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What made woolly mammoths unique?

A team of scientists led by the Center for Paleogenetics in Stockholm has recently compared the genomes of woolly mammoths to those of modern-day elephants to find out what made these mammoths unique. The analyses revealed that, although many of the mammoths’ trademark features, such as their wooly coats and large fat deposits, were already encoded in the earliest woolly mammoths, these and other traits became more defined during evolution.

“We wanted to know what makes a mammoth a woolly mammoth,” said study lead author David Díez-del-Molino, an expert in Population Genomics and Ancient DNA at the Center for Paleogenetics. “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.”

In order to identify genes that had accrued a large number of mutations in woolly mammoths, the experts compared the genomes of 23 Siberian mammoths with those of 28 modern-day African and Asian elephants. While 22 of the mammoths were relatively modern, having lived in the past 100,000 years, one of the genomes belonged to one of the oldest known woolly mammoths, Chukochya, who lived about 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,” explained senior author Love Dalén, professor of Evolutionary Genomics at the Center for Palaeogenetics. “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.” 

Many of the adaptive genes the experts discovered were related to living in cold environments and are shared by unrelated modern-day Arctic mammals such as reindeer or polar bears. Moreover, since Chukochya’s genome shared about 91.7 percent of the mutations that caused protein-coding changes in the more modern woolly mammoths, many of their defining traits – such as thick fur, fat metabolism, and cold-perception abilities – were most likely present when the woolly mammoths diverged from their ancestors, the steppe mammoths. 

However, these traits developed further as the mammoths evolved. “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,” Dalén explained.

Finally, more modern woolly mammoths had several immune mutations in T cell antigens which were not found in their ancestors, and may have been selected to confer them enhanced cell-mediated immunity in response to emerging viral pathogens.

Since all the mammoths investigated in this study were from Siberia, the scientists aim to include in future research North American woolly mammoths too. “We showed a couple of years ago that there was gene flow between woolly mammoths and the ancestors of Colombian mammoths, so that’s something that we will need to account for because North American woolly mammoths might have been carrying non-woolly mammoth genes as well,” Dalén concluded.

The study is published in the journal Current Biology.

By Andrei Ionescu, Staff Writer

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