In an achievement right at the limits of our scientific capabilities, an international team of geneticists 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, the researchers extracted extremely degraded DNA and put it back together to reveal a previously unknown genetic mammoth line.
Previously, the oldest DNA sample found was from a horse bone found in the Yukon permafrost, dating back to between 560,000 and 780,000 years ago.
“This DNA is incredibly old,” said evolutionary geneticist Love Dalén of the Center for Paleogenetics in Sweden. “The monsters are a thousand times older than the Viking remains, and even predate the existence of humans and Neanderthals.”
About a million years ago, even woolly mammoths (Mammuthus primigenius) did not yet exist. The known and beloved beasts didn’t start appearing until about 800,000 years ago, lived in Earth’s frozen climates until they eventually became extinct about 4,000 years ago.
Because this is relatively recent, in geologic time, and because they preferred cold habitats (which preserve remnants better), we know quite a bit about these ancient creatures.
Woolly mammoths coexisted with Colombian mammoths (M. columbi), which lived in North America and became extinct about 11,500 years ago.
Their predecessors, the mammoths from which woolly mammoths evolved, are less known. We know woolly mammoths must descend from steppe mammoths (M. trogontherii), which roamed most of Eurasia until about 200,000 years ago. We also thought that Colombian mammoths descended from steppe mammoths that crossed to North America about 1.5 million years ago.
In an effort to learn more about this ancestor, the scientists turn the mammoth genealogy upside down.
The three mammoth teeth from which they extracted DNA were excavated decades ago and carefully preserved in a museum collection. The youngest, 700,000 years old, belonged to a woolly mammoth – one of the earliest known. The older two, which were over 1 million years old, were expected to belong to the steppe mammoth.
Meticulous restoration and comparative efforts allowed the researchers to assemble and sequence the DNA stored in the hard enamel of the animals’ teeth. The second oldest of the three, found in Adycha, conveyed this: it appeared very close to the steppe mammoth in morphology and DNA.
The oldest specimen, found in Krestovka and dating from about 1.6 million years ago, was more surprising. It turned out to belong to a previously unknown genetic mammoth line that diverged from a common ancestor more than 2 million years ago.
“This came as a complete surprise to us,” said geneticist Tom van der Valk from Uppsala University in Sweden.
“All previous studies have shown that there was only one mammoth species in Siberia at the time, called the steppe mammoth. But our DNA analyzes now show that there were two distinct genetic lineages, which we here call the Adycha mammoth. the Krestovka mammoth. We can’t say for sure yet, but we think these two represent different species. “
It gets even more interesting. By comparing the DNA of these ancient mammoths with those that came later, the researchers found that it could be the Krestovka mammoth that crossed the Bering Land Bridge to North America 1.5 million years ago, not the steppe mammoth.
The Colombian mammoth’s DNA contains a mixture of Krestovka and woolly mammoth, suggesting that the two were breeding when woolly mammoths migrated to North America and produced a hybrid.
“This is an important discovery,” said paleogeneticist Patrícia Pečnerová of the University of Copenhagen in Denmark. “It appears that the Colombian mammoth, one of North America’s most iconic Ice Age species, evolved through a hybridization that took place about 420 thousand years ago.”
The Adycha mammoth, while more in line with expectations, also had some secrets to reveal. By comparing its genome with that of woolly mammoths from 700,000 to a few thousand years ago, the team sought to understand how the woolly mammoth adapted to a frozen Arctic environment.
The traits associated with that adaptation – genes related to thermoregulation, hair growth, circadian rhythm, and white and brown fat deposits – were already present in the Adycha genome long before the woolly mammoth emerged. But the animals also continued to evolve; the gene involved in measuring temperature, for example, had more variants in later woolly mammoths.
The team’s techniques will not work for all remains. The cold temperature of the permafrost slows down DNA degradation, so age-matched remnants from other locations would likely have degraded too much; and within permafrost, there is a limit to how far back that DNA can be repaired.
“One of the big questions now is how far back in time we can go. We haven’t reached the limit yet,” says molecular archaeologist Anders Götherström from the Center for Paleogenetics.
“An educated guess would be that we could recover DNA that is 2 million years old, and possibly as much as 2.6 million years ago. Before that, there was no permafrost where ancient DNA could be preserved.”
Many preserved creatures have been excavated from Earth’s permafrost. The research reveals some remarkable discoveries lurking in bones previously considered too old to attempt to study.
The research is published in Nature