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About 9,200 years ago, when the ice had retreated for several millennia from most of the northern hemisphere, a herd of woolly mammoths — no more than 10 — were isolated in the far north of Siberia. The melting of ice raised the waters and what was previously connected to the continent became an island, today called Wrangel. It was the last refuge of this imposing animal. Being so few, the laws of evolution condemned them to be wiped out. But the study of their genome shows that the group overcame the profound inbreeding, and grew bigger: in just 20 generations, there were 300 mammoths, a number that would have given them a decent shot at survival. However, just as they had mysteriously thrived for 6,000 years after disappearing from the rest of the planet, they mysteriously vanished. Now, a genetic analysis of two dozen Mammuthus primigenius has provided clues about what could have happened to them, but it does not solve the mystery.
While human empires such as the Assyrian or Egyptian empires prospered, the last woolly mammoths lived on Wrangel Island. Found throughout the northern hemisphere during the Ice Age, their numbers fell millennium after millennium. The fact that the animal’s demise coincided with human expansion, as well as the end of the Ice Age, has divided scientists on the cause of their extinction. While some blame climate change, others believe it was human hunters. Today, advances in genomics and ancient DNA (which is better preserved in the tundra than in tropical rainforests) are opening new windows to the past and trying to find an answer to this question.
One of the most ambitious efforts has just been published in the scientific journal Cell. A group of researchers who have been excavating in Wrangel for years analyzed the genome of 21 mammoths. The genetic material belongs to specimens from more than 50,000 years ago, when the species lived in times of splendor, until just 4,300 years ago. Fourteen of them were on the island during the 6,000 years that were isolated. By comparing the islanders with each other and with those on the mainland, the researchers confirmed that, as expected, there was a very small genetic pool. All mammoths throughout that time descended from a single matriarch. The scientists estimate that the herd would have had about eight members. With this profound founding effect, inbreeding was inevitable. This resulted in a sharp decline in genetic diversity. Given this problem, the expectation would be for the species to be driven to extinction. But this was not the case.
“The population was very inbred. It’s hard to give an exact number or compare the amount of inbreeding with another species because this depends so much on the type of method or filtering you use. But if we compare the Wrangel mammoths with their direct ancestors from the mainland, we find that they had four times higher levels of homozygosity, a measure of inbreeding,” says the first author of the research, Marianne Dehasque from the Center for Palaeogenetics, a joint agency of the Swedish Museum of Natural History and Stockholm University.
Her colleague David Díez del Molino adds: “We are the first to be surprised. When we look at the variability within each individual, the genetic variability, the index we use is heterozygosity. That value was 0.75 before Wrangel. And it is very stable in mammoths of very different periods, 50,000 years ago, 20,000 years ago, 12,000 years ago. By the time we have mammoth 0 on the island, i.e., less than 10,000 years ago, the diversity value plummets to 0.42. That’s 43% less.”
Another of the results that has confused the scientists concerns mutations. In very small groups, with inbreeding, an increase in these genetic changes, some potentially harmful, is expected. In fact, they observed a 30% increase in deletions (mutation due to loss of genetic material). But again, that wasn’t what led to the woolly mammoth’s extinction. “Following classical models, we thought that, when a population is small, it accumulates mutations that are harmful, deleterious, because you can’t make them disappear. You have so few individuals that they have to reproduce, otherwise the population disappears,” says Díez del Molino. “When populations are larger, it is easier for the mutation to disappear, because when there are thousands of individuals, if one that has a negative mutation does not reproduce, nothing happens,” he explains. But what the researchers found is that while the most harmful mutations were purged, others that were not so harmful built up.
They found confirmation of this immediately: from a small herd, the woolly mammoths population grew to be between 200 and 300. That is a considerable population for an island that is a bit larger than Spain’s Basque Country. What’s most striking is that this growth occurred in just 20 generations. If comparisons to today’s elephants are valid, that means it took them only about 600 years to reach those numbers. Furthermore, for the next 5,000 years, the size of the population remained relatively stable.
Love Dalén, senior author of the research, who is also from the Center for Palaeogenetics, states in a press release: “We can now confidently reject the idea that the population was simply too small and that they were doomed to go extinct for genetic reasons.” In fact, he believes that “this means it was probably just some random event that killed them off, and if that random event hadn’t happened, then we would still have mammoths today.”
After millennia of relative stability, genetic data reveal that it took only 10 generations (about three centuries) for woolly mammoths to completely disappear. But the researchers were not able to identify why. The climate could not have been responsible for such a sudden disappearance. And according to Díez del Molino, “There is no evidence that humans hunted mammoths on the island.”
In fact, the human species only appears in the fossil record about 300 years after the last mammoth died. “Given that our results show that the population was demographically stable until its extinction about 4,000 years ago, we think that what caused the final disappearance of the mammoth must have been something brief and sudden,” says Dehasque. “This is where we enter the realm of speculation, but, for example, a disease outbreak, extremely bad weather affecting food availability or other catastrophic events could have caused the collapse.”
At least one clue supports the pathogen hypothesis: the Wrangel mammoths had very low diversity in a set of genes known as the major histocompatibility complex, which is usually very stable and plays a key role in the vertebrate immune response. This may have made them more vulnerable to diseases.
Juan L. Cantalapiedra, a paleobiologist at Spain’s National Museum of Natural Sciences, who did not participate in the research, highlights how much new information is becoming available thanks to the field of genetics. With respect to the study’s findings, he points out that “extinctions are very complex processes in which several factors are usually involved.” He is interested in the idea of a virus or bacteria being responsible for the extinction, but recalls that “pathological agents do not fossilize,” meaning researchers would have to look “in the animals frozen in the permafrost.”
Although the mammoth genomes analyzed in this study span a long period of time, they do not include the last 300 years of the species’ existence. However, researchers have recently unearthed fossils from the last period of their history on the island and plan to perform genomic sequencing in the future. Perhaps then the mystery of the last mammoths will be unraveled.
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