DNA Mutations May Have Doomed the Woolly Mammoth

This mural at the American Museum of Natural History in New York City shows woolly mammoths near the Somme River.

(Image credit: Charles R. Knight)

By the end of the ice age, the last remaining woolly mammoths had acquired so many genetic mutations that their numbers were practically guaranteed to spiral toward extinction, a new study has revealed.

Mammoths were once among the most common large herbivores that roamed across North America, Siberia and Beringia, a geographic area that once stretched from Siberia to the Canadian Yukon but is now mostly submerged under the Bering Strait. The giant beasts first appeared about 700,000 years ago. But, at the end of the last ice age, about 10,000 years ago, their population suddenly declined.

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Researchers from the University of California, Berkeley, recently compared the existing genomes of two different mammoth specimens. One was a 45,000-year-old woolly mammoth found in northeastern Siberia, and the other was a 4,300-year-old mammoth from Wrangel Island, off the coast of Russia. The scientists found that the DNA of the Wrangel Island mammoth, which represents the last members of the species, had multiple harmful mutations that would have interfered with normal functions and compounded the difficulties of survival.

In larger populations, mutations that occur naturally are weeded out by competition, Rogers said. But with such a small population, there would have been no mechanism to prevent these mutations from being passed on to the next generation of mammoths. As such, harmful changes in the mammoth genome that deleted large chunks of DNA, or messed up how genes were read and translated, would have accumulated, according to Rogers.

By looking at which genes were affected by these harmful mutations, Rogers and her colleague, Montgomery Slatkin, a population geneticist at the University of California, Berkeley, were able to guess what functions or behaviors might have been affected as mammoth populations dwindled. The animals probably lost many olfactory receptors, which detect odors, as well as urinary proteins, the researchers found. This could have affected their social status and mate choice, Rogers said. The genome also revealed that the island mammoth had certain mutations that likely created an unusual translucent satin coat, as well as several other mutations with effects that scientists don't fully understand yet.

"It makes sense that the researchers would find an accumulation of deleterious mutations in a population that was very small," said Beth Shapiro, an evolutionary biologist at the University of California, Santa Cruz, who was not involved in the new study. "[This] reveals that it's not necessarily just a small population size that is potentially dangerous for populations but also the content of those genomes that's important."

It might also provide an interesting twist to mammoth "de-extinction" experiments. If some mammoth genomes carry an overabundance of negative mutations, researchers need to carefully screen the genes before they are inserted into mammoth-elephant hybrid genomes, Rogers said. In fact, screening the mutations and testing what their functions are could also inform scientists about how the mutated genes affected mammoths just before they went extinct, she added.