About 3700 years ago, as Mesopotamian poets were composing the “Epic of Gilgamesh,” the last woolly mammoths on Earth were making their last stand on a remote Arctic island. A terminal colony persisted on tiny Wrangel Island north of the Siberian mainland thousands of years after the rest of its kind had disappeared. Now, a new study reveals the mammoths’ horrific final days: A series of harmful genetic mutations appears to have led to what authors call a “genomic meltdown” in the population.
Woolly mammoths by the tens of thousands once roamed across ice age grasslands spanning Europe, Asia, and the northern reaches of North America. But after the global climate began warming some 12,000 years ago, mossy tundra began to replace grasses, depriving the massive animals—roughly the size of modern African elephants—of their most important food source. Human hunters further culled their numbers. Woolly mammoths went extinct on the mainland about 10,000 years ago, but small pocket populations persisted on islands, isolated from human contact.
Hoping to learn more about the last lonely days of the Wrangel Island mammoths, bioinformatics researcher Rebekah Rogers of the University of North Carolina in Charlotte and biologist Montgomery Slatkin of the University of California (UC), Berkeley, compared the complete DNA sequence from a 4300-year-old mammoth bone found on Wrangel Island with that of a 45,000-year-old specimen that lived on the Siberian mainland.
The researchers identified a series of major detrimental mutations in the Wrangel Island mammoth. One combination of altered genes likely led to the loss of a large number of olfactory receptors for detecting smells. Another suite of mutations would have reduced the number and variety of the animals’ urinary proteins. Together, those changes would have wreaked havoc on the mammoths’ ability to mark and recognize territory, determine rank, and choose mates, if—like modern mammals—they relied on odors for these tasks. The result for the Wrangel Island mammoth community, which numbered about 300 based on population genetics models, could have been social chaos, researchers report today in PLOS Genetics.
In another bizarre twist, two peculiar mutations to a gene known as FOXQ1—well studied in rodents and rabbits—would have given the Wrangel Island mammoths a translucent, cream-colored, satiny coat. The hairs of its fur would have lacked an inner core, possibly robbing them of their insulating properties. Mice with this mutation also suffer from gastric irritation.
Harmful mutations like these are predicted to build up in small, isolated populations that become inbred, a phenomenon called genomic meltdown, according to most evolutionary biologists. If your options for a mate are limited, you can’t be too choosy about undesirable genetic traits, so those don’t get weeded out. The Wrangel Island mammoths provide a rare opportunity to see that theory play out in a real population, Rogers says.
Though the study included only a single specimen from the island, Rogers is confident its genetics would closely match the island’s other mammoths. That’s because the number and types of differences between its genome and the older mainland mammoth snugly fit mathematical predictions for how much genetic variation should exist between two individuals from the same species over time.
Beth Shapiro, a paleogenomics researcher at UC Santa Cruz, who wasn’t involved with the study, agrees. “I think it’s a great example of what evolutionary theories would predict, but it’s rare and great to see in a natural setting,” she says.
It is unlikely that any single mutation doomed the Wrangel mammoths—it still isn’t clear exactly what led to their eventual extinction. Rather, their deteriorating genetics would have made it difficult for them to adapt to new social and environmental conditions, Rogers says. Shapiro sees it as a lesson for modern populations that are dwindling and isolated. “By the end, the Wrangel Island mammoths were genetically screwed. Understanding how that happened might help us figure out which species today are most at risk of the same thing happening.”