An international team of scientists have discovered ancient fossil chromosomes preserved in a glass-like state in the skin of a 52,000 year old female woolly mammoth, revealing a treasure trove of genetic data on the colossal mammal, which some are working to bring back from extinction.
“We’ve known that tiny fragments of ancient DNA can survive for long periods of time,” said Dr. Marcela Sandoval-Velasco of the University of Copenhagen’s Center for Evolutionary Hologenomics, and co-first author of the new study detailing the discovery. “But what we found here is a sample where the three-dimensional arrangement of these DNA fragments was frozen in place for tens of millennia, thereby preserving the structure of the whole chromosome.”
Chromosomes are thread-like structures composed entirely from DNA that reside in the cells of all living things. Each one of these biological databanks contains a wealth of genetic information, which scientists can use to glean insights into the history and evolution of life on Earth. Under normal circumstances, the remains of dead creatures degrade with the passage of time, causing DNA to fragment. Because of this, most of the ancient animal DNA discovered to date has been incomplete, often being comprised of fewer than 100 base pairs, out of the billions that would once have made up the full sequence of the organism prior to breaking apart.
We could even reconstruct the mammoth chromosomes in 3D. All this worked spooky good. It seemed physically impossible (given the Stokes-Einstein-Sutherland relation) that such small molecules could possibly diffuse so slowly. We were pretty stuck on this problem. (10/18) pic.twitter.com/uMaB6H6UcU
— Juan A. Rodríguez (@jrotwitguez) July 11, 2024
These base pairs — or letters — are themselves made up from combinations of organic molecules called nucleotides, which come together to form the ‘steps’ in the ladder-like double helix structure of animal DNA. When taken in the context of the surrounding structure, these base combinations can be analyzed to reveal a wealth of genetic information on long extinct animals, assuming that enough of the DNA has survived to the present day.
The 52,000 year old skin at the heart of the new research published in the scientific journal Cell was taken from behind the ear of a mammoth, the remains of which was discovered in Northern Siberia back in 2018. An intensive analysis of the sample revealed the presence of complete fossil chromosomes in the ancient remains — each of which measured billionths of a meter in length — which had seemingly been frozen in a glass-like state for tens of thousands of years.
“This is a new type of fossil, and its scale dwarfs that of individual ancient DNA fragments — a million times more sequence,” explained Erez Lieberman Aiden, a corresponding author on the new study who serves as director of the Center for Genome Architecture at the Baylor College of Medicine. The researchers were able to use sophisticated tools to reconstruct the mammoth’s 3D genomic architecture, while revealing the existence of tiny ‘chromatin’ loops in the structures of the chromosomes that measured less than 50 nanometers in length.
“Fossil chromosomes are a game-changer, because knowing the shape of an organism’s chromosomes makes it possible to assemble the entire DNA sequence of extinct creatures,” explained co-first author of the study Dr. Olga Dudchenko, who is an assistant professor at the Baylor College of Medicine. “This enables the types of insights that would not have been possible before.”
The team quickly discovered that the mammoth had 28 chromosome pairs, which numbered the same as those found in the herbivore's closest living relative, the asian elephant. They were also able to identify a number of ‘active’ genes in the ancient beast that were inactive in its modern-day decendants, which may account for the mammoth’s woolly appearance.
Recent times have seen a number of research groups take an active interest in bringing the woolly mammoth back from extinction, and it seems that the wealth of genomic information provided by the fossil chromosome could significantly impact those efforts.
“These results have obvious consequences for contemporary efforts aimed at woolly mammoth de-extinction,” commented Dr. Thomas Gilbert, director of the Center for Hologenomics at the University of Copenhagen, and co-corresponding author of the new paper. Both Gilbert and fellow study author Aiden serve as advisors to the Dallas-based company Colossal Biosciences, which is currently working towards a number of ambitious, and controversial goals, including the preservation of endangered species, and the resurrection of the woolly mammoth, and the dodo.
But how is it that the mammoth skin at the heart of the new study was able to survive the passage of 52,000 years in such magnificent condition? Well, according to Dr. Dudchenko, the mammoth was likely spontaneously freeze dried very soon after its death in a process not dissimilar to that used to create beef jerky from animal carcasses. This effectively locked the chromosomes in a durable, glass-like state, known as ‘chromoglass, in which the ancient DNA had little room for movement, even on a molecular level.
“We confirmed this theory by doing experiments on old, freeze-dried beef jerky, which is much easier to find than woolly mammoth jerky,” explained Dr. Cynthia Pérez Estrada of Rice University’s Center for Theoretical Biological Physics, who served as co-first author of the Cell study. “We fired a shotgun at it. We ran over it with a car. We had a former starting pitcher for the Houston Astros throw a fastball at it.”
In each instance, the brittle material would shatter into tiny pieces. However subsequent analysis revealed that the chromosomes embedded in the jerky had survived unscathed. Moving forward, the scientists hope that similar fossil chromosomes could be discovered in subjects ranging from mammoths, to Egyptian mummies where similar dehydrating processes have taken place, which could lead to similar breakthroughs in understanding.
For more science news, why not read about a recent NASA supercomputer simulation that reveals what it would look like to fall into the event horizon of a supermassive black hole, or read up on the fascinating geological secret connecting a string of ancient egyptian pyramids.
Anthony is a freelance contributor covering science and video gaming news for IGN. He has over eight years experience of covering breaking developments in multiple scientific fields and absolutely no time for your shenanigans. Follow him on Twitter @BeardConGamer
