The great hairy beasts of the Pleistocene disappeared during sharp warming spikes, a new study in Science reports. The timing of these extinctions may absolve humans of responsibility, but one of the paper's authors says that our activities may still have been a contributing factor.
The history of life on Earth is one of extinctions and new arrivals, but the last 50,000 years have seen a great loss of land-based megafauna, defined as animals weighing more than 45 kilograms (100 pounds), everywhere outside of Africa, without replacement.
One theory, popularized by Jared Diamond, is that our technology made humans lethal to anything large, tasty-looking and unsuited to being tamed. Consequently, when we arrived in new locations we proceeded to wipe out many of the large herbivores, and carnivores subsequently starved to death. African animals, however, had a chance to get used to us as we were evolving, and produced defenses of their own. An alternative view blames changes in climate.
The disappearances of Australia's three-ton wombats and demon ducks of doom have produced famously hostile camps, although many paleontologists think that both contributed. Now some of the most prominent scientists in this debate have turned their attention to the disappearance of Eurasia's mammoths, giant bears and woolly rhinoceroses, and placed the blame on dramatic bursts of warming known as Dansgaard-Oeschger events.
“The Ice Age wasn't one continuous cold era,” coauthor Professor Chris Turney, of the University of New South Wales, told IFLScience. Turney acknowledges that Dansgaard-Oeschger events' causes are unknown, but says the matching between the temperature upswings and disappearance of large species is striking.
Dating extinction is hard, Turney admits. This is not just because “There is always a question of whether you have the last specimen that ever lived,” but because the radioactive carbon that paleontologists use to date bones is better at ordering events than at matching them to exact dates. Dansgaard-Oeschger events, on the other hand, can be dated precisely from the record of annual Greenland snowfalls.
Turney and his colleagues used sediment cores from Venezuela's Cariaco Basin to bring the two timelines together. These cores, they claim, “capture a climate record via shifts in the trade winds associated with northward migration of the Intertropical Convergence Zone in the tropical Atlantic, alongside a comprehensive suite of radiocarbon ages from planktonic foraminifera in the sediment core.”
This produced a remarkable match between warming events and extinctions, but surviving species appear to have shrugged off subsequent bouts of cooling. Turney says that this may be because temperatures went up by the elevator and down by the stairs, taking much longer to come back down than they did to rise. However, he also says that the greater vulnerability to warming should arouse concerns today.
Turney agrees that humans may have proven a problem for species trying to migrate to escape climatic changes, but thinks that the warming influence was dominant. In this context the more stable tropical temperatures over much of Africa may have had more to do with megafauna survival than familiarity with humans.