Ancient DNA found in frozen soil suggests the woolly mammoth may have been around for thousands of years longer than we thought.
Credit: Mauricio Anton
SYDNEY: Extinct woolly mammoths and ancient American horses may have been grazing the North American steppe for several thousand years longer than previously thought.
An international team of researchers found ancient DNA from both species locked in permafrost samples dated to between 7,600 and 10,500 years ago, according to a study published in the U.S. journal Proceedings of the National Academy of Sciences.
This new evidence suggests that at least one population of these now-extinct mammals endured longer in the continental interior, challenging the conventional view that these and other large species, or megafauna, disappeared from the Americas about 12,000 years ago.
Exctinctions in real time
"We don't know how long it takes to pinch out a species," said researcher Ross MacPhee of the American Museum of Natural History in New York.
"Extinctions often seem dramatic and sudden in fossil records, 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."
At the end of the Pleistocene - the geological epoch spanning 12,000 to 2.5 million years ago - many of the world's megafauna, such as giant sloths, saber-toothed cats, dire wolves, stag-moose, and mammoths, vanish from the geological record.
This sudden disappearance - inferred from radiocarbon dating of fossilised bones and teeth - has led some scientists to propose cataclysmic explanations, like overhunting by humans, an extraterrestrial impact, and the introduction of new infectious diseases.
But hard remains of animals are rarely preserved and difficult to date due to physical degradation. So the researchers decided to tackle the problem by dating the last survivors through dirt.
Dating through dirt
The team collected soil cores from undisturbed permafrost in central Alaska. These frozen sediments preserve small fragments of DNA exeptionally well, even in the absence of any visible organic remains, such as bone, skin or hair.
Two independent methods of dating - radiocarbon and optically stimulated luminescence - were used to date plant remains and individual mineral grains found in the same layers as DNA. One of the cores collected confirmed the presence of both the woolly mammoth and ancient horse between 7,600 and 10,500 years ago.
According to Richard Roberts of the University of Wollongong in Sydney, "With these two techniques, we can be confident that the deposits from which the DNA was recovered haven't been contaminated since these lost giants last passed this way.
"It's a genetic graveyard, frozen in time," he said.
