In new research, scientists analyzed 23 ancient genomes of woolly mammoths (Mammuthus primigenius), including one of the oldest known specimens at 700,000 years old, to identify fixed mutations unique to the species and to obtain estimates of when these mutations evolved. They found that at the time of its origin, the woolly mammoth had already acquired a broad spectrum of positively selected genes, including ones associated with hair and skin development, fat storage and metabolism, and immune system function. They also identified a gene with several mutations that may have been responsible for the woolly mammoth’s miniscule ears.
An illustration of an adult male woolly mammoth (Mammuthus primigenius) navigates a mountain pass in Arctic Alaska, 17,100 years ago. Image credit: James Havens / University of Alaska Museum of the North.
The evolution of mammoths (genus Mammuthus) was characterized by a series of morphological transitions defined by increasing specialization to life in cold high-latitude environments with open landscapes and grassy vegetation.
This process culminated with the evolution of the woolly mammoth, which originated in northeastern Siberia during the early stages of the Middle Pleistocene, approximately 700,000 years ago, and had become extinct by the onset of the Holocene, 12,000 years ago, across the vast majority of its range.
The woolly mammoth had a Holarctic distribution and inhabited terrestrial environments up to 80 degrees north, even during full glacial conditions.
Compared to both its extant elephant relatives as well as earlier members of Mammuthus, it was uniquely adapted to life in the high Arctic.
The exceptional preservation of woolly mammoth remains recovered from permafrost deposits has enabled scientists to identify a wide range of morphological adaptations, such as thick woolly fur, small ears, short tail, and considerable fat deposits.
Moreover, genetic analyses have hinted at previously unknown physiological adaptations to the Arctic environment, including genes related to thermal sensation and hemoglobin structure.
However, recent work has indicated that only a small subset of these adaptations was unique to the woolly mammoth compared to its million-year-old ancestors.
“We wanted to know what makes a mammoth a woolly mammoth,” said Dr. David Díez-del-Molino, a researcher at the Centre for Palaeogenetics in Stockholm.
“Woolly mammoths have some very characteristic morphological features, like their thick fur and small ears, that you obviously expect based on what frozen specimens look like, but there are also many other adaptations like fat metabolism and cold perception that are not so evident because they’re at the molecular level.”
Genomes from 23 woolly mammoths and 28 living elephants revealed adaptive differences. Image credit: Díez-del-Molino et al., doi: 10.1016/j.cub.2023.03.084.
To identify genes that were highly evolved in woolly mammoths, meaning they had accrued a large number of mutations, Dr. Díez-del-Molino and colleagues compared the genomes of 23 woolly mammoths with 28 modern-day Asian and African elephant genomes.
Twenty-two of these woolly mammoths were relatively modern, having lived within the past 100,000 years, and 16 of the genomes had not been previously sequenced.
The twenty-third woolly mammoth genome belonged to one of the oldest known woolly mammoths, Chukochya, who lived approximately 700,000 years ago.
“Having the Chukochya genome allowed us to identify a number of genes that evolved during the lifespan of the woolly mammoth as a species,” said Professor Love Dalén, a researcher at the Centre for Palaeogenetics in Stockholm.
“This allows us to study evolution in real time, and we can say these specific mutations are unique to woolly mammoths, and they didn’t exist in its ancestors.”
Not surprisingly, many genes that were adaptive for woolly mammoths are related to living in cold environments.
Some of these genes are shared by unrelated modern-day Arctic mammals.
“We found some highly evolved genes related to fat metabolism and storage that are also found in other Arctic species like reindeer and polar bears, which means there’s probably convergent evolution for these genes in cold-adapted mammals,” Dr. Díez-del-Molino said.
While previous studies have looked at the genomes of one or two woolly mammoths, this is the first comparison of a large number of mammoth genomes.
This large sample size enabled the team to identify genes that were common among all woolly mammoths, and therefore likely adaptive, as opposed to genetic mutations that might only have been present in a single individual.
“We found that some of the genes that were previously thought to be special for woolly mammoths are actually variable between mammoths, which means they probably weren’t as important,” Dr. Díez-del-Molino said.
Overall, the 700,000-year-old Chukochya genome shared approximately 91.7% of the mutations that caused protein-coding changes in the more modern woolly mammoths.
This means that many of the woolly mammoth’s defining traits — including thick fur, fat metabolism, and cold-perception abilities — were probably already present when the woolly mammoth first diverged from its ancestor, the steppe mammoth (Mammuthus trogontherii).
However, these traits developed further in Chukochya’s descendants.
“The very earliest woolly mammoths weren’t fully evolved. They possibly had larger ears, and their wool was different — perhaps less insulating and fluffy compared to later woolly mammoths,” Professor Dalén said.
More modern woolly mammoths also had several immune mutations in T cell antigens that were not seen in their ancestor.
“These mutations may have conferred enhanced cell-mediated immunity in response to emerging viral pathogens,” the authors said.
Their paper was published in the journal Current Biology.
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David Díez-del-Molino et al. Genomics of adaptive evolution in the woolly mammoth. Current Biology, published online April 7, 2023; doi: 10.1016/j.cub.2023.03.084
