Spanish scientists have discovered the first fossil mammoth chromosomes in a 52,000-year-old specimen.

We could start by saying that we are close to reviving the woolly mammoths, and there will probably be newspapers that will write headlines about it. However, a study just published in the journal cell It tells us a lot about what is already discovered, not about milkmaids’ tales of millionaires and Jurassic Park fans. And while we already had fragments of woolly mammoth DNA, what we didn’t have were their chromosomes, the three-dimensional shapes that these long strands of DNA take on when they pack up to take up less space.

The credit goes to an international team that includes researchers from the United States, Denmark and our country, in particular from the National Centre for Genomic Analysis (CNAG) and the Centre for Genomic Regulation, both in Barcelona. And this milestone is even more significant because these are the first chromosomes found in a long-extinct species. A necessary slogan because, although many media outlets portray it as the first chromosomes discovered in an extinct species, that is not entirely true.

Some species have become extinct only recently, and their tissues have been preserved in formalin. The marsupial tiger, or thylacine, is one of the most striking examples.. In any case, if we are talking about species that went extinct before we had the technology to preserve them, yes, we are dealing with the first chromosomes discovered. It’s hard to formulate a sentence that limits a milestone to extinct species we haven’t saved, which is perhaps why most media outlets miss these details. However, none of this overshadows the great achievements made by these researchers.

A small detail compared to the huge advance this discovery represents for paleogenetics. Dr. Juan Antonio Rodríguez, the study’s lead author and researcher at the National Center for Genomic Analysis in Barcelona, ​​explains that they found, in particular, “(…) 28 pairs of chromosomes, which makes a lot of sense because that’s what modern elephants have and they are the closest living relatives of the woolly mammoth. (…) It was extremely exciting (…). “It’s not usually that much fun just counting from one to twenty-eight.”

Every now and then, the press reports on a private company that promises to bring back mammoths using DNA preserved in their bodies, frozen under the Siberian ice. You may have read this news, and if so, you may be wondering why this discovery is so important. Well, the truth is that while we already had mammoth DNA, they were short fragments and, so to speak, decontextualized, as if they were puzzle pieces, most of which were lost. We don’t know where they went, what their relationship was to the rest of the DNA.

Indeed, some of this prehistoric DNA could be activated by inserting it into elephant skin cells, but its incompleteness remained a major limiting factor in understanding woolly mammoth genetics. What this team has achieved is complete chromosomes.“We found a sample in which the three-dimensional arrangement of these DNA fragments was frozen, remaining intact for tens of millennia,” 52,000 years, says co-author Dr. Marcela Sandoval-Velasco of the Center for Evolutionary Research Hologenomics at the University of Copenhagen.

And that’s not all, because by finding 28 complete chromosomes, experts were able to analyze which parts were particularly expressed. Because although all the cells in our body have the same DNA, not all of them express all the “information” that their genetic material contains, so some become brain cells, others heart cells, and so on. In this case, the genetic material was so preserved that experts were able to see a phenomenon known as chromosomal compartmentalization, in which active and inactive DNA are separated within the cell nucleus, remaining adjacent but distinguishable.

This allowed the researchers to verify that most of the genes that were active in their mammoth skin were the same as those that are typically active in modern elephant skin. Moreover, the key exception made sense: “It turns out that there are key genes regulating hair follicle development whose pattern of activity is completely different from that of elephants.”clarified co-author Dr. Thomas Gilbert, director of the Center for Hologenomics. Or, in other words: the expression of this gene in the mammoth, which is inactive in elephants, explains why some have hair, while others are practically bald.

And that’s not all, because according to co-author Marc A. Marty-Renom, head of the structural genomics group at the National Center for Genome Analysis and also at the Center for Genomic Regulation: “The conservation of the loops in these ancient chromosomes is perhaps the most impressive part (…) they are only 50 nanometres in size and are important because they bring activating DNA sequences closer to their genetic targets. “So these fossils not only tell us which genes were active, but also why.”

What’s no less interesting than the chromosomes themselves is that they’ve survived. Normally, DNA has a half-life of 521 years. That means that after that time, half of your “information” will be gone. How is it possible that this one has survived so well, for so long? The truth is that we have much older DNA, 2 million years old, but here, despite the time and harsh conditions that our mammoth endured, not only a good part of the “information” has been preserved, but also its structure. We could compare this to a filing system full of documents. It would be lucky if, after 52,000 years of floods, hurricanes and earthquakes, we could find most of the documents legible, but it would be even more amazing to find them in their proper folders, perfectly organized.

The key appears to be the freezing and drying process their tissues have been subjected to over all these millennia, something similar to what happens to the jerks, what we know as the “glass transition,” which makes the tissues more fragile but allows them to better preserve the structure of their chromosomes. In fact, experiments have tested this by mistreating beef jerky.

“We shot him with a shotgun. We drove over it. The former Houston Astros pitcher threw him a fastball. Each time, the dried meat shattered into tiny pieces, breaking like glass. But at the nanoscale, the chromosomes remained intact, unaltered,” said Dr. Cynthia Perez Estrada, a co-author of the study and a research scientist at Rice University’s Center for Genome Architecture and Center for Theoretical Biological Physics.

So, without a doubt, this discovery is a major step forward in paleogenetics and a wonderful opportunity to learn about the extinct woolly mammoths and, in general, about the history of life on Earth, of which we are just one small speck on the road.

UNKNOWN:

• We don’t necessarily have to be aiming to bring a woolly mammoth back to life to be interested in studying its DNA in detail. De-extinction is a complex ethical issue that we shouldn’t trivialize, but knowing more about the genomes of other species can open doors we didn’t even know were closed. It helps us ask ourselves the right questions and, hopefully, answer some of them from time to time. Research like this improves our understanding of genetics in general and the conservation of living tissue.

REFERENCES (MLA):

• Olga Dudchenko et al.: “Three-dimensional genome architecture is preserved in a 52,000-year-old woolly mammoth skin sample” https://doi.org/10.1016/j.cell.2024.06.002.