Ancient DNA rewrites the history of the woolly mammoth
Fifteen thousand years ago, woolly mammoths and North American horses roamed the cold grasslands that then covered much of Alaska and the western Yukon. They're extinct now, but we know quite a bit about them because of the fossils they left behind in places like Old Crow, the Klondike, and parts of Alaska.
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Duane Froese sits beside a treasure trove of fossils near Old Crow, holding a mammoth tooth in his left hand. He plans to return to Old Crow in search of DNA traces of woolly mammoths in the Yukon.
(photo: Grant Zazula)
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However, we don't know when they disappeared or why. The conditions that preserve bits of dead animals as fossils are rare, and the chances of finding the fossil remains of the last woolly mammoth or the last North American horse are so small that they make winning the lottery look like a sure thing.
But animals leave more than bones behind – and that other evidence is rewriting the prehistory of North America. Animals shed bits of their genetic material as they move through a landscape. It comes from their feces, their urine, dropped hairs, and even sloughed-off skin cells. Modern instruments and lab techniques can detect that genetic material and identify its source.
If the circumstances are right, the genetic material – the DNA – can be preserved for thousands of years. The frozen sediments underlying parts of the Yukon watershed provide exactly those circumstances. University of Alberta researcher Duane Froese and an international team are analyzing sediment cores from several places in the Yukon and Alaska in search of the last of the big ice age mammals, the megafauna.
The first of the evidence is in, and it's startling. Sediment cores from a site near Stevens Village, on the Yukon River in Alaska, show that woolly mammoths and North American horses survived in that area until at least 10,500 years ago, and maybe even later. That's well after the end of the ice age, and several thousand years later than previous estimates – based on fossil bones and teeth – for the disappearance of both animals from North America.
The findings throw doubt on a couple of theories developed to explain the extinction of the ice age megafauna: the Blitzkrieg hypothesis and speculation about an extraterrestrial impact.
Froese says the Blitzkrieg hypothesis argues that when humans arrived in the Americas, they hunted the large mammals to extinction very quickly. However, the DNA evidence shows that humans coexisted with the ice-age animals for thousands of years.
"So the 'blitzkrieg' part is not supported, but humans could have had a hand in their eventual demise," he says.
The DNA evidence also argues against the theory that an extraterrestrial impact – possibly a meteorite strike about 12,900 years ago – caused the sudden mass extinction of big mammals in North America. They simply didn't die out that quickly or that early.
While the DNA research hasn't revealed what actually did drive the animals to extinction, it has turned up some fascinating information, Froese says. The mammoth DNA was particularly revealing.
"Interestingly, even though the DNA is fragmented and quite short, we could establish that there were two different mammoths that contributed DNA to the sample, and that they both belonged to a particular herd that were related to the last surviving populations recovered in Siberia on Wrangell Island."
The DNA itself can't be dated, but its age can be determined by dating the sediment layer in which it occurs. In the parts of Alaska and the Yukon where the glaciers didn't reach, tens of thousands of years of sediment layers have accumulated beneath the current ground surface. Froese and his colleagues have been sampling and dating those sediment layers for many years, but the idea of searching them for DNA first came up in 2003.
"Eske Willerslev, now a Professor at the University of Copenhagen, and I were doing field work together in the Klondike in 2003," Froese explains. "At that time he had some initial results from some Siberian permafrost that showed you could detect large mammals from DNA recovered from sediments. It was really an exciting discovery to think that only a few grams of sediment could potentially link a specific part of the geologic record to the large mammal community."
They decided to try out DNA analysis at the Stevens Village site, which Froese had studied several years earlier. It took a while to figure out how to use the technique, Froese says. "That's really why it's taken quite a few years to bring this together."
Now that they have an idea of what sedimentary DNA can reveal, Froese and his colleagues are bringing their research to the Yukon.
"We have a very exciting site in the Klondike, which our group has been working on. It covers the interval from 13,000 to 7,500 years before present. In fact, it's the best site I know of for this time interval in Yukon, Alaska, or Siberia."
The researchers have spent two summers sampling and dating the Klondike sediments. The abundant plant material trapped in the sediments makes it possible to date them accurately. It also provides information about the environment of the past. Froese says they're trying to track environmental change over those thousands of years and link it to what's known about changes in large mammal populations in the same period.
In the summer of 2010, the researchers will return to the Klondike sites. They'll also take a look at some likely sites in the Old Crow area. Sedimentary DNA – and what it can tell us about the last of the ice age megafauna – is high on the to-do list for next summer's research.
"We still have a lot to learn about how it can be applied," Froese says. "That's part of the motivation in expanding to new sites in the Yukon, to test some of the results from Alaska."
- For further information, see the research team's article about the Stevens Village DNA work, "Ancient DNA reveals late survival of mammoth and horse in interior Alaska," published in the December 2009 issue of the journal Proceedings of the National Academy of Sciences.
