Where cars now drive along congested roads in the heart of modern Los Angeles, sabre-toothed cats once roamed. They stalked prey that ranged from hoofed mammals to creatures resembling elephants. The ferocious cats competed with dire wolves, American lions and short-faced bears.
During the Pleistocene, the geological period that began just over 2.5 million years ago, North America experienced a series of ice ages. During these freezes and thaws, giant mammals thrived in the woodlands and savannahs of Southern California. Primitive cheetahs chased antelope-like pronghorns over miles of grasslands, while mastodons roamed in dense herds.
But suddenly, around 11,000 years ago, almost all of these species died. Mammoths, giant sloths and camels all disappeared completely from the Americas. Only one large plant-eater remained, nearly unchanged since it first began racing through the south-west 30,000 years ago: the pronghorn.
Nobody is really sure what caused the extinction event. It has variously been blamed on fluctuating temperatures and climates, the encroachment of man, invasive plants or new bacteria, or all of the above.
But perhaps the bigger question is, why did some species survive while so many died out? In particular, why was the pronghorn able to outlast almost every Ice Age herbivore and persist into the present day – with pretty much the same build and look as it did in prehistory, to boot?
It is not just an academic question. Figuring out how the pronghorn survived the mass extinction could help us understand the mass extinction that is now underway, and offer tips on how to rescue today's threatened species.
Pronghorns, technically known as Antilocapra americana, look much like they did thousands of years ago. Blindingly quick, with keen senses and adaptations, they persisted when so many others perished.
Their hair comes off as a predator defense
To understand their survival tools, I visited the Los Angeles Zoo and spoke to Josh Sisk, a curator specialising in pronghorns and other hoofed species ("ungulates"). We watched a dozen pronghorns as they fed on grain, a few feet away in an enclosure modelled after their desert habitat.
"They're the last remaining species from the Ice Age," Sisk says.
During the dry season, pronghorns regularly make round trips of 300 miles in search of greener pastures. Theirs is the longest land migration in the continental U.S.
They are also the fastest land mammal in the United States, with a top speed of nearly 60mph. This evolved to keep them out of the jaws of their old adversary, the cheetah.
But as that cat departed, wolves and coyotes became their main foe.
To battle these new enemies, Sisk says, the pronghorns developed some strange and effective adaptations. "Their hair comes off as a predator defense." That way predators get a mouthful of hair, not flesh.
The La Brea Tar Pits are a living museum surrounded by a city of 13 million people
That hair is hollow, Sisk adds, perfect for extreme cold and heat as they roam from Southern Canada down the Baja peninsula to Mexico. Padded hooves keep them light and sure-footed in rocky terrain, and they can go days without water, surviving off the mist that collects from the ocean. They have the largest tracheas for their body size among ungulates, allowing them to breathe in enough air to maintain top speed.
Finally, they have the same size eyes as an African elephant, proportionally quite large, in order to give them added defense against carnivores. "They've got to have that evolutionary adaptation from the Pleistocene and the Ice Age," Sisk says. Back then, there were many types of meat-eaters stalking the pronghorns. Bigger eyes mean better sight, he says, and better protection from attack, an adaptation that took root.
Most of us do not think of these antelope-like creatures as prehistoric, but they are living fossils. To find out how they managed to persist into the present day, I headed to one of the world's most famous fossil sites.
The La Brea Tar Pits are a living museum surrounded by a city of 13 million people. Buried in the asphalt-spewing oil is the richest collection of Pleistocene fossils found anywhere. Between 3 and 5 million fossils, representing over 600 species of animals and plants, have been uncovered since oil prospectors discovered bones here in the 1800s.
Just a few inches of tar is enough to totally immobilise an animal the size of a horse
Today, researchers are still uncovering fossils preserved by the sticky black tar, which continues to bubble up throughout the museum's 23 acres.
It is a weird juxtaposition of the very new and very old. Just across the street from the museum is a 99 Cents Only Store and a shiny underground subway, plus scores of businesses, apartments and people. There is also L.A.'s bustling Wilshire Boulevard, a street that sees hundreds of thousands of cars a day. Those cars are running on the same oil that spelled the end for so many creatures, but that also preserved their remains.
