Humans are truly self-important creatures. No matter what happens, we think it must have been our doing. Lightning cracks and earthquakes? We obviously angered God. Your team wins a football game? It’s because you wore your lucky jersey. Woolly mammoths go extinct thousands of years ago? Well, we must have killed them.
That last example is called the “overkill hypothesis.” At the end of the Pleistocene, around 11,000 years ago, many large mammals, like the woolly mammoth and the giant ground sloth, died out. Paleontologists have argued for centuries about what killed these so-called “megafauna.” The overkill hypothesis, proposed 40 years ago by geoscientist Paul Martin, holds that humans swept into their habitats and hunted the giants to extinction. (This idea is also called the “blitzkrieg model,” but let’s stick with “overkill hypothesis”—I prefer not to liken our hunter-gatherer ancestors to the Nazis.)
The theory had a good run. Humans got to spend several decades imagining themselves as the descendants of super-hunters who swept aside the strongest beasts nature had to offer.
Unfortunately, the likely truth is less impressive. The overkill hypothesis is wrong—or at least incomplete. A series of more recent studies shows that forces much larger than humans were primarily responsible for the disappearance of the great mammals of the Pleistocene. First, archaeologists have pointed out that only two of the 36 animals that went extinct during this period show evidence of having been hunted by humans. Last year, researchers showed that many of the megafauna in the American Northeast were already or almost gone when humans rolled through.
If humans didn’t single-handedly kill those great beasts, then who or what did? Rapid climate change has long been a prime suspect, but some of the pieces didn’t quite fit. Paleontologists could find little evidence of mass extinctions during the coldest part of the last ice age—the period during which you’d expect climate change to be taking its toll.
It now seems that we were looking at the wrong end of the process. For some reason, people are inclined to think of cold as the more dangerous temperature extreme. Perhaps it’s hard-wired into the human brain. But heat is just as deadly, if not more so. And it now looks as though rapid warming at the end of the ice age—not the frigid cold of the ice age itself—was largely responsible for the megafauna extinction.
Today, a team of geneticists reported in Science that DNA evidence from the late Pleistocene shows the major turnover of species lines up perfectly with a period of sudden climate oscillations identified in ice cores. These samples of ancient ice carry a record of the composition of the earth’s atmosphere going back hundreds of thousands of years.
The study doesn’t end the extinction mystery entirely, since warming at the end of the Pleistocene probably couldn’t have killed the megafauna without help. It’s not as though a few degrees of temperature increase gave the mammoths heat stroke, and they crumpled into a lifeless woolly pile.
One possibility is that temperature shifts changed the food supply. Woolly mammoths, giant ground sloths, and other megafauna appear to have been herbivores. (At least, that’s what their dung suggests.) Last year, a team based in Europe argued that the ice age reduced the population of protein-rich plants called “forbs,” a family that includes the modern sunflower. When the temperature eventually rebounded, new plants more suited to the warmer climate replaced the forbs and greatly limited the availability of plant-based protein.
Humans probably also played a supporting role.
“The abrupt warming of the climate caused massive changes to the environment that set the extinction events in motion,” says Chris Turney, a paleoclimatologist at Australia’s University of New South Wales and a coauthor of today’s study. “But the rise of humans applied the coup de grace to a population that was already under stress."
Think of humans as the matador, the guy who comes in at the end of a bullfight and takes all the credit for killing a bull that has been weakened to the edge of death by a bunch of other guys. Suddenly, we don’t sound so tough.
onEarth provides reporting and analysis about environmental science, policy, and culture. All opinions expressed are those of the authors and do not necessarily reflect the policies or positions of NRDC. Learn more or follow us on Facebook and Twitter.