A 52,000-year-old woolly mammoth has had its entire genetic code mapped in a move it's hoped will further efforts to bring the giant ancient beasts back to life.
It is the first time such a feat has been achieved for any ancient fossilised remains.
It's hoped the same strategies could also be used on other ancient DNA samples, such as Egyptian mummies, in the future.
While many ancient DNA samples of fossilised ancient animals and people exist, most specimens consist of very small, scrambled DNA fragments.
However, this particular woolly mammoth discovered in Siberia happened to be freeze-dried by the weather soon after it died, preserving its DNA in a glass-like state.
The fossilised chromosomes extracted from the mammoth are around one million times longer than most ancient DNA fragments.
A team of US and Italian researchers set out to create a 3D map of these chromosomes in the same way scientists reconstruct the human genome.
"This is a new type of fossil, and its scale dwarfs that of individual ancient DNA fragments - a million times more sequence," study co-author Erez Lieberman Aiden, director of the Centre for Genome Architecture at Baylor College of Medicine, said.
Aiden thought that if the right ancient DNA sample could be found - one with the 3D organisation of the fragments still intact - it would be possible to use the same strategies as used on modern-day humans to assemble the ancient genomes.
The researchers tested dozens of DNA samples over the course of five years, before landing on an unusually well-preserved woolly mammoth excavated in northeastern Siberia in 2018.
The researchers extracted DNA from a skin sample taken behind the mammoth's ear.
Using a technique called Hi-C, they were able to detect which sections of DNA were likely to be located close together and to interact with one another in their natural state in the nucleus.
"Imagine you have a puzzle that has three billion pieces, but you don't have the picture of the final puzzle to work from," co-author Marc A. Marti-Renom, a structural genomicist at the Centre Nacional d'Anàlisi Genòmica in Barcelona, said.
"Hi-C allows you to have an approximation of that picture before you start putting the puzzle pieces together."
They were then able to use the genomes of modern-day elephants as a template to assemble that of the ancient species, which roamed the Earth's northern hemisphere for at least half a million years.
The analysis revealed that woolly mammoths had 28 chromosomes - the same number as present-day Asian and African elephants.
They also found the woolly mammoth's skin cells had distinctive gene activation patterns compared to those of its closest relative, the Asian elephant, including for genes potentially related to its woolly-ness and cold tolerance.
The scientists say that the next step for the woolly mammoth is to examine the epigenetic patterns of tissues other than the skin.
"These results have obvious consequences for contemporary efforts aimed at woolly mammoth de-extinction," M. Thomas Gilbert, a paleo-genomicist at the University of Copenhagen, said.
