Between around 10,000 and 12,000 years ago, many large animal species around the world went extinct. For decades, scientists have debated the cause of these extinctions, with blame often directed at either climate change or human activity.
In a new study published August 18 in Science, researchers use fossilized animal remains and sediment cores to argue that in Southern California humans and a shifting climate worked hand in hand to contribute to extinctions. People lit fires that were fueled by a warmer and drier climate, leading to drastic changes in the plant and animal life in the area.
“I really like that this is getting away from the dichotomous arguments we’ve had forever about megafauna extinction in the Americas, that it’s an either-or choice between climate- or human-caused,” Angela Perri, a zooarchaeologist at Texas A&M University who didn’t contribute to the findings, tells Science’s Michael Price. “The most likely scenario is a combination.”
The researchers first examined fossils from the La Brea tar pits, naturally occurring pools of asphalt that trapped and preserved bones from dozens of mammal species and over a hundred bird species over a period of thousands of years. The team dated 172 samples from eight different animal species, including saber-toothed cats, dire wolves and ground sloths.
The tar pits trap collagen, a connective tissue that enables researchers to precisely date the specimens. The remains dated to between 15,600 and 10,000 years ago. But a little over 13,000 years ago, camels and sloths disappeared from the record, followed a few hundred years later by the rest of the species except for coyotes, which still live today.
“No one in the study was prepared for what we found,” F. Robin O’Keefe, a co-author of the study and biologist at Marshall University, tells Scientific American’s Meghan Bartels. “The coyotes keep being deposited, but the megafauna just, poof, disappear. And for most of them it is like a ‘poof’—it’s a pretty dramatic event.”
Next, the researchers compared these results to sediment cores from Lake Elsinore, also in Southern California, which give a sense for how vegetation, fires and climate changed in the area over time, writes Science News’ Jake Buehler.
Lisa N. Martinez, one of the co-authors and a paleoclimatologist at the University of California, Los Angeles, had found a dramatic increase in charcoal indicating fire activity around 13,200 years ago in the cores, per the Los Angeles Times. The cores also indicate that at the same time, a drastic shift in local flora occurred. Oaks and juniper trees disappeared and were replaced by plants better adapted to fires, including pines, grasses and chaparral plants, writes Science.
The climate was also undergoing change during this period—temperatures were rising and the area was experiencing drought. The cores showed that temperatures climbed 5.6 degrees Celsius over the previous thousand years, per the Los Angeles Times.
This confluence of evidence suggested to the researchers that human-caused fires could have been particularly damaging due to the climate at the time. O’Keefe tells Science News that humans hunting herbivores would have also made the ecosystem more vulnerable.
“You get this vicious cycle,” O’Keefe tells the publication. “You add more people and it gets hotter and drier, and you’re killing more herbivores. So there’s more fuel [to burn].”
Jacquelyn Gill, a paleoecologist at the University of Maine who did not contribute to the study, tells Scientific American that she was not surprised by the findings. “We know that in modern systems, extinction is very rarely unicausal,” Gill tells the publication. “You often need to have some force that’s stressing this population. Then there’s often an element of bad luck or some other stressor that comes in. We see that over and over again.”
The findings also hint at how our planet today, warming at a much faster pace and dealing with a greater impact from human activity, could similarly face a massive shift in the plants and animals living on it.
“This paper provides a picture of how climate change can completely transform ecosystems,” Jarmila Pittermann, a plant physiologist at the University of California, Santa Cruz, who wasn’t involved in the research, tells the Los Angeles Times’ Corinne Purtill. “It is super-convincing and a massive warning to all of us.”