About 9,200 years ago, when the ice had retreated for several millennia from most of the northern hemisphere, a herd of mammoths, no more than ten, 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 disappear. But the study of their genome shows that they overcame the profound inbreeding they incurred, moving forward: in just 20 generations there were already about 300, a number that would have given them many chances to survive. However, just as they thrived for 6,000 years after disappearing from the rest of the planet, they suddenly vanished. The genetic analysis of two dozen Mammuthus primigenius It gives clues about what could have happened to them, but does not solve the mystery.
While human empires such as the Assyrian or Egyptian empires prospered, the last woolly mammoths lived on the island of Rangel. Habitual throughout the northern hemisphere, including the Iberian Peninsula, throughout the Ice Age, they disappeared millennium after millennium. The temporal parallel between its disappearance and human expansion, on the one hand, and the end of the ice age, on the other, has been dividing scientists in the field for years. For some, climate change was the determining factor. For others, it was the hunters who delivered the final stab. In these, advances in genomics and ancient DNA (which is better preserved in the tundra than in tropical forests) are opening new windows to the past and trying to settle the issue.
One of the most ambitious efforts has just been published in the scientific magazine cell. A group of researchers who have been excavating in Wrangel for years have analyzed the genome of 21 mammoths. The genetic material belongs to specimens from more than 50,000 years ago, the oldest, when the species lived in times of splendor, until just 4,300 years ago. 14 of them, the most recent, are of animals from the island that cover the period of 6,000 years that they endured in Wrangel. By comparing the islanders with each other and with the continentals, they have confirmed that, as expected, a very narrow genetic bottleneck occurred. All mammoths throughout that time descended from a single matriarch. They estimate that the herd would have about eight members. With this profound founding effect, inbreeding was inevitable. This has resulted in a sharp decline in genetic diversity. With this, one would have expected that genetics would condemn them. But it was not like that.
“The population was very endogamous. It is difficult to give an exact number or compare the amount of inbreeding with another species because this depends a lot on the type of method or filtering used. But if we compare Wrangel’s mammoths with their direct ancestors from the continent, we found that they had four times higher levels of homozygosity, a measure of inbreeding,” says the first author of the research, Marianne Dehasque, from the Paleogenetics Center, a joint organization of the Swedish Natural History Museum and Stockholm University. His partner David Díez del Molino adds: “The first ones surprised are us. When we look at the variability that exists within each individual, the genetic variability, the index we use is heterozygosity. That value was 0.8 before Wrangel. And it is very stable in mammoths from very different periods, from 50 thousand years ago, from 20 thousand years ago, from 12 thousand years ago. By the time we have mammoth 0 on the island, that is, less than 10,000 years ago, the diversity value plummets to 0.4. It is 40% less,” explains Díez del Molino.
Another of the results that has confused them has to do with mutations. In very small groups, with inbreeding, an increase in these genetic changes, some potentially harmful, is expected. In fact, they observed an increase in deletions (mutation due to loss of genetic material) of 30%. But again, that didn’t condemn them. “Following classical models, we thought that, when a population is small, it accumulates mutations that are harmful, deleterious, because they cannot be made to disappear. You have so few individuals that they have to reproduce, because if not the population disappears,” says Díez del Molino. “When populations are larger, it is easier for the mutation to disappear, because when it comes to thousands of individuals, if one that has a negative mutation does not reproduce, nothing happens,” he details. But what they have found is that while the most harmful mutations were purged, others that were not so harmful accumulated.
They get confirmation straight away: from a small herd, they grew to a number estimated at between 200 and 300 mammoths. Such a large number for an island that is slightly larger than the Basque Country or smaller than the Community of Madrid, is a considerable population. What is most striking is that this growth occurred in just 20 generations. If comparisons with present-day elephants are valid, this means that it took them only about 600 years to prosper in this way. What is more, over the next 5,000 years, the number of mammoths remained relatively constant.
“It had to be a random event that killed them and if it hadn’t happened, we would still have mammoths today”
Love Dalén, from the Center for Paleogenetics at the National Museum of Natural History and Stockholm University
Love Dalén, senior author of the research and also from the Center for Paleogenetics, states in a note: “We can now safely reject the idea that the population was too small and that they were doomed to become extinct for genetic reasons.” In fact, he claims that “this means it probably had to be a random event that killed them, and if that random event hadn’t happened, we would still have mammoths today.” That is why it is another of the results of this work. After millennia of relative stability, genetic data reveal that it took only ten generations (about three centuries) for woolly mammoths to completely disappear.
But they cannot go further to identify the culprit. The weather, with its weather, could not be responsible for such a sudden disappearance. “There is no evidence that humans hunted mammoths on the island,” highlights the Spaniard Díez del Molino. In fact, our species only appears in the 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 mammoth’s final disappearance must have been something brief and sudden,” adds his partner 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,” she adds. The possibility of the pathogen has at least one clue: Wrangel’s mammoths had a very low diversity in a set of genes known as the major histocompatibility complex, which is usually very stable and plays a fundamental role in the immune response of the animals. vertebrates. This could have made them more vulnerable.
“Extinctions are very complex processes in which several factors usually intervene”
Juan L. Cantalapiedra, paleobiologist at the National Museum of Natural Sciences
The paleobiologist at the National Museum of Natural Sciences, Juan L. Cantalapiedra, who has not participated in this research, highlights the amount of new information that genetics provides, something that a few years ago was impossible. Regarding the results, he recalls that “extinctions are very complex processes in which several factors usually intervene.” The role of some virus or bacteria attracts him, “but pathological agents do not fossilize,” he recalls. We would have to look, he adds, “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 latter period of its history on the island and plan to conduct genomic sequencing in the future. Perhaps then the mystery of the last mammoths will be revealed.
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