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- The La Brea tar pits captured dire wolves, sabertooth cats, and other megafaunal for millennia.
- Their fossils may show an extinction event 13,000 years ago.
- Some researchers blame human-set fires for their extinction, but others are skeptical.
From about 50,000 years ago, animals, vegetation, and other debris managed to get stuck in the sticky substance of the Rancho La Brea tar pits in present-day Los Angeles.
The millions of fossils found in the pits give scientists a time machine into the ancient past and reveal a mysterious mass extinction about 13,000 years ago.
Species including dire wolves, sabertooth cats, and camels simply vanished from the local archeological record.
Scientists have various theories about what caused so many megafauna — large animals that weigh at least 97 pounds as adults — to go extinct at around the same time, from the warming climate to human hunters to disease.
A new paper says that, at least in this region of Southern California, human-made fires may be to blame.
About 13,000 years ago, there was a huge spike in the amount of carbon found in a nearby lake. Fires likely caused the increase, lead author F. Robin O'Keefe said, and humans could've set those fires.
"Humans have a direct effect in lighting fires, but they also have an indirect effect by killing off the herbivores," O'Keefe said. "And then while all that's happening, it's also getting a lot warmer and a lot drier."
Missing dire wolves and constant coyotes
There are over 100 pits in Rancho La Brea's 23-acre park, Regan Dunn, assistant curator at the La Brea Tar Pits and Museum and a co-author on the paper, said.
The tar is actually naturally occurring asphalt, a low-grade crude oil bubbling up from the oil field below through. Eventually, water or leaves covered the muck. Unwitting animals stopped for a drink or tried to cross, becoming trapped.
But if too much water flooded a tar pit, it no longer captured animals. So, each pit is its own snapshot of an era, some 50,000 years old, some more recent. "They're kind of these discreet little tornadoes of fossil bone concentrations," Dunn said.
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Pit 61/67 is where the majority of the animals in the study came from. Researchers excavated it between 1913 and 1915, finding a trove of fossils.
The study's radiocarbon dating of the pit's bones showed an interesting trend. "There's lots and lots of megafauna, dire wolves, saber tooth cats, horses, bison, and coyotes," O'Keefe said. "And then suddenly that just stops and it's just coyotes."
By around 13,000 years ago, they were all gone. The pit had captured not just the animals, but evidence of an extinction event that the researchers think was due to human-made fires. But other experts are skeptical that humans were the cause.
For this paper, the authors wanted to study anything in the environment that may have contributed to the megafaunal extinction, including temperature, precipitation, vegetation changes, and fire events.
To get a fuller picture of the megafauna's landscape, they looked at a nearby lake — Lake Elsinore. Elsinore is the name of the castle in "Hamlet," a play where nearly every character dies. There's something poetic about using a lake with the same name in extinction research.
Around 13,200 years ago, charcoal accumulation rates at Lake Elsinore increased by 30 times earlier amounts. The spike lasted about 300 years. There may have been catastrophic fires or lots of smaller fires, O'Keefe said. During heavy rainfall, runoff thick with charcoal found its way to the lake.
The paper connects the dots between the elevated levels of carbon, increased fire activity, and humans. But paleontologist Ross MacPhee, who wasn't involved in the study, said there isn't direct proof to make such a claim. "I'm very cautious, very skeptical that the evidence is sufficient in any way to implicate humans," he said.
But if humans were starting fires in Southern California, there should be examples of that behavior in other areas, too. "If it was a good idea to cook all the meat that you possibly could by starting these fires, then you'd expect to see the same phenomenon repeated" in places like Northern Canada, which had large numbers of megafauna, MacPhee said.
In addition to the lake's charcoal levels, the researchers used its sedimentary, geochemistry, and pollen data to learn about the region's temperature and precipitation. Based on the pollen and sedimentology records, a severe drought was affecting the area 200 years before the megafauna disappeared, Dunn said. Changing climate also played a role in their dwindling numbers and the severity of the fires.
Around 14,700 years ago, the woodland began transitioning into shrubland. The leaves that the camels and the giant slots ate disappeared, and then so did those animals. Grass, flowering plants, and shrubs cropped up, all more suitable for horses and other grazing animals than camels.
The result was a less tree-filled, more open landscape than before. Several studies have shown an increase in wildfires after megafauna went extinct in certain regions. The loss of herbivores at La Brea could've contributed to a feedback loop, making fires — however, they started — more intense, the paper's authors say.
A decades-long debate among the scientific community
Since the 1960s, experts have debated the role of humans in wiping out mammoths, mastodons, and other large mammals thousands of years ago.
A 2018 study found that archaeologists often cite a combination of factors for Pleistocene megafaunal extinctions, while ecologists more often say humans were the tipping point or driving force behind it, a theory known as overkill.
Paelongotolist MacPhee cautions that there could be other explanations for why these animals in the 61/67 tar pits stopped appearing. "It's entirely reliant on whether you have grounds for believing that these really are first appearances or last appearances, as opposed to just the ones that you were lucky enough to find," he said.
Whatever was happening in Southern California may not have been occurring elsewhere, either. "You can't use conditions at one site to generalize to the entire continent," MacPhee said. For example, some of the mammals that disappeared from Rancho La Brea, like wild horses, were still found in the Yukon until about 6,000 years ago.
This isn't the first time researchers have linked humans' use of fire to an area's megafaunal extinction. A 2005 paper by biologist Guy Robinson and his colleagues found an up-to-10-fold increase in charcoal levels from 12,300-year-old sediments near Orange County, New York. The authors concluded that a combination of fire and changing climate were culpable in the local animals' demises.
It was a tough sell for some experts at the time. "Indeed, some paleontologists accuse Robinson of reading too much into ancient remnants of charred grass," Sharon Levy wrote in her book "Once and Future Giants: What Ice Age Extinctions Tell Us about the Fate of Earth's Largest Animals."
Humans on the move
Whether there was a sufficient number of people to do substantial damage to the ecosystem is MacPhee's question with any overkill theory. "So far, it's extremely difficult to estimate human density many thousands of years ago, especially in North America," Amelia Villaseñor, an assistant professor in the Univesity of Arkansas's anthropology department, said in an email.
"The human record in Southern California is really terrible," O'Keefe said. The only contemporary fossilized human from the area is the 13,0000-year-old Arlington Springs Man. His co-authors estimate there was a sharp increase in population around 13,200 years ago, enough to have a devastating impact on the ecosystem with the fires they set.
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While she agrees the loss of camels, horses, and megafauna had an effect on Southern California's environment 13,000 years ago, Villaseñor said she wouldn't draw the same parallels the paper does with fires in the past with those affecting the region today. "The controlled fires set by people [about] 13,000 years ago and even less than 10,000 years ago, at low population densities, are very different than the high density, fire-suppression caused fires of today," she said.