Around 34,000 years ago, woolly mammoths went extinct from parts of Europe, only to be replaced by… woolly mammoths. The two groups—the disappearing individuals and their substitutes—belonged to the same species. If you looked at their fossils, you probably couldn’t tell them apart. Their genes, however, reveal them to be part of two genetically distinct lineages, one of which suddenly displaced the other.
Alan Cooper, who specialises in studying the DNA of extinct animals, first noticed this pattern ten years ago, and not just among mammoths. The steppe bison—a huge cow—was quickly replaced by a different bison species. Later, one population of cave bear was swapped out for another genetically distinct population. Later still, the giant short-faced bear disappeared and the modern grizzly took its place. “This all happened very abruptly,” says Cooper. “They’re weren’t all happening at the same time, but the patterns were there, and many of these changes were only detectable through DNA.”
These giant beasts—the woolly mammoth, the cave and short-faced bears, the steppe bison, the Eurasian cave lion, the woolly rhino, and the Irish elk—are now extinct. There have been some two centuries of debate over what killed them. Was it a blitzkrieg of incoming humans, with our insatiable appetites and sharp spears, or was it a change in climate?
The turnovers that Cooper saw were extinctions of a kind, and they certainly didn’t seem to be due to hunting. At least six of them took place tens of thousands of years after humans arrived on the scene. If we hunted these animals to extinction, we sure took our sweet time over it, and in some cases, we seemingly ignored a virtually identical group or species that then took the place of the vanquished. An environmental event seemed more likely. But what kind of event?
To find out, the team used ancient DNA to discover cases of turnover that are hard to observe from fossils alone. They then carbon-dated the bones of the ingoing and outgoing animals to work out when these turnovers happened. Finally, they plotted these dates against an accurate record of North Atlantic climate over the past 60,000 years, gleaned from Greenland ice and Venzeuelan sediments.
Most of the replacement events lined up with sudden bursts of warming called Dansgaard-Oeschger (D-O) events, which happened a lot between 12,000 and 60,000 years ago. In each one, average temperatures abruptly rose by anywhere from 4 to 16 degrees Celsius in just a few decades, before gradually falling again over several millennia. If you plot the temperatures on a graph, you see a distinctive sawtooth pattern—sharp upward spikes of warming and then gentle downward slopes of cooling. “They’re the most abrupt changes in climate that you got in the Pleistocene—those transitions from cold to warm,” says Cooper.
When one group of large beasts cycled into another (and, eventually, into nothing), it usually happened during the warm periods, or interstadials, that followed the D-O events. “Periods of cold are often held up as a key reason for megadeath, but we see that our extinctions and genetic transitions didn’t fall in those periods of time,” says Cooper. During the cold, the animals may have retreated into warmer refuges, and their populations may have contracted, but they didn’t die out entirely. That only happened after sudden warmth.
“In the last two and a half million years, ice ages have been the rule for the earth’s climate system — the warm periods are the exception,” says Jacquelyn Gill from the University of Maine. “Given that, it absolutely makes sense that the authors found evidence for more turnover during warmer climates, rather than cold events.”
“What we don’t know is whether it’s the warming that’s doing the damage or the pace of change,” Cooper adds. It’s probably not as simple as cold-adapted species losing out to warm-adapted ones. The sudden temperature rises would also have caused dramatic changes in rainfall and other weather patterns. They might also have altered the ranges of different animals, bringing separate populations or species into new contact and new conflict. Finally, humans could still have played a role in finishing off these giant beasts, after changing climates had weakened them. Supporting that idea, Cooper’s team found that transition events were less common during earlier parts of the Pleistocene when D-O events were common but humans were not.
“These and previously published data argue convincingly that, sometimes, extinctions happened in the absence of humans—especially local extinctions followed by re-establishment of populations from elsewhere, as more ideal climate conditions returned,” says Beth Shapiro from the University of California, Santa Cruz. But “it’s hard to imagine that humans did not at least contribute somewhat,” she adds. If climate change corralled animals into ever narrower ranges, those “refuges” would have been fantastic hunting grounds—more like slaughterhouses than sanctuaries.
“As animals became stressed due to rapid changes in climate, and consequent reduction in habitat and loss of connectivity between whatever patches of habitat remain, humans are poised to have the biggest potential negative impact on these populations,” says Shapiro. “It’s about timing—poor timing, if you’re a mammoth.”
The synergistic threats of rapidly warming climate and relentlessly destructive humans has lessons for us in the present, says Cooper. “We should really be aware that if you go back 10,000 years, the climate goes apesh*t,” he says. “People generally have no idea about this, but these warm-cold-warm-cold patterns are standard for the Pleistocene. The stability of the Holocene, which human civilisation was built upon, is totally anomalous. That makes me very concerned about prodding the global climate system with a stick”, as we are now doing.
“I think these results are most sobering when applied to the present, where the few remaining megafauna survivors are some of our most threatened,” adds Gill. “When it comes to the conservation of elephants, rhinos, or tigers, it’s clear that we need to be conserving the genetic diversity that may be critical to their survival through the coming centuries of warming.”
Reference: Cooper, Turney, Hughen, Brook, McDonald, Bradshaw. 2015. Abrupt warming events drove Late Pleistocene Holarctic megafaunal turnover. Science http://dx.doi.org/10.1126/science.aac4315