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DNA in Alaskan Permafrost Offers Insights into Mammoth Extinction

NEW YORK (GenomeWeb News) – Mammoths and ancient Yukon horses survived for thousands of years longer than fossil evidence indicates, according to a new genetic study.

An international research team used a combination of mitochondrial DNA sequencing and soil dating to assess mammoth and prehistoric horse sequences in Alaskan permafrost samples. The research, scheduled to appear online this week in the Proceedings of the National Academy of Sciences, suggests the animals roamed central Alaska some 7,600 to 10,500 years ago — thousands of years after the last known fossil evidence for these species was deposited.

"Our findings show that the mammoth and the horse existed side by side with the first human immigrants in America for certainly 3,500 years and were therefore not wiped out by human beings or natural disasters within a few hundred years, as common theories otherwise argue," senior author Eske Willerslev, director of the University of Copenhagen's Centre for GeoGenetics, said in a statement.

The most recent fossil evidence available suggests mammoths and ancient horses went extinct between 13,000 and 15,000 years ago. Such patterns — combined with the disappearance of many other large animal species — have fueled speculation that an extra-terrestrial impact or excessive hunting by humans wiped out a slew of organisms at the end of the Pleistocene period.

Even so, the researchers explained, it's unlikely that the fossilized bones and teeth discovered so far came from the very last surviving animals. That means mammoths and other animals may have survived longer than suggested by the fossil record alone.

"Ideas about the timing and causes of late Pleistocene extinctions have been based, so far, on recovery of fossil bones and teeth and radiocarbon dating the fossils," co-author Duane Froese, an earth and atmospheric sciences researcher at the University of Alberta, explained in a statement. "We know that we're unlikely to find the last fossils — meaning that the last dates on fossils are almost certainly too old — but by how much is hard to know."

For the current study, the researchers focused on trying to find ancient DNA from animal skin, hair, and waste, extracting DNA from permafrost core samples collected in the Alaskan community of Stevens Village, located north of Fairbanks on the banks of the Yukon River.

"Whilst an animal leaves only a single corpse when it dies, it leaves quantities of DNA traces through urine and feces whilst it is still alive," Willerslev explained in a statement. "It is these DNA traces which we find in the soil."

The researchers used Sanger and Roche 454 GS 20 FLX instruments to sequence "sedimentary ancient." Meanwhile, they also determined the age of these sediment samples using radiocarbon dating and another method called optically stimulated luminescence.

In the process, the team identified mtDNA sequences matching the woolly mammoth, horse, bison, moose, and snowshoe hare.

But while mtDNA from bison, moose, hare, and other animals living in the area today were detected in more recent samples, the mammoth and prehistoric horse sequences were found in just one sediment layer: dated at 7,600 to 10,500 years old. In contrast, no such sequences were detected in control samples from the soil surface.

The findings suggest mammoths and prehistoric horses survived for thousands of years after the most recent fossil evidence was deposited, the researchers explained, co-existing with humans for far longer than previously appreciated.

"Extinctions often seem dramatic and sudden in fossil records," co-author Ross MacPhee, the American Museum of Natural History's mammalogy curator, said in a statement, "but our study provides an idea of what an extinction event might look like in real time, with imperiled species surviving in smaller and smaller numbers until eventually disappearing completely."

In the future, the researchers noted, similar soil DNA analyses may prove useful for studying both living and extinct species — and contribute to forensic investigations. For example, Froese said the team is in the process of looking at sites in the Yukon to see if they can detect mtDNA from mammoths and other mammals.

"Dirt DNA has lots of exciting potential to contribute to extinction debates in other parts of the world too, as well as a range of archaeological questions," Willerslev said. "This kind of information is really valuable for studies of animals that are hard to detect, and there are some neat forensic applications too."