Sunday, December 19, 2021

Reconstructing prehistory from a teaspoonful of soil

Yukon Photo credit Tyler Murchie
Prehistoric horses survived longer in North America than previously thought, according to a new study.

Researchers at McMaster University, Hamilton, Canada, used DNA capture-enrichment techniques, that they had developed previously, to find ancient DNA from plants and animals in as little as a teaspoonful of soil. The soil for the study was taken from core samples collected from the permafrost at four sites in the Klondike region of central Yukon, in northwest Canada. 


The work is published in the journal Nature Communications.


Tyler J. Murchie  and co-authors explain that environmental samples, such as soil, contain fragments of genetic material. Most ancient environmental DNA (eDNA) is broken down by bacteria or by physical or chemical processes. However, some of this eDNA becomes bound to sedimentary minerals, which protects it, especially when it is frozen. 


By extracting and analysing this sedimentary ancient DNA (sedaDNA), the research team was able to build up a picture of the plant and animal ecosystems from 30,000 to 4000 years ago. 


By analysing the DNA, the research team could rebuild the fluctuating animal and plant communities at different time points. Of particular interest was the Pleistocene-Holocene transition, an unstable climatic period 11,000-14,000 years ago when a number of large species such as mammoths, mastodons and sabre-toothed cats disappeared.

The analysis reveals that mammoths and horses were already in steep decline prior to the climatic instability, but they did not immediately disappear due to human overhunting as previously thought. In fact, the DNA evidence shows that both the woolly mammoth and North American horse persisted until as recently as 5,000 years ago.

The authors explain that through the early Holocene (starting about 11,000 years ago), the Yukon environment continued to experience massive change. Formerly rich grasslands - the “Mammoth Steppe”- were overrun with shrubs and mosses, species no longer held in check by large grazing herds of mammoths, horses and bison. Today, grasslands do not prosper in northern North America, in part because there are no megafaunal “ecological engineers” to manage them. 

“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” says evolutionary geneticist Hendrik Poinar, a lead author on the paper and director of the McMaster Ancient DNA Centre.

This work builds on previous research by McMaster scientists who had determined woolly mammoths and the North American horse were likely present in the Yukon approximately 9,700 years ago. Better techniques and further investigation have since refined the earlier analysis and pushed forward the date even closer to contemporary time. 

“Now that we have these technologies, we realize how much life-history information is stored in permafrost,” explains Tyler Murchie, a postdoctoral researcher in McMaster’s Department of Anthropology and a lead author of the study.

“The amount of genetic data in permafrost is quite enormous and really allows for a scale of ecosystem and evolutionary reconstruction that is unparalleled with other methods to date” he says.

“Although mammoths are gone forever, horses are not” says Ross MacPhee of the American Museum of Natural History, another co-author. “The horse that lived in the Yukon 5,000 years ago is directly related to the horse species we have today, Equus caballus. Biologically, this makes the horse a native North American mammal, and it should be treated as such.” 

Scientists also stress the need to gather and archive more permafrost samples, which are at risk of being lost forever as the Arctic warms.


For more details, see:


Collapse of the mammoth-steppe in central Yukon as revealed by ancient environmental DNA

Tyler J. Murchie, Alistair J. Monteath, Matthew E. Mahony, George S. Long, Scott Cocker, Tara Sadoway, Emil Karpinski, Grant Zazula, Ross D. E. MacPhee, Duane Froese & Hendrik N. Poinar 

Nature Communications (2021) vol 12, Article number: 7120


For a video presentation, see:

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