The oldest known RNA, a molecule essential to the majority of biological processes, was found in a woolly mammoth that lived in Siberia approximately 39,000 years ago. This discovery demonstrates that RNA can survive longer than previously thought and opens up new avenues for research into extinct organisms.
The muscle tissue in the left front leg of a young male mammoth, possibly aged five to ten, whose carcass was found in 2010 in the Siberian permafrost in the Russian Far East along the Oyogos Yar coast bordering the Laptev Sea, contained the RNA that was successfully isolated and sequenced.
One of the best-preserved frozen carcasses of this extinct species is the mammoth, known as Yuka. Among other things, its RNA showed which genes had been "turned on" in Yuka's tissue at the time of death, indicating cell stress.
Skeletal fossils provide the majority of our knowledge about prehistoric organisms, but their biological information is limited. However, new insights into these organisms have been made possible by the increasing capacity to extract distinctive biomolecules from ancient remains.
The new RNA discovery supports recent developments in the study of ancient proteins, which construct and operate a large portion of a cell's machinery, and ancient DNA, which contains the genetic instructions for all living things.
The signals that an organism's genome sends to its cellular machinery are known as ribonucleic acid, or RNA. These signals tell cells which genes to activate or deactivate, how and when to regulate their function, and which proteins to produce.
Emilio Mármol, a geneticist, veterinarian and bioinformatician at the University of Copenhagen's Globe Institute, lead author of the study published on Friday, November 14, in the journal Cell, said, "With RNA, you can access the actual biology of the cell or tissue happening in real time within the last moments of life of the organism."
"This gives us direct access to the functional landscape of the cell metabolism of woolly mammoths when they were alive, something that is not possible - at least not in the lengths we report - just by using DNA or proteins. Adding this layer of information provides a more comprehensive view to the biology of woolly mammoths," Mármol added.
RNA is more brittle than proteins and DNA. The earliest DNA found to date came from microorganisms, plants, and animals that lived in Greenland sediment around two million years ago. The dental remains of a hornless rhinoceros that lived in the Canadian High Arctic approximately 23 million years ago contained the oldest proteins.
The oldest RNA ever found was from a wolf cub that lived in Siberia some 14,000 years ago.
"I think the key finding is that we can recover RNA from such an old sample. It's a proof of principle that opens the door for much more detailed analyses of gene expression patterns in Ice Age megafauna," said geneticist and study co-author Love Dalén of Stockholm University and the Centre for Palaeogenetics in Sweden.
Before it died, Yuka suffered trauma; deep cuts on its hide showed that it had been attacked by a cave lion, an extinct species that was a larger, cold-adapted relative of the modern African lion.
"None of the marks are severe enough to have caused the death, so it is still a bit unclear why Yuka died," Dalén said.
In Yuka, the researchers found RNA molecules that code for proteins involved in metabolic regulation and muscle contraction under stress, which may or may not have been caused by this attack.
Researchers previously believed that RNA degraded only minutes or hours after death, but this study and others demonstrated that it can live for a much longer period of time in the correct conditions. Siberia's cold climate was especially favorable for RNA preservation.
These researchers and others have previously sequenced Yuka's DNA. Yuka, who was previously believed to be female, was actually male, with an X and a Y chromosome, according to the genetic data. Yuka stood approximately 5-1/4 feet (1.6 meters) at the shoulder.
The researchers noted that ancient RNA offers many opportunities and believe that RNA can be recovered from even older remains under the correct circumstances.
"We hope our work elicits a renewed interest in exploring RNA in other old remains, not necessarily from the Ice Age or extinct species, but also from medieval or historical remains of both extinct and extant organisms," Mármol concluded.
