How Early Humans May Have Transformed L.A.’s Landscape Forever

FLORA LICHTMAN: This is Science Friday. I’m Flora Lichtman, filling in for Ira this week. For the next hour, it’s old things considered. We’re going way back in time to meet ancient creatures– think megalodon and T-rex– and hear what life was like in the really olden days. And we’ll revisit some of the world’s deadliest, scariest catastrophes, like the eruption of Mount Vesuvius, and try to survive them– using science, of course.

But first, let’s take a trip to Southern California, about 15,000 years ago. It was the ice age, and saber-toothed cats, woolly mammoths, and dire wolves prowled the landscape– until they didn’t. And for years, scientists have been trying to answer the question, why did these animals go extinct? A study offers new clues and suggests that wildfires might have been the nail in these critters’ coffins.

Joining me are two authors on this study, Dr. Emily Lindsey, Paleoecologist, curator, and Excavation Site Director at the La Brea Tar Pits and Museum, and Dr. Reagan Dunn, Paleobotanist and Curator, also at the La Brea Tar Pits and Museum in LA. Welcome to Science Friday. Thank you for being here.

EMILY LINDSEY: Thanks for having us.

REGAN DUNN: Hello, Flora. Thank you.

FLORA LICHTMAN: OK, Emily, take me to La Brea around 15,000 years ago, before the extinctions. What was it like there?

EMILY LINDSEY: So up until really around 15,000 years ago, you would have seen a sort of juniper woodland environment here. You had oak trees as well. And you had a landscape that looked something like the African savanna does today. You had big elephant relatives, the mammoths and mastodons, of course. You had big cats, like our iconic saber-toothed cat as well as American lions and jaguars, dire wolves. You had camels and horses and bison and even enormous hippopotamus-sized sloths wandering around the landscape.

FLORA LICHTMAN: Hippopotamus-sized sloths?

EMILY LINDSEY: It was incredible.

FLORA LICHTMAN: So Regan, what was it like in the after times?

REGAN DUNN: After the megafauna had perished, things were quite a bit different. All of those trees suffered. And what you had in their replacement was a chaparral vegetation, or shrubs and short-statured trees and also a lot of grass that are plants that are fire-adapted today. And these are the plants that dominate our Southern California chaparral ecosystems today.

FLORA LICHTMAN: What about animal life?

REGAN DUNN: Animal life following the extinction is much like the animal life of California today. So we have things like mountain lions and skunks and weasels and deer and foxes and some of these smaller-statured critters, especially coyotes, the sneaky coyote. It’s been around for a long time. And it visits the tar pits today even.

FLORA LICHTMAN: Well, I was going to ask, actually, how– do La Brea’s Tar Pits– are they a part of this story? Are they helping you sort of reconstruct what these ecosystems were like?

EMILY LINDSEY: Yes. So this is actually one of the most important fossil sites in the world, that just happens to be in the middle of modern Los Angeles. And the reason it’s so important is that, for about 60,000 years, these shallow pools of liquid asphalt have been trapping tens of thousands of animals as well as plants. So we have an incredible record of the southern California ecosystem more or less continuously from about 60,000 years ago up to the present.

FLORA LICHTMAN: Just a big pool of fossils?

EMILY LINDSEY: Many big pools of fossils that turned into underground asphalt preserved fossil deposits. And the preservation of the asphalt is so good that we can actually get a lot of chemical information out of the original bone and plant material that’s preserved, including very precise radiocarbon dates.

FLORA LICHTMAN: Wow. Let’s get to the– this megafauna extinction. Is it a mystery why these animals went away? I guess I assumed it was, like as usual, humans killing them or killing the food they ate. But is it more complicated than that?

EMILY LINDSEY: You know, this is a topic that has been debated for about 70 years now. And the reason it’s debated is that there were actually two processes happening at about the same time. So you have the Earth is coming out of the last major ice age, where we had more than half of North America covered in glaciers, for instance– same situation in Eurasia. So the Earth is undergoing these pretty extreme and pretty rapid climate warming, which of course is having significant impacts on plant communities and animal communities that depend on them.

And related to these processes, you also see human populations spreading out around the world. In some cases, they’re arriving on continents for the first time, or their numbers are increasing in places where they’ve been living for some time. And so there’s been these sort of two camps of scientists arguing about whether the climate changes or the human activities that were primarily to blame for this extinction event. And the reason we haven’t really been able to tease it out is that you have these processes happening almost simultaneously, and we haven’t, up until now, had a very good chronology or radiocarbon-dated record to know exactly when many of these species disappear.

