Prehistoric big-game hunters may be off the hook in the latest twist of a whodunit that tries to explain why bison populations sharply crashed thousands of years ago.
Proponents of the overkill theory blamed the first Americans to cross an ice-free corridor — connecting what is now Alaska and Siberia — for hunting bison within a whisper of disappearance. Those super hunters also are faulted for pushing massive mammals, such as woolly mammoths, short-faced bears and North American lions into extinction.
A team of 27 scientists used ancient DNA to track the hulking herbivore’s boom-and-bust population patterns, adding to growing evidence that climate change was to blame.
“The interesting thing that we say about the extinctions is that whatever happened, it wasn’t due to humans,” said the paper’s lead author, Beth Shapiro, a Wellcome Trust Research Fellow at Oxford University. By the time people arrived, “these populations are already significantly in decline and on the brink of whatever was going to happen to them in the future.”
The story written into the bison’s DNA is one of an exponential increase in diversity with herd sizes doubling every 10,200 years. Then, 32,000 to 42,000 years ago, the most recent glacial cycle kicked in, beginning a lengthy cooling trend. Bison genetic diversity plummeted. A significant wave of humans didn’t appear in the archaeological record at eastern Beringia — the land connection between Alaska and Siberia that was created when growing glaciers locked away water, dropping sea levels by as much as 450 feet — until more than 15,000 years later, the authors write in today’s edition of Science.
The Science paper refers to dates in radiocarbon years, a dating technique that doesn’t match up precisely with conventional calendars. For instance, 12,000 years before the present in radiocarbon years equates to 14,000 years ago according to calendars, Miss Shapiro said.
Cold “and arid conditions increasingly dominated, and some component of these ecological changes may have been sufficient to stress bison populations across Beringia,” the authors note.
About the same time, brown bears and a type of horse went extinct in Alaska. The results “offer the first evidence of the initial decline of a population, rather than simply the resulting extinction event,” the authors write.
Tapping genetic information gives scientists the means to assess the health of bison over thousands of years, said Russell Graham, director of the Earth & Mineral Sciences Museum at Pennsylvania State University.
At given points in the distant past, the researchers could tell whether bison were thriving or decimated and attach a firm date to that health check.
“The real importance of the paper, at least from my perspective, is it provides us a way of measuring what is happening to a population of animals through time,” Mr. Graham said.
John Alroy, a University of California, Santa Barbara, research biologist and overkill proponent, remains unconvinced. The near-extinction of bison would not have happened without the handiwork of human hunters, he said, adding: “I think the interpretation is off-base and inappropriate, and I’m not persuaded at all by their claims.”
Researchers have looked at modern animals to flip back in time to better understand how their ancestors fared during the peak of the last ice age. Because of severe populations crashes, though, modern bison have lost much of the genetic diversity locked up in the bones of their ice-age ancestors.
Only two subspecies remain in North America, the plains and the wood bison. “If you just used modern bison populations and tried to figure out what was going on in the past, you wouldn’t get the right answer,” Miss Shapiro said.