In a feat just to the limits of our scientific capabilities, an international team of geneticists has recovered and sequenced the oldest DNA to date.
From the teeth of three ancient mammoths that roamed Siberia between 700,000 and 1.2 million years ago, researchers extracted extremely degraded DNA and reconnected it to reveal a previously unknown genetic mammoth progeny.
Previously, the oldest recovered DNA sample came from a horse bone found in the Yukon permafrost, dating back to 560,000 to 780,000 years ago.
“This DNA is incredibly old,” said evolutionary geneticist Love Dalén of the Center for Paleogenetics in Sweden. “The samples are a thousand times older than the remains of the Vikings and even precede the existence of humans and Neanderthals.”
About a million years ago, even woolly mammoths (Mammuthus primigenius) does not yet exist. The well-known and beloved beasts did not begin to appear until about 800,000 years ago, living in the frozen climates of the Earth until they disappeared about 4,000 years ago.
Because this is relatively recent, in geological time, and because they preferred cold habitats (which better preserve the remains), we know little about these ancient creatures.
Wool mammoths existed alongside Colombian mammoths (M. columbi), which inhabited North America and disappeared about 11,500 years ago.
Their predecessors, the mammoths from which the wool mammoths evolved, are less known. We know that wool mammoths come from steppe mammoths (M. trogontherii), which roamed most of Eurasia until about 200,000 years ago. I also thought that Colombian mammoths came from steppe mammoths that passed into North America about 1.5 million years ago.
In an attempt to learn more about this ancestor, scientists have turned mammoth genealogy into a headache.
The three mammoth teeth from which they extracted DNA were dug decades ago and have been carefully preserved in a museum collection. The youngest, at 700,000 years old, belonged to a woolly mammoth – one of the oldest known. The older ones, over 1 million years old, were expected to belong to the steppe mammoth.
Through careful restoration and comparative efforts, the researchers were able to collaborate and sequence the DNA that had been preserved in the hard enamel of the animals’ teeth. The second oldest of the three specimens, found in Adycha, proved this: it was very close to the steppe mammal in morphology and DNA.
The oldest specimen, found in Krestovka and dating to about 1.6 million years ago, was more surprising. It turned out to belong to a previously unknown genetic mammoth line, which moved away from a common ancestor more than 2 million years ago.
“This was a complete surprise for us,” said geneticist Tom van der Valk of Uppsala University in Sweden.
“All previous studies have shown that there was only one mammoth species in Siberia at that time, called the steppe mammoth. But our DNA analysis now shows that there were two different genetic lines, which we refer to here as the mammoth Adycha and the mammoth Krestovka. We can’t say for sure yet, but we think they can be two different species. “
It gets even more interesting. Comparing the DNA of these ancient mammoths with those who came later, the researchers found that it could have been the Krestovka mammoth that crossed the Bering land bridge in North America 1.5 million years ago, not the steppe mammoth.
The DNA of the Colombian mammoth has a mixture of Krestovka and wool mammoth, suggesting that the two grew when wool mammoths migrated to North America, producing a hybrid.
“This is an important discovery,” said paleogeneticist Patrícia Pečnerová of the University of Copenhagen in Denmark. “It seems that the Colombian mammoth, one of the most iconic species of the ice age in North America, evolved through a hybridization that took place about 420 thousand years ago.”
The mammoth Adycha, although more in line with expectations, also had some secrets to reveal. By comparing its genome to that of woolly mammoths from 700,000 to several thousand years ago, the team tried to understand how the woolly mammoth became adapted to a frozen Arctic environment.
The traits associated with this adaptation — genes associated with thermoregulation, hair growth, circadian rhythm, and white and brown fat deposits — were already present in the Adycha genome long before the woolly mammoth appeared. But animals also continued to evolve; the gene involved in detecting temperature, for example, had several variants in wool mammoths later.
The team’s techniques will not work for all the remnants. The cold temperature of the permafrost slows down the degradation of DNA, so that remnants of a similar age from other locations would probably be too degraded; and, within permafrost, there is a limit to how much DNA is recoverable.
“One of the big questions now is how far back we can go. We haven’t reached the limit yet,” said molecular archaeologist Anders Götherström of the Center for Paleogenetics.
“An educated assumption would be that we could recover 2 million-year-old DNA and possibly even up to 2.6 million. Before that, there was no permafrost in which ancient DNA could have been preserved. “.
Lots of preserved creatures have been excavated from Earth’s permafrost. Research shows what remarkable discoveries might be hidden in bones previously considered too old to attempt to study.
The research was published in The nature.