FLORA LICHTMAN: Hm. OK, and so in your new study, you found that fire played a role. Will you tell me more about that, Regan?

REGAN DUNN: Yes, absolutely. Prior to the megafaunal extinction and, in fact, in the last 33,000 years of history in the Southern California area, there is very little evidence for fire from that early time period up until the extinction event about 13,200 years ago. There’s a little bit of charcoal, and that’s how we measure the wildfire occurrence from the bottom of Lake Elsinore. You just see this little blip in charcoal.

But then, beginning around 13,200 years ago, you see this dramatic spike in the amount of charcoal in the bottom of the lake. And what that represents is really about a 400-year-long period between 13.2 and 12.8 thousand years of pretty intense wildfire, or certainly a very enhanced wildfire regime. And it’s not like that any time previous in those 33,000 years. And following the extinction event, also, you see this fire regime continues into the Holocene.

FLORA LICHTMAN: Do you have a sense of what caused the fires?

REGAN DUNN: Yeah, the timing of the fires is certainly suspicious. We know that there had been significant warming in the 1,000 years before the fires began. And we know that there was significant drought pretty much coincident with the time period of the fires.

But also, the number of humans that were living in North America at that time was really increasing during that exact time period. And so we hypothesized that we have this climate that is primed for wildfire activity, and then we have the arrival of human beings, who brought with them their most effective tool, which they carry to every continent they go to. And that is fire. And in Southern California, these ecosystems, prior to this, were fairly naive to an ignition source like humans.

FLORA LICHTMAN: Hm. So basically, climate change set up the conditions, and humans lit the match. Is that the hypothesis?

REGAN DUNN: Yep, that’s exactly our hypothesis.

FLORA LICHTMAN: And how big a role did these fires play in these– in these animals’ extinction? I mean, would we still have woolly mammoths trundling around Beverly Hills if people weren’t lighting fires 13,000 years ago?

REGAN DUNN: [LAUGHS] That’s– well, that’s a good question. I was just thinking that today and wondering. I’m not really sure. There were a lot of different things going on in different areas. And what we describe in the paper is the extinction event that happened here in Southern California. And so what we hypothesize is that these animals, especially large animals, are going to be affected by these fires because a lot of these large animals migrate through these areas.

And so if you have intensive burning, you’re burning migration routes. You’re burning cover. The animals are lacking shade. You’re changing hydrologic regimes, so the water on the surface runs off. It could have created a more arid climate to begin with, just burning off all the vegetation– of course, a lack of resources just in the sense of just food. And so if you have that happening, these impacts happening, in enough places, that you’re going to slowly sort of windle– dwindle populations to a point where they have problems reproducing.

And then they– these isolated populations can’t reach each other. And you have this fragmentation going on, which then you have a reduction in the genetic drift. And so you can imagine how widespread fires could reduce the habitat in such a way that you would have reduction in the populations to the point where the other stresses– perhaps human hunting could have really taken them out, in addition.

FLORA LICHTMAN: Hm. Emily, does this study tell us anything about how we should be managing Southern California’s ecosystems today?

EMILY LINDSEY: You know, I think a real surprise that all of us came out of this study with was the realization that what we consider to be the native landscape or native ecosystem of Southern California– that’s our fire-adapted chaparral– never existed before humans showed up. And we know that, certainly going back millennia, and now it looks like potentially going back more than 12,000 years, humans have been actively managing that landscape. But it looks now like it was actually a human-created landscape.

And so I think when conservation biologists and land managers are thinking about how are we going to manage sort of the native vegetation and support native wildlife in this habitat, we really have to go back and consider, well, what is the role of humans in developing and managing this ecosystem because I think conservation biology has traditionally viewed sort of the best possible conservation conditions as removing all humans from a landscape and just leaving everything alone like it was before any humans were there. And we know that, at least in the case of our ecosystem here in Southern California, that’s simply not realistic or possible.

FLORA LICHTMAN: Totally fascinating. That’s all we have time for, but I want to thank you both for joining me.

REGAN DUNN: Thank you, Flora. It’s been a pleasure to be on the show this afternoon.

EMILY LINDSEY: Thanks for having us.

FLORA LICHTMAN: Dr. Emily Lindsey, Paleoecologist, Curator, and Excavation Site Director at the La Brea Tar Pits and Museum and Dr. Regan Dunn, Paleobotanist and Curator also at the La Brea Tar Pits and Museum in LA.

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