For the first time, preserved DNA has been recovered from animal remains over a million years old. The DNA belonged to two mammoths that lived around 1.2 million years ago.
The genetic sequences change our understanding of mammoth evolution. They reveal that, at that time, Siberia was home to two distinct groups of these animals. The mammoths of North America were the product of a hybridisation event between these two groups, and obtained half of their DNA from each.
“Instead of there being one species [or lineage] of mammoth up in Siberia around 1-2 million years ago, it now looks like there are two,” says Love Dalén at the Centre for Palaeogenetics in Stockholm, Sweden.
The first mammoths evolved in Africa about 5 million years ago. “It was originally a tropical species,” says Dalén. But over the next few million years, some mammoths moved out of Africa.
A key species was the steppe mammoth (Mammuthus trogontherii), which evolved in northern Eurasia about 1.7 million years ago. Later, North America was home to Columbian mammoths (Mammuthus columbi). The famous woolly mammoths (Mammuthus primigenius) lived in Eurasia more recently, with the last ones dying out just 4000 years ago.
Quite how these species are related, and why they evolved in the ways they did, are tricky questions to answer.
Dalén’s colleague Patrícia Pečnerová, now at the University of Copenhagen in Denmark, extracted DNA from three mammoth teeth found in north-east Siberia. They were collected in the 1970s by the late Russian palaeontologist Andrei Sher. Two of the teeth, from Krestovka and Adycha, look like they belonged to steppe mammoths and are respectively 1.1-1.2 and 1-1.2 million years old. The third, from Chukochya, seems to be a woolly mammoth and is 500-800,000 years old.
The older teeth are the first specimens greater than a million years old to have their DNA read. That is far older than the previous record for ancient DNA, a 700,000-year-old horse, although it has proved possible to obtain protein sequences from even older remains, including a 1.9-million-year-old Homo erectus tooth.
“This looks supercool,” says Rebekah Rogers at the University of North Carolina at Charlotte. “It’s bringing together palaeontology and genetics on a deeper timescale than ever before.”
The team found that the Adycha and Chukochya teeth both looked like ancestors of the later woolly mammoths. But the Krestovka tooth was a surprise. Despite being about the same age as the Adycha one, its DNA was quite different. The mammoth it belonged to was a member of a separate lineage that branched off from the other Eurasian mammoths at least 1.78-2.66 million years ago.
Dalén’s team believes that the Krestovka mammoths were the ones that first colonised North America, crossing a land bridge to what is now Alaska perhaps 1.5 million years ago. “According to our model, the mammoths in North America between 1.5 and 0.5 million years ago were exclusively this Krestovka type,” says Dalén.
But the later Columbian mammoths of North America, whose DNA had previously been sequenced, weren’t simply the descendants of the Krestovka mammoths. Instead, the story took a twist. In Siberia, the steppe mammoths gradually gave rise to woolly mammoths. Much later, “a small band of woolly mammoths crossed the Bering land bridge and entered North America, and there they hybridised with the rest of the Krestovka mammoths”, says Dalén.
The result was Columbian mammoths, which were a 50:50 mix of Krestovka and woolly mammoth ancestry.
Such interbreeding seems to be common in the origin of new species, says co-author Tom van der Valk at Uppsala University in Sweden. “This is one of the major things that has changed in the past decade when we talk about speciation. It seems more and more, whenever we look, we do find gene flow between lineages that have been separate for some time and then intermixed again.”
“What this piece of work is showing is that biology is messy,” says Tori Herridge at the Natural History Museum in London. The formation of a species is rarely as simple as a population splitting in two, she says.
The analyses also shed new light on the evolution of woolly mammoths.
Previously, it was assumed that these animals evolved thick coats and other adaptations in response to the cold climate of the ice age. “It’s always seen as the epitome of cold-adapted mammoths,” says Herridge.
But the DNA shows that many of the woolly mammoth’s adaptations to the cold were already present in the earlier steppe mammoth populations. “It has been more of a gradual process, slowly getting better at this environment, rather than one rapid, single burst of adaptation,” says van der Valk.
This means we may need a new explanation for the evolution of some woolly mammoth traits, says Herridge. “Maybe the cold adaptation is not the main driver.”
Journal reference: Nature, DOI: 10.1038/s41586-021-03224-9
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