When the pueblo of Los Angeles was founded in the late 1700s, much of the surrounding environs were parcelled out as Mexican land grants. One such parcel, a mile east of el pueblo, encompassed parts of the future neighbourhoods of Beverly Hills, Hollywood, and Hancock Park. It would prove to be some of the rich city's richest real estate.
Explorers christened this plot "Rancho La Brea", or "tar ranch," after asphalt was first discovered by a 1769 Portuguese expedition. The name stuck.
The tar has been spewing for thousands of years. Early Native Americans used it as adhesives to line baskets and caulk canoes. But long before that, it was a threat to wildlife.
In 2006 an intact mastodon tusk was found by accident during the construction of an underground car park
Just a few inches of tar is enough to totally immobilise an animal the size of a horse. Ancient herbivores got trapped on the surface like flies on flypaper, and this attracted carnivores, who in turn became trapped. This cycle continued for millennia. It is no wonder the area is a treasure trove of bones.
When the oil boom hit California in the 1800s, the first fossils were discovered here in an asphalt mine owned by the Hancock family. They thought the bones were trapped domestic animals.
In 1875, William Denton, a geology professor at Wellesley College, was presented with a strange canine tooth. He realised that he was looking at an ancient fossil of a long-extinct sabre-toothed cat. He published his findings, but no one paid him or his work much mind, perhaps because Denton and his wife claimed the bones talked to them.
Real excavations began in 1906, and again in 1912-1913, led by researchers from the University of California Berkeley. The Southern California Academy of Science also dug from 1909 to 1911. The bones they discovered were in part the impetus for founding the LA County Museum, which itself performed over 100 excavations from 1913 to 1915, recovering over a million bones.
In general, smaller species with faster reproductive times tend to survive
More recently, in 2006 an intact mastodon tusk was found by accident during the construction of an underground car park.
Scientists from the La Brea Tar Pits and Museum are now busy cataloguing and unearthing thousands of tonnes of boxes of fossil-rich deposits, a painstaking endeavour known as Project 23.
"There's thousands, potentially hundreds of thousands of bones exposed just right here," says Sean Campbell, a site preparator. He shows me wolf and coyote bones, which sit next to smaller bird and rodent remains. Their sheer quantity is overwhelming.
This huge collection of fossils allows scientists to pick out patterns, in particular which species died and which survived. They have found quite a few pronghorns here, including one pronghorn subspecies that did go extinct.
There were actually two pronghorns: a dwarf that reached about 2ft tall and weighed about 20 pounds; and what we would think of as a modern pronghorn, with a white and orange short-coat, weighing upwards of 100 pounds. I saw some of those bones as Beau Campbell, the assistant lab supervisor and identical twin to Sean, led me through the cavernous collection.
Why did the dwarf fail and the larger-size pronghorn survive? Feverish debate continues.
The larger-size pronghorn now had the ideal environment, because grasslands do not provide much cover for predators
Even the museum's new assistant curator, Emily Lindsey of the University of California Berkeley, admits it is hard to track. There are even conflicting studies on just how important climate change was in the extinction, even among studies from La Brea, Lindsey told me.
In 2016, she co-authored a paper arguing that the North American megafauna died off, at least in part, as a result of human impact. But Lindsey says that is not the whole story.
Survival was also based on size. "In general, smaller species with faster reproductive times tend to survive," Lindsey says. But strangely, one of those smaller species, the dwarf pronghorn, did not hang on. Why?
Under curator John Harris, Lindsey's predecessor, studies were published suggesting that plants were stressed by low levels of carbon dioxide in the air. In other words, plants were starving because they could not photosynthesise enough. Plants then had a hard time growing and reproducing.
That may be one of the reasons the dwarf died out. No trees means no cover, and for a small creature, that can make them an open target. As the large plants stunted, and herbivores died because there was not enough food to support them, so did the large predators that preyed on them.
But even among the predators, not every species lost out.
Some larger predators shrank and ditched their specialised traits, adopting more generalised survival strategies. Wolves, bears, coyotes and cougars all came through. These "winning" members of the Carnivora order underwent successful and minimal evolutionary changes.
