persisted for 6,000 years despite genetic obstacles. Their sudden disappearance remains a mystery, providing lessons for modern conservation efforts. Credit: Beth Zeiken
Genetic analysis of the last woolly mammoths on Wrangel Island has revealed a population that managed to survive for 6,000 years despite heavy inbreeding and low genetic diversity.
Originally originating from no more than eight individuals, this group expanded to 200-300 members. Although genetic problems did not directly cause their extinction, it remains unclear what ultimately led to their demise. The study provides insight into how such populations can inform current conservation strategies for the endangered kinds today.
Ten thousand years ago, the last population of woolly mammoths became isolated on Wrangel Island, located off the coast of Siberia, due to rising sea levels that separated the mountainous island from the mainland. A new genomic study shows that this isolated population, which lived on the island for the next 6,000 years, began with no more than eight individuals and expanded to between 200 and 300 within 20 generations. Published in magazine cell on June 27, the study showed that Wrangel Island mammoths showed signs of inbreeding and low genetic diversity, but these factors alone did not explain their mysterious and final disappearance.
Siberian Continental Tusk. Credit: Love Dalén
A reassessment of extinction theories
“We can now confidently reject the idea that the population was simply too small and that they were doomed to extinction for genetic reasons,” says senior author Lav Dahlen, an evolutionary geneticist at the Center for Paleogenetics, a collaboration between the Swedish Museum of Natural History and Stockholm university. “That means it’s probably just some random event that killed them off, and if that random event hadn’t happened, then we’d still have mammoths today.”
I love Dalen. Credit: Ian Watts
Insights into ongoing conservation efforts
In addition to shedding light on woolly mammoth population dynamics, this analysis of Wrangel Island mammoths can help inform conservation strategies for today’s endangered animals.
“Mammoths are an excellent system for understanding the ongoing biodiversity crisis and what happens genetically when a species goes through a population bottleneck because they mirror the fate of many modern populations,” says first author Marian Dehask of the Paleogenetics Center .
Marianne DeHask, working in the Ancient DNA Laboratory. Credit: Love Dalén
Genetic challenges and lasting legacy
To understand the genomic consequences of the Wrangel Island predicament on the mammoth population, the team analyzed the genomes of 21 woolly mammoths—14 from Wrangel Island and 7 from the mainland population that preceded the predicament. Overall, the samples span the last 50,000 years of the woolly mammoth’s existence, providing a window into how mammoth genetic diversity has changed over time.
Compared to their mainland ancestors, the genomes of mammoths from Wrangel Island show signs of inbreeding and low genetic diversity. In addition to overall low genetic diversity, they showed reduced diversity in the major histocompatibility complex, a group of genes known to play a critical role in the vertebrate immune response.
Tusk from Wrangel Island. Credit: Love Dalén
Long-term genetic effects and future research
The researchers showed that the genetic diversity of the population continued to decline during the 6,000 years that the mammoths inhabited Wrangel Island, albeit at a very slow rate, suggesting that the population size was stable until the very end. And although the island’s vast population gradually accumulated moderately harmful mutations throughout its 6,000-year reign, the researchers showed that the population slowly purged the most harmful mutations.
“If an individual has a highly deleterious mutation, it’s essentially not viable, so those mutations gradually disappeared from the population over time, but on the other hand, we see that mammoths accumulated mildly deleterious mutations almost until they went extinct,” says Dehask . “It is important for modern conservation programs to bear in mind that it is not enough to bring the population back to a decent size; you also have to monitor it actively and genetically because these genomic effects can last more than 6000 years.
The final mystery of the extinction of the woolly mammoth
Although the mammoth genomes analyzed in this study span a large period of time, they do not include the last 300 years of the species’ existence. However, researchers have found fossils from the mammoth’s latest period and plan to conduct genome sequencing in the future.
“What happened at the end is still a bit of a mystery – we don’t know why they disappeared after being more or less fine for 6,000 years, but we think it was something sudden,” Dahlen says. “I would say there is still hope to find out why they disappeared, but no promises.”
Reference: “Temporal dynamics of pre-extinction woolly mammoth genome erosion” 27 Jun 2024, cell.
DOI: 10.1016/j.cell.2024.05.033
