Believing that a species went extinct because of a catastrophic event such as an ice age or a meteor is easier when you are a child. However, for scientists, extinction is not as simple to explain with just a big explosion. Woolly mammoths once roamed vast areas of ice age in Europe, Asia, and North America. With the global climate warming about 12,000 years ago and increasing threats from human hunters, they retreated northwards. They vanished from the mainland about 10,000 years ago, but a population on Wrangel Island survived for another 6,000 years, being isolated by rising sea levels. Because of this data, the scientists raised a question to why the mammoths went extinct.
Initially, scientists thought that the reason behind the extinction was harmful genetic mutations from inbreeding, which caused a "genomic meltdown" in the isolated population. A genomic meltdown refers to how mutations in small or repeatedly bottlenecked populations are expected to accumulate by genetic drift, causing fitness declines. In mutational meltdown models, such fitness declines further reduce population size, thus accelerating additional mutation accumulation and leading to extinction. Specifically for Mammoths, The animals had lost many olfactory receptors, which detect odors and urinary proteins. This can impact their social status and mate choice. The genome also revealed that the island mammoth had specific mutations that likely created an unusual translucent satin coat.
Scientists initially considered genetic factors as a primary cause for the extinction of the last woolly mammoths due to several observations and findings: Low Genetic Diversity, Genomic Meltdown Theory, Population Bottlenecks, Immunogenetic changes, and Comparison with Other Extinct Species: The "genomic meltdown" theory suggests harmful mutations accumulated in such populations, reducing their fitness. Evidence of population bottlenecks was supported by the drastic decreases in population size, further reducing genetic diversity and increasing inbreeding. Immunogenetic changes indicated compromised disease resistance. Comparisons with other extinct species living simultaneously, like the woolly rhinoceros and Irish elk, which exhibited similar genetic issues, reinforced this hypothesis. Recent studies, however, suggest a catastrophic event likely delivered the final blow.
Although the theory was strongly supported for several years, recent genetic analysis now suggests that a catastrophic event, such as an extreme storm or a plague, may have been responsible for their extinction. The latest research indicates that despite low genetic diversity, a stable population of several hundred mammoths lived on the island for thousands of years before their abrupt disappearance, meaning genes could not be the only reason behind their extinction. "We can now confidently reject the idea that the population was simply too small and doomed to extinction due to genetic reasons," said Prof. Love Dalén, an evolutionary geneticist at the Centre for Palaeogenetics, run jointly by the Swedish Museum of Natural History and Stockholm University. "It was likely a random event that killed them off, and without that event, we might still have had mammoths today."
A scientist working on this research, Love Dalén, and his colleagues analyzed the genomes of 13 mammoth specimens from Wrangel Island and seven earlier specimens from the mainland, covering a span of 50,000 years. The research, published in the journal Cell, reveals that the Wrangel population experienced a severe bottleneck, gradually diminishing their size to just eight breeding individuals at one point. However, they recovered to a stable population of 200-300 within 20 generations, maintaining stability until their sudden extinction as the species always did. Compared to their mainland ancestors, the Wrangel Island mammoths exhibited signs of inbreeding and low genetic diversity, particularly in genes crucial for immune response. This vulnerability to new pathogens, such as a plague or bird flu, may have contributed to their demise. However, it was not the main reason behind the extinction.
"Mammoths are an excellent system for understanding the ongoing biodiversity crisis and the genetic impacts of population bottlenecks," said Marianne Dehasque of Uppsala University, the study's first author. Dr. Vincent Lynch, who is a biologist at the University at Buffalo and not involved in the research, noted that the findings shed new light on the mammoths' final days. He suggested that a genetically compromised group would struggle to cope with environmental changes, such as new pathogens. "Extinction, especially when not caused by humans, typically results from a combination of factors like inbreeding, small population size, harmful mutations, and sometimes, bad luck," Lynch said. This comprehensive genetic study underscores the complexity of extinction events and provides valuable insights into the challenges faced by species with reduced genetic diversity in changing environments, helping us understand more of the world we live in.