A series of catastrophic fires killed off many large mammals in southern California by 13,000 years ago, and they were largely due to the arrival of humans
By Michael Le Page
17 August 2023
A sabre-toothed cat skull in the Page Museum at the La Brea tar pits, California
Robert Landau/Getty Images
A series of catastrophic fires was the immediate cause of the extinction of many large mammals in southern California 13,000 years ago, according to a study of fossils from the La Brea tar pits. The findings suggest these extreme fires were probably a result of humans abruptly changing the ecosystem by killing off herbivores – meaning there was more vegetation to burn – and deliberately starting fires.
“It’s a synergy of the drying climate and the humans, and the fact that they are killing herbivores and increasing fuel loads, and all of those things go together to make a feedback loop that takes the ecosystem to a chaotic state,” says Robin O’Keefe at Marshall University in West Virginia. “The fire event is really catastrophic.”
The tar pits at La Brea in Los Angeles have trapped numerous animals over the past 50,000 years and preserved their bones, providing an extraordinary window into the past. Many of the bones have never been precisely dated because radiocarbon dating was more expensive in the past and required destroying large chunks of bone, and also because results were skewed by the tar inside the bones.
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Now, costs have fallen, only tiny quantities of bone are needed and the tar contamination problem can be solved by extracting preserved collagen and dating only this material. As a result, O’Keefe and his colleagues were able to precisely date 172 bones from eight species.
Seven of these species are extinct, including the sabre-toothed cat , the dire wolf , the western camel and the ancient bison , which was even larger than surviving bison. The team also dated coyote bones as a control.
The dating shows that the seven species were all gone from the La Brea area by 13,000 years ago, though some survived elsewhere in North America for another millennium or so. Their disappearance from La Brea coincides with massive spikes in the number of charcoal particles in lake sediments, which are deposited during wildfires.