NEW YORK — Woolly mammoths likely died out after a shifting climate made the Arctic tundra where they lived wetter and led to a decline of the vegetation they ate, according to a new environmental DNA analysis.
An international team of researchers collected more than 500 sediment samples from different Arctic sites and sequenced the DNA fragments found within them. By comparing these snippets of DNA to publicly available plant and animal DNA databases, they teased out which plants and animals were present in different parts of the Arctic at the Last Glacial Maximum between 26,500 years and 19,000 years ago, during the Pleistocene about 12,000 years ago, and into the Holocene. As they reported in Nature on Wednesday, the woolly mammoth largely disappeared about 4,000 years ago at a time when climatic patterns shifted quickly and led the Arctic to become wetter.
“Scientists have argued for 100 years about why mammoths went extinct,” senior author Eske Willerslev from the University of Cambridge and Copenhagen University said in a statement, noting that humans had long been blamed for hunting them to extinction, as mammoths had withstood the ebb and flow of previous ice ages.
“We have finally been able to prove that it was not just the climate changing that was the problem, but the speed of it that was the final nail in the coffin — they were not able to adapt quickly enough when the landscape dramatically transformed and their food became scarce,” he added.
The researchers generated an environmental DNA metagenomic dataset based on 535 sediment samples collected at 74 different sites around the Arctic Circle in North America, Siberia, and the North Atlantic. The samples ranged in age and encompassed the past 50,000 years. In total, the dataset included 10.2 billion sequencing reads.
At the same time, they developed a reference database by combining NCBI-nt and NCBI-RefSeq with reference data for a dozen Arctic animals and more than 1,500 Holarctic plants.
By reconstructing the plant data, the researchers pieced together how vegetation in the Arctic changed over time and in response to climatic shifts. Overall, plant diversity increased from 50,000 years ago until the onset of the Last Glacial Maximum about 26,500 years ago when the climate became cold and dry. Herbaceous plants dominated until 19,000 years ago and vegetation turnover was high during this period of decline in diversity. After the LGM when warming began, the vegetation also changed, with an increase in woody plants.
At the same time, there were differences by region. For instance, the North Atlantic experienced a quick rise in temperature that was coupled with a sharp rise in taxonomic richness, while the changes were less pronounced in northeast Siberia and North America.
The researchers further modeled how abiotic and biotic factors influenced the distribution of Arctic mammals. The presence of eDNA from herbivores like caribou, hares, and voles were strong co-indicators for the presence of horse and woolly mammoth eDNA. The distribution of humans, meanwhile, was unrelated to that of the herbivores, with the exception of hares.
The distribution patterns of certain plants were also associated with the presence of certain mammals. A plant signature dubbed NMDS2, which reflects woody plants rather than herbaceous ones, was positively associated with the distribution of mammoths but had the reverse effect on horses, suggesting horses might have been more restricted to grasslands and mammoths might have had a greater flexibility in diet.
Still, the researchers traced the decline of mammoths to about 7,300 years ago in northeast Siberia and 8,600 years ago in North America but noted that they survived until more recently in some smaller pockets — indicating that they co-existed with humans for 20,000 years. The extinction of woolly mammoths apparently came as the Arctic became warmer and wetter and no longer supported the vegetation they relied upon, they wrote.
“This is a stark lesson from history and shows how unpredictable climate change is — once something is lost, there is no going back,” Willerslev added.
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