How an age of fire wiped out California’s mammoths

Before the fires came, Southern California was ruled by giants. 

Thirteen thousand years ago, the lush coastal forest of what is now greater Los Angeles was patrolled by a menagerie of large mammals that have long since vanished from the continent.

Dire wolves, camels, cave bears, bison and the great mastodon — all these creatures shared the California forest for thousands of years with a relatively recent arrival: a clever primate with an aptitude for hunting and a deep love of fire.

They stalked the southern half of the great coastal forest that today extends from San Francisco to the Alaska panhandle and that once stretched all the way down the coast. And then, as those Oregon-like forests vanished to be replaced by the dry, scrubby and fire-prone chaparral of modern Southern California, they disappeared. 

The question of why those megafauna died off has long bedeviled paleontologists and archaeologists, with researchers suggesting that the climate killed them off, or an asteroid impact, or human hunting bands.

But a new paper published Thursday in the journal Science suggests the key ingredient in the extinction was human-caused fires — positing that as the climate changed, those fires helped light the match that blew up the ancient forests and the large animals that had called it home.

That transition, the paper indicates, represented a dramatic fire-induced tipping point between the prehistoric “megafaunal woodland” that once defined Southern California to today’s human-dominated chaparral, a land of scrub and more regular wildfire. 

The findings offer a grim reminder of the extent to which apparently stable ecosystems of the Earth can flip in a period of decades — a point that is particularly pertinent in light of the ongoing change in the northern half of that primordial forest belt, the coastal forests of the Pacific Northwest, which are becoming ever more vulnerable to fire as they dry out under climate change.

“It’s a classic behavior of dynamical systems: starting at one stable state, going through a chaotic moment, and then stabilizing somewhere else,” said co-author and archaeologist Robin O’Keefe of Marshall University. 

In this case, “chaotic moment” is a euphemism for a new age of apocalyptic fire that wiped away the failing remnants of the woodlands — and many of the largest animals that had lived there.

While many factors set the stage for this shift — like a generally drying climate — researchers now believe that prehistoric humans inadvertently kicked it off in what O’Keefe called “a perfect storm.”

“Humans are the big red arrow. I don’t think you’d get a punctuated change in ecosystems without humans,” he said.

The theory the Science team presented in some ways resembles a much older one for why the megafauna of North America went extinct — the so-called Overkill Hypothesis, which holds that prehistoric humans wiped out cave bears, giant sloths and mammoths through overenthusiastic hunting.

“But it’s not that easy,” O’Keefe said. “North America is a really big place. And even if you show up with 10 of your friends, you know, it’s gonna take thousands of years for you to get a population size that’s big enough to have any kind of an impact.”

By the time the megafauna vanished from Southern California 13,000 years ago, humans had had thousands of years to build up their numbers — a July study found 18,000-year-old human remains in a rock shelter in Oregon, and a 2021 study found 23,000-year-old fossil footprints in Arizona.

But the biggest flaw in the Overkill Hypothesis is that humans lived for millennia alongside — and off the flesh of — the big mammals of North America before they disappeared. Why, then, did the system finally collapse?

To answer that question, scientists took advantage of a grim collection of relics that endured through the collapse of the coastal forests: the fossilized bodies of hundreds of unfortunate prehistoric mammals trapped in and slowly swallowed by the La Brea Tar Pits, near what is now the Miracle Mile of central Los Angeles. 

Recent advances in carbon dating have transformed the bubbling hydrocarbons of La Brea — and the animals that died there — into a treasure trove of quantitative data.

“It used to be this big pot of bubbling oil. So there was no, you had no idea what things have been deposited, because you can have a really old bone next to a really young bone,” O’Keefe said. 

But recent advances in radiocarbon dating — which tracks the predictable decay of radioactive carbon isotopes in bones to determine precisely how old they are — helped the science team to create a relatively precise timeline of what animals were present in the region, and when they disappeared.

By comparing this with the fossil record from the rest of the continent, researchers concluded that these animals went extinct in Southern California a thousand years before they did in the rest of North America.

And in the sediments of nearby Lake Elsinore, they found the residue of a contemporary culprit: a thick layer of charcoal that is the residue of ancient fire.

Fire would not have been unknown in the lush woodlands of Los Angeles, said O’Keefe — in fact, it would have been common, brought by human migrants on their long maritime and overland journey down the coast.

For at least 6,000 years after their presumed arrival in the West — dates that many native nations dispute — humans would have shared the California forests with now-vanished animals like camels, a situation they encouraged by burning the forest to create good grazing and easy kills.

While human numbers were relatively small — and in particular, while the climate was relatively wet — this may have been sustainable, O’Keefe said.

But he said that the hunters likely did not realize that, even as the land changed around them, they too were changing the land. Every dead herbivore meant — because there was one less grazer — more grass piled up to burn; every human baby meant more dead herbivores, and more dead grass.

And ultimately — and perhaps imperceptibly — that meant more fuel, which meant more destructive fire, which gradually reduced the ability of the landscape to retain water, which made it more vulnerable to fire in turn. 

Something that humans also set, directly, almost everywhere in our long global exodus out of Africa. 

In complex systems theory, this is called “hysteresis,” or a positive feedback loop: a kind of system in which every swing of the pendulum in one direction drives an even wilder swing in the other direction — until the system finally snaps.

“The ecosystem gets stressed more and more with humans, and the number of humans goes up and up — and then you have the phase transition and chaotic interval,” O’Keefe said.

From a systems theory perspective, he said, “Those fires are just the sign of a chaotic climate,” part of a process by which one ecosystem shifted to another. 

But “just” in that case conceals catastrophe, he emphasized. “The fire when it happens was severe enough that it’s enough to directly cause enough mortality — and destroy enough food — that it’s killing off the megafauna.”

O’Keefe said this ecosystem transition would likely have happened without human influence — it’s part of the long-term cycles of glaciation and thawing that dominate North America.

“Southern California has been through glacial periods and interglacials many times over the last million years,” O’Keefe said. 

“And every time we saw very similar vegetation changes — but the megafauna never went extinct. It would dry out, and it would get wet again, and this happened a bunch of different times.”

When the final axe fell, only one thing was different, he said: “Humans were present. And we think it was [necessary] for them to light the match, you know, so as climate change made the ecosystem more vulnerable, that humans were there to push it over the edge.”

It is possible for that conclusion to lead to grim places, he acknowledged. “In my not-better days, I really freak out about — our entire species has seen the increase in wildfires over the last 50 years. And is this the beginning? Are we getting into another catastrophe, or can we change our trajectory into a more stable kind of pathway?”

But of all the differences between us and these prehistoric Californians, one in particular stands out, O’Keefe said: “We have knowledge. People before us maybe didn’t have that. But if we can try to be more intentional about our trajectory ecologically, as a species, maybe we can avoid a catastrophic interval.”

He added that “from a policy standpoint, again, it is in society’s best interest to avoid another chaotic regime.”