A study comparing the mammoth genome to modern elephants revealed key characteristics that defined the species, such as furry fur, large fat deposits and small ears. These traits already existed in early mammoths and became more defined over the course of their more than 700,000-year existence.
A team of researchers compared the genomes of mammoths and modern elephants to find out what makes mammoths unique as individuals and species. Investigators report today (April 7) in the journal Current Biology that many of the iconic features of mammoths – including their fur coats and large fat deposits – were already genetically encoded in the earliest mammoths, but these and other traits have become more connivalent in the species' more than 700,000 years of existence. They also identified a gene with several mutations that may be responsible for the mammoth's tiny ears.
David Díez-del-Molino, paleogeneticist and first author at the Centre for Paleogenetics in Stockholm, said: "We wanted to know what makes a mammoth a woolly mammoth. Long-haired mammoths have some very characteristic morphological features, such as their thick fur and small ears, which can obviously be expected based on what the frozen specimen looks like, but there are many other adaptations, such as fat metabolism and cold perception, that are not so obvious because they are at the molecular level. "
This is a photograph of woolly mammoth tusks, which the authors sequenced throughout its genome. The tusk was discovered in northeastern Siberia in 2015 and was measured by radiocarbon as about 18,000 years old.
To identify genes that were "highly evolved" in mammoths — meaning they accumulated a large number of mutations — the team compared the genomes of 23 Siberian mammoths with those of 28 modern Asian and African elephants. 22 of these mammoths are relatively modern and have lived in the last 100,000 years, and 16 of them have genomes that have not been sequenced before. The genome of the twenty-third mammoth belongs to one of the oldest known mammoths, Chukochya, which lived about 700,000 years ago.
Senior author Love Dalén, professor of evolutionary genomics at the Centre for Paleogenetics in Stockholm, said: "With Chukochya's genome, we can identify some of the genes that evolved during the life cycle of mammoths as a species. This allows us to study evolution in real time, and we can say that these specific mutations are specific to mammoths, which did not exist in its ancestors. "
This is a photo of study co-author Love Dalén with a Yuka mammoth, whose genome was included in the analysis.
Not surprisingly, many genes that are adaptable to mammoths are linked to living in cold environments. Some of these genes are shared by unrelated modern Arctic mammals. Díez-del-Molino said: "We found some highly evolved genes involved in fat metabolism and storage, which are also found in other Arctic species such as reindeer and polar bears, which means that there may be a convergent evolution of these genes in mammals adapted to the cold." "
While previous studies have examined the genomes of one or two mammoths, this is the first time a large number of mammoth genomes have been compared. This large sample size allowed the team to identify genes that are common in all mammoths and therefore may be adaptive, rather than genetic mutations that may occur only in a single individual.
Díez-del-Molino said: "We found that some of the particular genes that were previously thought to be mammoths are actually variable between mammoths, which means they may not be as important. "
This is a photo of study co-author Marianne Dehasque working in the ancient DNA lab at the Centre for Paleogenetics in Stockholm.
Overall, the 700,000-year-old Chukochya genome shared about 91.7 percent of the mutations that caused changes in the protein coding of more modern mammoths. This means that when mammoths first diverged from their ancestors, steppe mammoths, many of the mammoth's defining traits – including thick fur, fat metabolism and cold perception – may have already existed.
However, these characteristics were further developed in the descendants of Chukchia. "The earliest mammoths hadn't fully evolved," Dalén said, "and they probably had larger ears, and their fur was different—perhaps less insulated and fluffy than later mammoths." "
More modern mammoths also have several immune mutations in T cell antigens that were not seen in their ancestors. The authors speculate that these mutations may confer enhanced cell-mediated immunity against emerging viral pathogens.
Working with ancient mammoth DNA comes with a series of obstacles. Díez-del-Molino said: "From field work to laboratory work to bioinformatics, there are some difficulties at every step. "
All of the mammoths whose genomes were included in the study were collected in Siberia, but the researchers hope to branch out and compare mammoths from North America in the future. "We showed a few years ago that there was gene flow between the ancestors of woolly mammoths and Colombian mammoths, so that's something we need to consider as well, because North American woolly mammoths may also carry genes from non-woolly mammoths," Dalén said. "