(CN) — The world record for sequencing ancient DNA has been shattered thanks to a trio of mammoth molars, shedding new light on the evolution of these ice-age icons.
A team of researchers from the Centre for Palaeogenetics in Stockholm successfully sequenced 1.2-million-year-old mammoth DNA recovered from Siberia, beating the previous record holders by a healthy margin. The DNA came from three mammoth molars frozen in permafrost and provides hope that even more ancient DNA can be recovered and sequenced in the future.
Million-year-old DNA had never been sequenced before this; the still-respectable previous record holder came from a 700,000-year-old Canadian horse. DNA degrades over time, and the older the DNA the more difficult it is to sequence. Fortunately, permafrost acts as a sort of time capsule.
"This DNA is incredibly old. The samples are a thousand times older than Viking remains, and even pre-date the existence of humans and Neanderthals", said senior author Love Dalén, a professor of evolutionary genetics at the Centre for Palaeogenetics, in a statement accompanying the study.
The teeth originally came from three related yet distinct types of mammoth, which researchers describe in a study published Wednesday in the journal Nature. Their analyses reveal the Columbian mammoths which roamed North America until around 10,000 years ago were actually a hybrid of the iconic wooly mammoth and another previously unknown lineage.
The authors provide new details about the evolution of these enormous creatures and how they were able to adapt to such blistering cold where other species failed. Wooly and Columbian mammoths didn’t even exist a million years ago — both evolved much later from a common ancestor known as the steppe mammoth. Still, a number of key details remain missing from the mammoth family tree, and these million-year-old teeth provide a few new clues.
Two of the molars were found to be over a million years old, while the third clocked in at a paltry 700,000. Scientists determined the age of the samples through the molecular clock, which estimates the average rate a species accumulates genetic mutations, and compared the result with geological data to come up with the correct ages.
The oldest specimen, a previously unknown variety known as the Krestovka mammoth due to its location in Siberia, was found to be 1.2 million years old and split genetically from its forebearers more than 2 million years ago.
"This came as a complete surprise to us. All previous studies have indicated that there was only one species of mammoth in Siberia at that point in time, called the steppe mammoth,” said lead author Tom van der Valk. “But our DNA analyses now show that there were two different genetic lineages, which we here refer to as the Adycha mammoth and the Krestovka mammoth. We can't say for sure yet, but we think these may represent two different species.”
The authors believe the Krestovka mammoths were the first to walk to North America around 1.5 million years ago, later breeding with wooly mammoths to form the Columbian mammoths found around the continent. They think this hybridization occurred about 420,000 years ago, when North America was still covered in ice.
One million years ago marked a pivotal time in mammalian evolution — Earth’s magnetic poles flipped during that time period, and climate change was dramatically reshaping the planet. Scientists believe studying DNA from this period could reveal the answers to other important evolutionary questions, and since frozen specimens twice that age exist there remains quite a bit to explore.
"One of the big questions now is how far back in time we can go,” wondered co-author Anders Götherström, a professor and joint research leader at the Centre for Palaeogenetics. “We haven't reached the limit yet. An educated guess would be that we could recover DNA that is 2 million years old, and possibly go even as far back as 2.6 million. Before that, there was no permafrost where ancient DNA could have been preserved.”
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