A team of Swedish scientists has successfully extracted and reconstructed the world's oldest DNA from the tooth of a Siberian mammoth, which roamed Earth over a million years ago. Also known as deoxyribonucleic acid, the all-important molecule — which contains the genetic instructions for the development and function of living things. — provides new insights into the evolution of the ancient Ice Age giants. Prior to this, the oldest DNA sequenced came from the bone of a horse that trotted around Canada about 700,000 years ago.
The path leading to the momentous feat began in 2017, when the scientists led by Dr. Tom van der Valk, a postdoctoral researcher at the Centre for Palaeogenetics in Stockholm, received three woolly mammoth teeth for analysis. The samples belonged to three ancient individuals whose remains had been discovered in northeastern Siberia by the late Russian paleontologist Andrei Sher in the 1970s.
The researchers carefully drilled out a tiny amount from each tooth — around a pinch — and extracted DNA from the powder. While that was simple enough, analyzing DNA from fossilized remains, especially ones so old, is a complicated task. As time passes, the genetic material within even the most well-preserved remains breaks down and degenerates. Hence, instead of obtaining a long, single DNA string, researchers end up with a pile of short fragments. The scientists then have to reconstruct the pieces to resemble the original DNA the best they can.
Study co-author and evolutionary geneticist Love Dalén likens the process to that of solving a jigsaw puzzle with billions of tiny pieces. "It helps to have the picture on the top of the puzzle box to sneak peek at," he says. The template, in this case, was obtained from elephants, which share vast stretches of their genome with mammoths. "And that is exactly how we use the reference genome from the elephant … it is the cover of the box," Dalén explains.
Their research showed that the youngest tooth dated back around 700,000 years, while the oldest was an astonishing 1.2 million years old. The fossils' ages were verified by dating the layers of rock around the areas they were unearthed. "This is by a wide margin the oldest DNA ever recovered," said Dalén.
Once the ages had been determined, the researchers tried to ascertain to which mammoth species each tooth belonged. The youngest individual, dubbed Chukochya, was clearly a woolly mammoth (Mammuthus primigenius). The second-oldest — a 1.1 million-year-old specimen they named Adycha, was a close cousin — the steppe mammoth (Mammuthus trogontherii), which was native to the Siberian region. However, tracing the lineage of the 1.2 million-year-old mammoth that the scientists called Krestovka proved to be much more difficult. The DNA samples seemed to indicate a relation to both the Columbian mammoth — which roamed the temperate regions of southern and central North America around half a million to 11,000 years ago — and the woolly mammoth, which preferred the cold Arctic.
After further genetic analysis and hours of discussion, the researchers realized they had stumbled upon a new lineage of steppe mammoths — one that most likely explained the origin of the Columbian mammoth. They believe that the woolly mammoth mated with the Krestovka mammoth about 420,000 years ago, leading to the hybrid Columbian mammoth. Identifying the new species helps bridge the gap between the Siberian steppe mammoths, which lived in the cold tundra, and the much larger Columbian mammoths that preferred temperate weather.
The researchers, who published their findings in the journal Nature on February 17, 2021, now hope to search the permafrost for even older DNA evidence. However, since the oldest permafrost only dates back to the Early Pleistocene age – around 2.6 million years ago — those hoping they discover dinosaur DNA will be disappointed.
Resources: theconversation.com, abcnet.au.com