The frozen ground in permafrost regions has proven once again to be an amazingly good time capsule: Researchers have successfully extracted and sequenced genetic material from mammoth remains over a million years old. Animal DNA is the oldest that has been successfully tested. Additionally, it also reveals news about the tribal history of Khartoum Ice Age. Because the oldest mammoth was not a woolly mammoth, but a Eurasian species previously unknown. North American mammoths could have emerged from this by crossing with the Siberian woolly mammoth.
Mammoths are the largest and most famous representatives of the fauna of the Ice Age. The hairy and cold-adapted Khartoum ancestors probably originated in Africa about five million years ago before spreading north and finally colonizing the entire northern hemisphere. About 2.5 million years ago, the southern mammoth (Mammuthus meridionalis), which still looked like an elephant, was developed first and then the steppe mammoth (Mammuthus trogontherii). According to popular theory, the steppe mammoth that migrated to North America via the Bering Strait became the prairie mammoth (Mammuthus columbi) about 1.5 million years ago, while the woolly mammoth (Mammuthus primigenius) spread into Siberia and northern Europe. These two species survived the longest Ice Age and were especially well adapted to the harsh climate of the Ice Age steppe and tundra thanks to their dense coat and tolerance to the cold.
DNA reconstructed for the first time over a million years ago
However, it is not yet clear how and when the wild mammoth appeared and how it relates to the woolly mammoth. To gain more clarity, Tom van der Valk of the Center for Paleogenetics in Stockholm and colleagues have now attempted to obtain primitive DNA from the fossilized teeth of three mammoths. These are found in permafrosts in Northeast Siberia. Because this permanently frozen ground prevents cell components and molecules from decomposing, there was hope for being able to isolate and sequence portions of genetic material from dental material. The oldest remains of the three mammoths, named “Krestovka” in relation to the place where they were found, are between 1.2 and 1.6 million years old and show characteristics of a steppe mammoth. The second, Adycha, was also similar to the steppe mammoth and was about 1.3 million years old. On the other hand, the third tusk, Chukochya, comes from a woolly-haired mammoth and is around 680,000 years old.
The researchers succeeded in isolating enough genetic material fragments from the three traces of near-complete mitochondrial DNA sequences of these three mammoths. This type of DNA does not belong to the genome of the cell nucleus, but is located in the mitochondria, the power stations of the cells. Since it is only passed on through the maternal line, this DNA is especially suitable for rebuilding family relationships. Extracting this DNA from fossils over a million years old is a first: the oldest DNA sequence to date was isolated in 2013 from a 700,000-year-old horse bone. The current study now shows that permafrost can also preserve ancient genetic material well enough. “One of the big questions is how far back in time we can go,” says Anders Guterstrom, a colleague of Van der Wock. “I think we can recover DNA that is two million years old, maybe even 2.6 million years ago.” Since there was no permafrost before, this is the limit.
The prairie mammoth was a hybrid
However, it is also interesting what a comparison of three mammoth genomes revealed about the evolution of Ice Age proboscis. Because it was revealed that Krestovka, the oldest mammoth, belonged to a previously unknown tribal line of mammoths. This must have separated from the streak of all other Siberian mammoths by at least two million years ago. “It was a complete surprise for us,” says Van der Wock. All previous studies indicated that there were only steppe mammoths in Siberia at that time. But our DNA analyzes reveal the existence of two different genotypes – perhaps two different species of mammoth. “According to genetic comparisons, it was the Krestovka mammoth, not the steppe mammoth that migrated from Siberia to North America about 1.5 million years ago. The first mammoths on this continent belonged to this newly discovered line – and from this line the wild mammoths later arose.
However, another representative was involved in creating a prairie mammoth, the researchers reported. Accordingly, about 40 percent of its genome comes from the Krestovka mammoth, while about 60 percent comes from the woolly mammoth. “This is an important discovery,” says co-author Patricia Beckenerova of the Swedish Museum of Natural History in Stockholm. “The prairie mammoth, a symbol of Ice Age animals in North America, appears to have evolved through crossbreeding that occurred about 420,000 years ago.” It appears that woolly mammoths from Siberia migrated to North America via the Bering Strait, where they were then moved with the Krestovka mammoths who already live there mating.
The genome comparison of the three mammoth fossils also revealed that, contrary to previous assumptions, even the oldest forms of mammoths were adapted to colder climates. Because the genetic variants that gave Khartoum its thick envelope, its insulating layers of fat, tolerate coldness and other adaptations to the arctic environment, were already present in Adycha, a mammoth fossil that still corresponds to the steppe mammoth species. “Our analyzes show that most of the cold adaptations were already present in the ancestors of the woolly mammoth,” says David Diez del Mulino, a colleague of Van der Walk.
Source: Tom van der Valk (Center for Palaeogenetics, Stockholm) et al., Nature, Doi: 10.1038 / s41586-021-03224-9
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