In 2010, small cores of permafrost sediments were collected by a team at the from in the Klondike region of central Yukon. They had remained in cold storage until paleogeneticists at the applied new genomics techniques to better understand the .
These tiny sediment samples contain an immense wealth of ancient environmental DNA from innumerable plants and animals that lived in those environments over millennia. These genetic microfossils originate from all components of an ecosystem — including bacteria, fungi, plants and animals — and serve as a time capsule of long-lost ecosystems, such as the , which disappeared around 13,000 years ago.
How exactly these ecosystems restructured so significantly, and why large animals seem to have been the most impacted by this shift has been an .
We can now use environmental DNA to help fill the gaps that have driven this debate.
Ancient DNA, cutting-edge technologies
Bacterial, fungal and unidentifiable DNA make up over 99.99 per cent of an environmental sample. In our case, we wanted a way to selectively recover the much smaller fraction of ancient plant and animal DNA that would help us better understand the collapse of the mammoth-steppe ecosystem.
For my , I was part of a team that developed a .
We analyzed these DNA fragments to track the shifting cast of plants and animals that lived in central Yukon over the past 30,000 years. for the late survival of woolly mammoths and horses in the Klondike region, some 3,000 years later than expected.
We then to include 21 previously collected permafrost cores from four sites in the Klondike region that date between 4,000 to 30,000 years ago.
With current technologies, we not only could identify which organisms a set of genetic microfossils came from. But we those fragments into genomes to study their evolutionary histories — solely from sediment.
Tremendous environmental change
The was a period of tremendous change across the globe. In , this period saw the collapse of the and its gradual replacement with the as we know it today.
This brought about the loss of iconic ice age megaherbivores like the , and , along with predators such as the and , among many others.
We found ancient environmental DNA from a diverse spectrum of ancient fauna, .
We were also able to observe how ecosystems shifted with the rise of woody shrubs around 13,500 years ago, and how that correlated with a decline of DNA from woolly mammoths, horses and steppe bison. With this remarkably rich dataset, we observed four main findings.
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There was a surprising consistency in the signal between sites, suggesting our data was representative of ecological trends in the region.
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Woolly mammoth DNA declines prior to the , a warm period at the end of the last ice age, suggesting that megafaunal losses may have been staggered.
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make up a substantial component of the mammoth-steppe ecosystem alongside grasses.
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There is a consistent signal of woolly mammoth and Yukon horse persistence into the Holocene, as much as 7,000 years after their disappearance from fossil records.
When paired with other records, our genetic reconstructions suggest that the transition out of the last glacial period may have been more drawn out than dated bones alone would suggest.
Mammoths, for example, may have declined in local population abundance thousands of years earlier than other megafauna, which is potentially correlated with the first of humans in the area. Further, , despite the environmental shift.
Woolly mammoths alongside humans
Our data suggest that horses and woolly mammoths may have persisted in the Klondike until approximately 9,000 years ago and perhaps as recently as 5,700 years ago, outliving their supposed disappearance from . However, it is possible for , which could mix the genetic signals of different time periods, necessitating a degree of caution in our interpretations.
Until recently, there was no evidence of mammoth survival into the mid-Holocene. But studies have now shown that mammoths survived until and years ago on Arctic islands.
Researchers at the found evidence for the late survival of horses and mammoths in Alaska until as recently as as . They also found evidence of mammoths surviving as recently as 3,900 years ago in Siberia, alongside to at least 9,800 years ago.
Steppe bison, which were thought to have disappeared and been replaced by the during the Pleistocene, have likewise been found to have survived even as recently as perhaps just . We were able to observe in the same sediment samples, which suggests that there were likely distinct populations of these animals living in the same area.
There is a growing body of evidence that many ice age megafauna probably survived well into recorded human history, roaming the north during the and while builders worked on the .
Genetic archives of our ecological past
The growing sophistication of environmental DNA methods to study ancient genetic microfossils highlights just how much information is buried in sediments.
Permafrost is ideal for preserving ancient DNA, but as this , so too will the genetic material preserved within, and the evolutionary mysteries they once held.
Advances in paleogenetics continues to push the boundaries of what was once relegated to science fiction. Who knows what undiscovered evolutionary information remains frozen in ordinary sediments, hidden in microfossils of ancient DNA?
Tyler J. Murchie currently receives funding from the CANA Foundation, a non-profit organization with horse rewilding initiatives.