An international team of scientists has sequenced and analyzed DNA from three mammoth specimens, two of which are more than one million years old. The results show that two distinct mammoth lineages were present in eastern Siberia during the Early Pleistocene; one of these lineages gave rise to the woolly mammoth (Mammuthus primigenius) and the other represents a previously unrecognized lineage that was ancestral to the first mammoths to colonize North America; the Columbian mammoth (Mammuthus columbi) traces its ancestry to a Middle Pleistocene hybridization between these two lineages.
Life reconstruction of the steppe mammoth (Mammuthus trogontherii). Image credit: Beth Zaiken / Centre for Palaeogenetics.
Mammoths appeared in Africa approximately 5 million years ago, and subsequently colonized much of the northern hemisphere.
During the Pleistocene epoch (2.6 million to 11,700 years ago), the mammoth lineage underwent evolutionary changes that produced the southern mammoth (Mammuthus meridionalis) and the steppe mammoth (Mammuthus trogontherii), which later gave rise to Columbian and woolly mammoths.
Although the exact relationships among these species are uncertain, the prevailing view is that the Columbian mammoth evolved during an early colonization of North America about 1.5 million years ago and that the woolly mammoth first appeared in northeastern Siberia about 700,000 years ago.
Mammoths similar to the steppe mammoth inhabited Eurasia from at least around 1.7 million years ago; the last populations went extinct in Europe about 200,000 years ago.
To investigate the origin and evolution of woolly and Columbian mammoths, Professor Love Dalén and colleagues recovered genomic data from three mammoth molars (dubbed Chukochya, Krestovka and Adycha) from northeastern Siberia that date to 700,000-1.2 million years ago — the oldest genomic data recovered so far.
“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 Professor Dalén, a researcher at the Swedish Museum of Natural History, Stockholm University and the Centre for Palaeogenetics.
The results show that the 1.2-million-year-old Krestovka mammoth belonged to a previously unknown genetic lineage that diverged from other Siberian mammoths 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 Dr. Tom van der Valk, a researcher at Stockholm University, the Centre for Palaeogenetics and Uppsala University.
“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.”
Chukochya (a), Krestovka (b), Adycha (c) mammoth molars. Image credit: van der Valk et al., doi: 10.1038/s41586-021-03224-9.
The researchers suggest that mammoths from the Krestovka lineage colonized North America some 1.5 million years ago.
The results also show that the Columbian mammoth that inhabited North America during the last Ice Age was a hybrid. Roughly half of its genome came from the Krestovka lineage and the other half from the woolly mammoth.
“This is an important discovery. It appears that the Columbian mammoth, one of the most iconic Ice Age species of North America, evolved through a hybridization that took place approximately 420,000 years ago,” said Dr. Patrícia Pečnerová, a researcher at Stockholm University, the Centre for Palaeogenetics and the University of Copenhagen.
The results also indicate that gene variants associated with life in the Arctic, such as hair growth, thermoregulation, fat deposits, cold tolerance and circadian rhythms, were already present in the 1-million-year-old mammoth, long before the origin of the woolly mammoth.
Most adaptations in the mammoth lineage happened slowly and gradually over time.
“To be able to trace genetic changes across a speciation event is unique,” said Dr. David Díez-del-Molino, a researcher at the Swedish Museum of Natural History, Stockholm University and the Centre for Palaeogenetics.
“Our analyses show that most cold adaptations were present already in the ancestor of the woolly mammoth, and we find no evidence that natural selection was faster during the speciation process.”
The findings were published today in the journal Nature.
T. van der Valk et al. Million-year-old DNA sheds light on the genomic history of mammoths. Nature, published online February 17, 2021; doi: 10.1038/s41586-021-03224-9