Despite surviving these climactic upheavals, pronghorns are now facing a new and potentially worse threat
Meanwhile, a world with less trees became a world of open grasslands. While many other herbivores who normally competed for food were now dying out, the larger-size pronghorn now had the ideal environment – because grasslands do not provide much cover for predators – and less competition.
To get a clearer picture of this time, and in particular why certain branches of the evolutionary tree stayed intact while others fell off, the museum is delving into the region's ancient food-web dynamics.
That will all help add pieces to the puzzle, Lindsey says. But it seems that despite surviving these climactic upheavals, pronghorns are now facing a new and potentially worse threat: us.
There are five subspecies: the relatively stable American and Oregon pronghorns (which are sometimes lumped together); the sonoran, with a population of about 500 in the U.S. and Mexico; the peninsular pronghorn, which counts as few as 50 to 150 left in the Vizcaíno Desert of Baja; and the Mexicana subspecies. The latter three are all endangered.
Thanks to heavy hunting by man and the loss of the once-vast grasslands and chaparral of Southern California, the peninsular subgroup that once lived in California is extinct. It is also critically endangered in Mexico, due to years of hunting, ranching and habitat destruction. Genetic testing shows that the animals in Mexico are identical to those formerly found in California, meaning a higher probability of successful mating and potential reintroduction.
The biggest problem facing them now is human encroachment and the inability to migrate
There were once over 35 million pronghorns ranging in the West. By the end of the 1800s, around the time Denton discovered fossils in La Brea, those herds were reduced by a staggering 99%. Early conservation boosted those numbers in the U.S. but populations in Mexico have dropped by 80%.
In total, there are about 700,000 pronghorns in the wild, with the World Wildlife Fund reporting that the species are on a steady decline. But it is a fractured group. While Wyoming boasts more pronghorns than anywhere else on the continent, elsewhere results have been nearly the polar opposite.
In Southern California, Arizona, and Mexico, wild populations have been decimated by infrastructure and development. "The biggest problem facing them now is human encroachment and the inability to migrate," says the curator Sisk.
In hopes of saving the remaining wild peninsular pronghorns in Mexico, the LA Zoo has teamed with a host of zoos in the U.S. and Mexico to raise and repopulate the subspecies.
The program began in 1998, starting with just five fawns. U.S. zoos became involved in 2002. Today there are over 450 captive animals.
There is talk of reintroduction, but it will likely take decades to build up a viable population
The pronghorns are first hand-reared in Baja. Once the fawns are weaned, they are driven up the coast to San Diego and then L.A., where they are raised to adulthood. Six fawns came to the Zoo in July 2016, Sisk tells me, pointing out the females he has reared.
It is a species insurance policy, I am told. The captive herd is a backup, should disease or famine wipe out the remaining wild pronghorns in Mexico.
But the work has not been easy. The team has tangled with red tape and the physical challenge of traveling to a remote region with no airport, and no paved roads for hundreds of miles, 14 hours from the border.
Will pronghorns return to the Southern California chaparral they roamed thousands of years ago?
Sisk hopes so, but the first goal is to bolster the population down south, genetic duplicates of the animals that once roamed here. There is talk of reintroduction, but it will likely take decades to build up a viable population, and heavy coin in a new era when federal dollars are not readily available.
The captive herd is a backup
What is more valuable to understanding this species, and perhaps the next step in our current climate shift, is the efforts of researchers like Sisk, Lindsey and the Campbell brothers, who are working to get a clearer idea of what kept this species going when so many others perished. Lindsey hopes that this approach could be an indicator that conservationists can employ with other animals.
"The major extinction event recorded in the Tar Pits, the disappearance of the large Ice Age mammals, was almost certainly caused by a combination of environmental changes and human actions," she adds. "If we can get a better sense of how these two factors interact to drive extinctions, that might help us determine what species and ecosystems are most at risk today."
In a strange twist, the animals buried in the ground may be just as important as the ones still roaming.
Join over six million BBC Earth fans by liking us on Facebook, or follow us on Twitter and Instagram.
If you liked this story, sign up for the weekly bbc.com features newsletter called "If You Only Read 6 Things This Week". A handpicked selection of stories from BBC Capital, Culture, Earth Future and Travel, delivered to your inbox every Friday.