But no inbreeding to the point of collapse? A genetic study sheds new light on the history and fate of the last mammoths that existed on the Siberian Wrangel Island until about 4000 years ago. They were characterized by low genetic diversity and mutations, but not to the fatal extent that had previously been assumed. Their population remained stable for over 6000 years and particularly harmful genetic variants were eradicated. A previously mysterious event has therefore probably destroyed the last mammoth population on earth, say the researchers.
They once stalked the cold steppes of the north in huge herds. But the climatic changes at the end of the last ice age meant that the woolly mammoths’ (Mammuthus primigenius) range became smaller and smaller. However, the shaggy giants were able to maintain a place of refuge for a surprisingly long time, as evidenced by finds: while they had already disappeared elsewhere, the mammoths were able to survive on Wrangel Island off the coast of Siberia until around 4,000 years ago. This isolated population came into being when the island separated from the Siberian mainland as a result of rising sea levels around 10,000 years ago.
But why have the animals not survived there to this day? There is no evidence to date that humans contributed to the extinction on the island. The oldest known traces of settlement date back to around 400 years after the disappearance of the last mammoths. One possible explanation, however, has been that the small, isolated mammoth population ultimately succumbed to a genetic collapse due to inbreeding. Increasing defects could therefore have impaired the animals’ vitality, causing their population to shrink more and more and finally disappear.
Tracking the consequences of inbreeding
Previous studies of DNA remains from mammoth finds have also provided evidence of a potentially critically low genome diversity of the island population. But now researchers led by Marianne Dehasque from Stockholm University are presenting a more comprehensive genetic study. They analyzed 14 genomes from dated remains of mammoth specimens from Wrangel Island. For comparison, the team used genetic material from seven mammoths that once lived on the Siberian mainland. In total, the samples span the last 50,000 years of the woolly mammoth’s existence.
As the researchers report, they were able to use certain clues in the genetic makeup of the Wrangel Island mammoths to determine the size of the founding population. It consisted of a maximum of eight individuals. It was therefore clear that there was a very narrow “bottleneck” in the population development of these animals. “In combination with previous findings, this suggests that the Wrangel Island mammoths only emerged from a single herd,” write the authors. According to the genetic clues, however, the animals managed to form a population of up to 300 individuals within 20 generations. It is also apparent that the Wrangel Island mammoths were apparently able to keep this population quite stable over the following millennia – even until shortly before their end.
Traces of inbreeding, but…
As for the genetic state of the population, the new analyses revealed that, compared to their ancestors on the mainland, the genomes of the Wrangel Island mammoths did indeed show clear signs of inbreeding and low genetic diversity. They also showed an accumulation of unfavorable genetic mutations. But this was not to a degree that could explain a collapse, say the researchers. According to the genetic findings, the most harmful mutations could even be purged from the population’s gene pool over the millennia. “If an individual has an extremely harmful mutation, it is essentially non-viable, so these genetic variants disappeared from the population over time,” Dehasque summarizes.
Nevertheless, a certain amount of genetic erosion can be observed in the evolutionary history of the island mammoths. However, this accumulation of moderately harmful mutations does not appear to have impaired the animals’ vitality to such an extent that their numbers ultimately declined significantly. Even the youngest mammoth, which lived about 300 years before the population died out, showed no signs of a declining population compared to older specimens.
“We cannot confirm the idea that the population was simply too small and that it was doomed to extinction for genetic reasons,” concludes senior author Love Dalén of Stockholm University. “This means that it was probably wiped out by an event without which the mammoths would still be around today,” says the scientist. Since there is no evidence so far of a human role, the researchers hypothesize: “A sudden event, such as an outbreak of disease or a sudden change in the environment, could possibly have caused the extinction of the Wrangel Island mammoths in combination with the reduced adaptability of the population,” write the authors.
Source: Cell Press, scientific article: Cell, doi: 10.1016/j.cell.2024.05.033