(mellow music) (Emily) This is my favorite critter here.

This is a giant ground sloth.

(Shane) Yeah, what makes this your favorite?

(Emily) It’s a sloth and it was giant.

(laughter) (Shane) Need you say more?

(bicycle bell rings) Meet paleontologist Emily Lindsey.

Her commute through L.A.’s urban jungle helps her connect with the present... (relaxing music) ...but when she gets to the La Brea Tar Pits, her thoughts turn to the ancient world.

♪ (Emily) This is called a short-faced bear.

(Shane) The idea that there was an animal this large is deeply disturbing to me.

How much bigger is this than, like, a polar bear?

(Emily) A lot.

(laughter) ♪ (Shane) Today, La Brea is almost as iconic as Disneyland or Hollywood, drawing visitors from around the world.

But at the start of the 20th century, people flocked here for another reason.

(Emily) The L.A. basin is the richest oil basin of its size in the world.

In the early 20th century, we were producing about a quarter of the world’s oil.

(Shane) In those days, Rancho La Brea was a rich source of the densest form of crude oil: asphalt.

(Emily) They were digging it up and shipping it up to San Francisco, where it was being used to pave the roads.

(Shane) In some places, finding asphalt didn’t even require digging.

(Emily) And because the other thing we have a lot of in L.A. is earthquakes, there are places where these fissures occur in the ground.

(Shane) Here, the asphalt seeps up to the surface in shallow, sticky pools.

Before long, miners noticed bones-- a lot of them.

They assumed the bones belonged to cattle from the ranch they were digging on until a traveling geologist took a closer look.

(Emily) And he was like, "That’s not a cow bone."

It was the saber of the saber-toothed cat.

Exactly what part of the cow do you think this came from?

(laughing) (Emily) Over time, plants, animals, big, small, get stuck in there.

(Shane) At La Brea, the miners had stumbled onto the world’s largest deposit of Pleistocene fossils-- an entire ecosystem sealed in an asphalt time capsule.

Soon, a new kind of excavation was underway.

(Emily) In about two and a half years, they collected about a million fossils.

Whoa.

(rhythmic music) Is, like, this density, like, normal for the pits?

(Emily) Yeah, it’s just bone on bone, on bone, on bone, on bone.

It makes them really challenging to excavate.

♪ So, imagine playing, pick-up sticks.

But every stick is, like, a different size and shape and also fragile and also priceless.

And then, somebody has dumped a bunch of cement on top of it.

(Shane) In the last century, scientists have excavated over five million fossils here, representing 40,000 years of ecological history.