It's not quite Jurassic Park, but scientists have successfully extracted and reconstructed million-year-old DNA from mammoth teeth — and solved a couple of mysteries about the creatures' evolutionary history along the way.
- Researchers extracted, sequenced and decoded DNA from three mammoth teeth
- They calculated the ages of the teeth to 1.65 million, 1.34 million and 870,000 years, making it the oldest DNA sequenced so far
- Genetic data showed evidence of hybridisation and a potential new species
The study, led by a Swedish team, smashed the record for the oldest DNA so far sequenced. The previous record-holder was DNA from a horse that trotted around Canada around 700,000 years ago.
Reporting in the journal Nature, the team also suspects the genes that allowed woolly mammoths to live in frigid conditions — and gave them long hair and deposits of fat — were already present in their ancestors, said Tom van der Valk, a palaeogeneticist at Stockholm's Uppsala University and a co-author of the paper.
To reach these conclusions, Dr van der Valk and his colleagues took three mammoth teeth, which were discovered in Siberia in the 1970s, drilled out a tiny amount from within each — around a pinch — and analysed the powder.
But it's the sheer age of the mammoth DNA that caught the eye of Morten Allentoft, an evolutionary biologist at Curtin University who works with ancient DNA, but who was not involved with the study.
In 2012, he and colleagues published a paper which suggested DNA could be reconstructed even after a million years.
"Then for a number of years, we talked about this one-million-year mark as sort of the 'magic barrier' that no-one had reached," Professor Allentoft said.
"So this [paper] is a fantastic result.
"Because when you have million-year-old DNA, you can truly understand evolution."
Jigsaw puzzle made of a billion tiny pieces
So how exactly do you extract DNA from a piece of an animal that died more than a million years ago?
It's only in recent years that technological advances have made it possible. That's because DNA has a habit of falling to bits, even if it's locked away in a tooth buried in the frozen soil of Siberian permafrost.
Microbes munch on it, and enzymes snip it up. Ultraviolet radiation and oxygen fragment it further.
So instead of having a long single DNA string, you end up with heaps of short scraps.
The job for Dr van der Valk and his crew, then, was to take those fragments and, using software, reconstruct them, as much as possible, to the original long string.
So how did they know which section went where?
Love Dalén, who was also part of the study, likens the process to putting together a jigsaw puzzle made of billions of tiny pieces.
It helps to have the picture on the top of the puzzle box "to sneak peek at", Professor Dalén said.
That picture, in this case, came from elephants. These beasts share huge stretches of their genome with mammoths, which can be used as a template.
"And that is exactly how we use the reference genome from the elephant … it is the cover of the box," Professor Dalén said.
"So we can have a look at that and have an idea of what we expect."
Validating the age of the teeth
Getting the DNA and reconstructing the genome as best they could was just one part of the study. The researchers also needed to validate the age of the teeth.
The first technique they used was a "molecular clock", which relies on the assumption that genetic mutations occur at a consistent rate over the aeons.
Of the DNA the researchers reconstructed, there were two kinds. There was DNA found in the nucleus, which holds the genetic blueprint of the animal, and DNA from mitochondria, which are minute structures that generate energy.
Mitochondrial DNA is more stable than nuclear DNA — it's in a ring, not a long strand — and there are more copies of it in each cell, so it's generally easier to work with.
While the researchers couldn't reconstruct an entire nuclear genome from each tooth, they did manage to piece together complete sets of mitochondrial DNA.
By counting the differences between the mammoth and elephant mitochondrial genomes, they calculated the age of the three teeth to be around 1.6 million, 1.34 million and 870,000 years old.
These ages were backed up by other dating methods which analysed not the teeth themselves, but the layers of rock around them.
For instance, fossils of rodents which lived during certain periods can pin a date on mammoth teeth in those same rocky layers.
New member of the mammoth family tree?
Once they had the genetic data, the researchers looked at where it fit in the mammoth family tree.
They realised that the oldest specimen, dubbed 'Krestova' after where it was found, was distinctly genetically different to the two younger specimens.
So much so that it could even be considered its own species, said Bastien Llamas, an evolutionary biologist at the University of Adelaide who, like Professor Allentoft, also works with ancient DNA but was not involved in the study.
"It's basically a brand new genetic lineage that they have identified."
The researchers also solved the mystery of how the Columbian mammoth, which lived in North America from around half a million to 11,000 years ago, came to be.
The new genetic data showed the Columbian mammoth was a hybrid, comprising half woolly mammoth and half Krestova.
Dr van der Valk said that now the team has cracked a million years, they're keen to push that limit further, "but how much longer is, of course, just a pure guess on our part".
While permafrost is the most promising place to search for super-old DNA, the oldest permafrost in the northern hemisphere is 2.5 million years old, "so beyond that is likely to be very difficult, if not impossible".