New genetic study reveals the woolly mammoths' journey towards extinction

Illustration: Beth Zaiken

A major issue in conservation biology concerns the viability of populations after sudden population declines. Previous studies have demonstrated that demographic bottlenecks are associated with a loss of genetic diversity and inbreeding depression. However, the long-term genomic effects of such sudden population declines, even after partial recovery, remain largely unexplored. The woolly mammoth (Mammuthus primigenius) is an excellent model system to investigate the genetic consequences such bottlenecks.

“Not only did the last mammoth population survive on a small Siberian island for 6,000 years before becoming extinct” explains Dr. Marianne Dehasque, lead author of the study in Cell, “but thanks to the cold arctic conditions, the DNA has been extremely well-preserved in many of the remains”.

To investigate what happened to the mammoths during the time leading up to the extinction, researchers from the Centre for Palaeogenetics, which is jointly funded by the Swedish Museum of Natural History and Stockholm University, sequenced high-quality genomes from 21 woolly mammoths. The DNA was recovered from radiocarbon dated remains that span the last 50,000 years of the mammoths' existence. The results show that up until 10,000 years ago, the genetic diversity of the mammoths in northeastern Siberia did not change much. 

Marianne Dehasque
Photo: Love Dalén

“This suggests that the number of mammoths remained remarkably stable throughout the last Ice Age, despite the arrival of modern humans as well as numerous climatic changes, including the Bølling-Allerød interstadial, that took place during this period”, says Dr. Dehasque.

However, the fate of the last surviving mammoths on Wrangel Island presents a different narrative. The results show that when the island was formed due to rising sea levels, less than ten mammoths survived and gave rise to the Wrangel Island population, which subsequently survived for another 6,000 years. The genomic data shows that the population quickly recovered in size after the initial bottleneck, and that this also led to a stabilization in genetic diversity. However, the results also show that despite this recovery, the mammoth’s were affected by harmful mutations for several thousands of years. 

"Despite recovering from a near-extinction event to approximately 300 reproducing individuals, our findings reveal persistent inbreeding depression. These findings suggest that the mammoths were suffering from genetic diseases for hundreds of generations after the recovery", says Love Dalén, a Professor of evolutionary genomics based at the Centre for Palaeogenetics.

According to Dr. Dehasque, the study's findings have broader implications for conservation biology. "Our research underscores the importance of long-term genetic monitoring in conservation efforts. Even after population recovery, the impact of deleterious mutations may persist for generations, compromising the viability of endangered populations."

What eventually caused the extinction of the Wrangel population some 4,000 years ago remains a mystery. The results from the study shows that, despite being subject to inbreeding depression, the population size remained at approximately 300 reproducing individuals up until just a few generations before the extinction. This suggests that whatever caused the extinction, it led to a very rapid decline in population size. 

“The quest to find out what happened to the very last mammoths continues” says Prof. Dalén. “It’s very much like a murder mystery, where we know something caused the extinction and we just have to keep collecting evidence to figure it out”.

Article in Cell: Temporal dynamics of woolly mammoth genome erosion prior to extinction