A lingering signal of horses and mammoths in North America, thousands of years after their supposed extinction, have been found by researchers examining ancient DNA preserved in the frozen tundra.
Canadian researchers set out to characterize the collapse of the mammoth-steppe in central Yukon as revealed by ancient environmental DNA.
Tyler Murchie and his fellow researchers in Canada acknowledged the complexities of attempting to disentangle the relative impacts of climate change, ecosystem restructuring, and human activities associated with the Late Quaternary extinctions of large mammals in North America.
Advances in the extraction and identification of ancient DNA shed into the environment and preserved for millennia in sediment now provide a way to augment what palaeontologists can learn from the fossil record, they said.
The study team, writing in the journal Nature Communications, have assembled a 30,000-year sedimentary ancient DNA record derived from permafrost silts in the Klondike region of Yukon, in Canada.
“We observe a substantial turnover in ecosystem composition between 13,500 and 10,000 calendar years ago with the rise of woody shrubs and the disappearance of the mammoth-steppe ecosystem,” they reported.
The most common DNA signatures from large mammals identified in the tested sediments were the steppe bison, the woolly mammoth and horses. Less abundant species included caribou/reindeer, and ovine species, most likely the Dall sheep.
There was a low biomolecular signal from predators, including the grey wolf and marten.
A variety of rodents were identified, among them the arctic ground squirrel, the taiga vole, and the northern collared lemming.
Human DNA was identified despite not being targeted in the study, but the scientists did not consider this human signal to be reliable without further investigation.
Overall, the DNA sedimentary record assembled by the team reflected a gradual decline in the megafauna signal through time.
The mammoth decline was seen first, from about 20,000 years ago. This was followed by declining signals for bison and horses, until a punctuated decrease near the Pleistocene-Holocene transition about 11,000 years ago. The DNA signals of these species almost disappear, while those for moose elk/wapiti enter the dataset.
The sedimentary DNA puts the disappearance of megafaunal grazing species in the Klondike at between 13,000 and 10,000 years ago. But they also observed a lag in the final appearance of horses and mammoths as late as 5700 years ago.
Sedimentary cores from three regions resulted in more than 100 DNA sequences assignable to horses and mammoths that were dated well beyond their last dated macrofossils (bones, teeth, and soft tissue).
These youngest DNA signatures for horses and mammoths are of great interest, they said, because they imply local survival of these animals long after the Pleistocene-Holocene transition.
“Small refugial populations might have survived in remote pockets at sizes too small to be readily detected by macrofossil collections derived largely from a small set of resource extraction and development sites,” they said, noting that palaeontological and archaeological records across much of the Arctic and Subarctic are notably sparse.
The authors said their work builds on the findings of Murchie and others in a 2021 study in which horse and woolly mammoth DNA was unexpectedly identified in a permafrost sample dating to about 9700 years ago. This post-dates the last macrofossil evidence of these animals in Alaska by some 3300 years.
The evidence, they said, points to a substantial “ghost range”, indicating a refugium for horses and mammoths in these northern latitudes, apparently outliving the functional extinction and complete loss of other continental populations.
Relative bison and horse DNA signals in the sediment decreased from about 15,000 years ago. Horse DNA in the sediment had remained relatively consistent until the rise of woody shrubs during the Allerød warming, about 13,500 years ago, the authors noted.
In summary, they said there are consistent, multi-site signals of late persistence for horses and mammoths, perhaps surviving some 7000 years longer than their last dated macrofossils in the region.
The degree to which humans may have been involved in any of the transformations seen in the region is hard to gauge from available evidence, they said. At this time, there is no clear evidence for an ecologically significant human presence in the region before 14,000 years ago.
Thereafter, any large mammals who had already undergone thousands of years of oscillating climatic and ecological pressures may have been vulnerable to human forces. However, evidence of human contributions to the ecological turnover in the area remains functionally absent, they said.
The human factor is at most a mysterious component in a set of compounding pressures on the animals that once inhabited the region, they said. The testing of Pleistocene permafrost for human DNA may prove crucial to addressing lingering unknowns in the peopling of the region, they added.
The study team comprised Murchie, George Long, Tara Sadoway, Emil Karpinski and Hendrik Poinar, with McMaster University in Hamilton, Canada; Alistair Monteath, Matthew Mahony, Scott Cocker, and Duane Froese, with the University of Alberta in Edmonton; Grant Zazula, with the Yukon Government’s Palaeontology Program; and Ross MacPhee, with the American Museum of Natural History.
Murchie, T.J., Monteath, A.J., Mahony, M.E. et al. Collapse of the mammoth-steppe in central Yukon as revealed by ancient environmental DNA. Nat Commun 12, 7120 (2021). https://doi.org/10.1038/s41467-021-27439-6