Fieldwork conducted about a decade ago is only now changing researchers’ understanding of large mammal extinctions during the Ice Age. An analysis of DNA locked away in frozen soil samples reveals that charismatic species like woolly mammoths and the wild horses of the Yukon stuck around for longer than previously believed.
The soil samples were taken from the Klondike region of Canada’s Yukon in the early 2010s, but no work on them had been published. Unlike traditional DNA samples, which might be taken from bone or hair of some organism, soils (even ancient ones) contain environmental DNA, which is genetic material locked away in the microscopic residue animals leave as they move through an environment.
The ice-cold cores from Klondike were later found in a McMaster University freezer by Tyler Murchie, an archaeologist specializing in ancient DNA at the university, who began to reinvestigate them. Murchie and his team’s work was published today in Nature Communications.
“I found them in the freezers while looking for a new project during my PhD,” said Murchie, lead author of the new paper, in an email. “One of my responsibilities at the ancient DNA centre is freezer maintenance, so I had a good idea of what cool stuff might be in there waiting for someone to study.”
One mystery the team sought to understand was the circumstances by which large North American species of the last Ice Age went extinct. Animals like woolly mammoths, steppe bison, and wild horses ranged across the continent for thousands of years, but the former two have disappeared from the planet. (Modern horses are directly related to Ice Age horses.)
What killed off the animals is typically attributed to one of two things: a warming climate that erased their food sources, or overhunting by humankind. Recent research has generally pointed to the former.
“I think a combination of climate, ecological, and anthropogenic pressures best explains the losses, but more research is needed to really solve that problem that Quaternary scientists have been grappling with for some 270 years,” Murchie said.
In the DNA contained in the ancient permafrost, the team found evidence that large mammalian species weren’t faring well even prior to the climate’s shift. In other words, the abundance of the DNA in samples began to diminish well before climatic shifts. (The team used radiocarbon dating of plant material in the soil samples to determine their ages.) But the animals didn’t vanish quickly; woolly mammoth and North American horse DNA remain present in the samples until as recently as 5,000 years ago, in the mid-Holocene, some 8,000 years later than the animals were once thought to have gone extinct.
“The rich data provides a unique window into the population dynamics of megafauna and nuances the discussion around their extinction through more subtle reconstructions of past ecosystems” said Hendrik Poinar, an evolutionary geneticist at McMaster University and also a lead author of the paper, in a university release.
Indeed it does, but that information is disappearing. As the climate warms, this time at alarming rate due to human causes, the permafrost is losing its permanence. Vast ponds are appearing in the planet’s northern reaches, causing swaths of the ground to collapse in massive sinkholes. The thaw also threatens the genetic information that has been cold-stored in the frozen earth. At the same time, though, the loss of permafrost has resulted in some incredible discoveries as preserved remains come out of the ice, including a still-furry cave lion cub and a 30,000-year-old wolf head.