FAIRBANKS - The case has been cold for thousands of years, but science might have finally cleared humans as a prime suspect in the demise of the woolly mammoth.
Work by a team of international researchers indicates that extinct species like mammoths and the Yukon horse were still roaming interior Alaska as recently as 7,600 years ago. That's about 5,000 years later than scientists previously thought, and the revelation casts serious doubt on theories that over-hunting was responsible for killing off the giant mammal species.
The reason why mammoths and other mammals vanished from the region near the end of the Pleistocene era has been an ongoing debate, generating a range of explanations including meteors, disease and humans. Hunters were thought to be a particularly good bet, since they had migrated to the area from Asia shortly before the extinction estimate, but the revised timeline for extinction now makes that seem unlikely.
"That one probably can't be supported," said Duane Froese, a researcher from the University of Alberta. "Humans and these large animals coexisted for a long period of time, according to this research."
The new timeline for the vanishing mammoths is because of a new dating technique. Froese was part of a team that mined prehistoric soil specimens from permafrost near Stevens Village, then looked for evidence of ancient DNA samples in areas where its age could be verified.
Froese said previous efforts to date mammoths relied on about 150 sets of fossils, with the youngest dated at about 12,000 years ago. But using fossils leaves plenty of room for record-keeping gaps, since they're rarely found and offer only a tiny slice of history.
Froese said scientists theorized they could also date mammoths by identifying the DNA in feces, urine and other remnants they left behind during their lifetimes. The permafrost-laden site near Stevens Village offered the ideal location to check, since Yukon River silt has frozen and accumulated there for thousands of years.
In permafrost samples ranging from 7,600 to 10,500 years old, they found what they were looking for - the presence of both mammoth and horse DNA. Evidence of species like moose and Arctic hare also were located in the sample site.
Other large mammals, such short-faced bears and saber-toothed tigers, might have also gone extinct during the same period. Froese said it's less likely that remnants from those predators will be found, since they appeared to be much less common.