Fossils from La Brea Tar Pits in Southern California suggest that sabertooth cats and other large North American mammals disappeared as a result of wildfires spurred by human activity.
Wildfires are getting worse. Parts of the United States, scientists say, are experiencing wildfires three times as often — and four times as big — as they were 20 years ago. This summer alone, smoke from Canadian blazes turned North American skies an unearthly orange, “fire whirls” were seen in the Mojave Desert and raging flames in Maui led to disaster.
Records of the distant past can reveal what once drove increased fire activity and what can happen as a result. In a new study published Thursday in the journal Science, a group of paleontologists that analyzed fossil records at La Brea Tar Pits, a famous excavation site in Southern California, concluded that the disappearance of sabertooth cats, dire wolves and other large mammals in this region nearly 13,000 years ago was linked to rising temperatures and increased fire activity spurred by people.
“We implicate humans as being the primary cause of the tipping point,” said Robin O’Keefe, an evolutionary biologist at Marshall University. “What happened in La Brea, is it happening now? Well, that’s a really good question — and I think we should figure it out.”
Earth has seen five mass extinction events so far; some scientists argue that the disappearance of large mammals at the end of the last ice age was the start of a sixth. “It was the biggest extinction event since an asteroid slammed into Earth and wiped out all the dinosaurs,” said Emily Lindsey, a paleoecologist at La Brea Tar Pits and Museum and author of the new study, adding that the disappearance could very well represent “the first pulse” in a sixth mass extinction.
Until now, researchers have not been able to pin down exactly what caused these animals to go extinct. La Brea Tar Pits is one of the few sites in the world with a large enough fossil record for scientists to investigate the question. The pits, which are still active across 13 acres of land, are filled with bubbling black asphalt that has seeped to the surface from inside Earth. Prehistoric animals that became stuck in this goo died of fatigue or predation, and the asphalt fossilized and preserved their remains. “And that’s still happening today,” Dr. O’Keefe said. “You can go out to La Brea and see a squirrel stuck in the tar — I’ve seen it with my own eyes.”
That’s bad luck for the animals, but good fortune for scientists: La Brea now boasts a continuous fossil record of the region stretching as far back as 55,000 years. Dr. O’Keefe and his team analyzed fossils for eight large mammal species — including the sabertooth cat, the American lion and Camelops hesternus, an ancient camel — that lived between 10,000 and 15,600 years ago. Using radiocarbon dating, the team determined that seven of these species went extinct around 13,000 years ago.
To figure out why, the researchers analyzed climate, pollen and fire records in the region alongside continental human population growth at the time. They found that human occupation began to rise rapidly around the same time that Southern California entered a period of severe drought and warming. Extreme fires ensued, and the vegetation, once rich in juniper and oak trees, was eventually replaced by grass and chaparral shrubs.
“What we see is that you have a 400-year-long period of massively elevated wildfire,” said Regan Dunn, a paleobotanist at La Brea Tar Pits and Museum and an author of the new paper. “And at the end of that period, you’re in a different ecosystem and all of the megafauna are gone.”
Dr. O’Keefe described the conditions as the perfect storm: “You have a bunch of different factors that are multiplying each other and giving you a huge increase in fires,” he said. Using a model similar to the ones that forecast trends in the stock market, the scientists determined that humans were the primary drivers of these fires, both through direct ignition and by the elimination of herbivores, which allowed flammable underbrush to spread uncontained. Shifts in the climate exacerbated this further, setting the stage for the extinction of species.
Dr. Dunn emphasized that this pattern could not account for the notable disappearance of large mammals elsewhere in the world at the end of the last ice age. “But in order to understand the global event, you really need to look at a regional scale,” she said. She added that what happened in Southern California 13,000 years ago “has striking parallels to the environmental and biodiversity crises we’re facing today.”
Climate records during the ice age extinction indicate a warming of about 10 degrees Fahrenheit over 1,000 years, Dr. Dunn said, whereas today, temperatures in Southern California have risen about 5.4 degrees Fahrenheit in only the past century. Increased fire activity after the arrival of humans has also been documented in other locations, including Australia, where fires have recently taken their own toll on the country’s unique wildlife.
“This study is a great example of how we can use the past to portend the future,” Anthony Barnosky, a paleoecologist at the University of California, Berkeley, who was not involved in the work, said in an email. “And what we are seeing today — increasing human pressures combined with and actually causing climate change — is like this lesson from the past on steroids.” Dr. Barnosky added that these changes are not gradual, but quick and catastrophic.
The researchers noted that it was hard to absorb the similarity of current events to those in the fossil record. “Many of the most threatened wildlife today are the remaining large-bodied mammals that didn’t go extinct” at the end of the last ice age, Dr. Lindsey said. But, she added, “because we caused this, we have the power to stop it.”