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.
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 (Smilodon fatalis), the dire wolf (Aenocyon dirus), the western camel (Camelops hesternus) and the ancient bison (Bison antiquus), which was even larger than surviving bison. The team also dated coyote (Canis latrans) 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.
“Some of those spikes for those fires are just enormous, orders of magnitude more than has ever happened before,” says O’Keefe.
Pollen in lake sediments shows that the vegetation had begun changing from woodland to a more open landscape around 16,000 years ago, as the area became drier due to the retreat of the ice sheets. But there was a sudden shift to fire-resistant vegetation around 13,000 years ago.
“The results of this study are consistent with humans increasing fire both directly though ignitions and indirectly through hunting of herbivores,” says Allison Karp at Yale University, who wasn’t involved in the study.
If the tiny number of people alive at the time could do this, the much greater number of people alive now can have a much bigger impact, says O’Keefe. “It’s super relevant to today,” he says.
More extreme wildfires are happening in many parts of the world as it warms, and O’Keefe says his findings show there is a risk this could lead to ecosystems flipping into another state, resulting in many species going extinct. “Hopefully, by learning these things about what happened at La Brea, maybe we can change our trajectory,” he says.
Earlier research had suggested that the development of the Clovis stone tool technology, whose distinctive feature is finely crafted large spear points for tackling big animals, enabled people in North America to wipe out the continent’s megafauna. However, these findings show that some large mammals were going extinct in places before Clovis tools appeared. O’Keefe and his colleagues think Clovis tools were instead a response to the loss of some megafauna.
“The things that seem to get hunted out first are the things that are easier to catch, like camels and horses and bison,” says O’Keefe. “It’s only when you start running out of those that we think that the Clovis technology evolves, because you have to do this really dangerous thing and try to take on a mastodon because all the easier to kill animals are gone.”
“Clovis wasn’t a driver of extinction. It evolves because the extinction was already under way,” he says.