Preserved in Tar, Relics From Long Before Freeways

Enjoy this article! Ever since I was a little girl I wanted to visit the La Brea Tar Pits and now I work right beside it.  LOVE IT!

LOS ANGELES — No one expects to stumble across a cache of Picasso’s works in the middle of a desert. So who would think that just off bustling Wilshire Boulevard, tucked between the Los Angeles County Museum of Art and the national headquarters of the Screen Actors Guild, lie buried some of the most exquisitely preserved fossils in the world?

The fossils of the La Brea Tar Pits are just that. They were first discovered in Maj. Henry Hancock’s asphalt mine in the 1870s, when Los Angeles was but a village. Since the early 20th century, more than one million bones have been excavated from the pits; when reassembled, they provide an extraordinary time capsule of the creatures that roamed Southern California 10,000 to 40,000 years ago.

Interest in these animals today, however, is more than a matter of prehistoric curiosity. Many of the species found at La Brea disappeared altogether as the planet warmed at the end of the last ice age. The reasons for their demise are not yet fully understood, but may be especially pertinent to understanding the effects of climate change on animal populations today.

The tar pits have so many fossils precisely because of the tar, which one can still see bubbling to the surface in spots throughout Hancock Park. The gooey asphalt that trapped and entombed the animals turns out to be a great preservative. Thousands of perfect skulls and nearly complete skeletons representing more than 200 vertebrate species have been retrieved from the death trap.

Among them are many giant beasts, including mammoths, mastodons and the short-faced bear. (Only its snout was short; the bear stood more than 11 feet tall, much larger than today’s grizzly, polar and brown bears.) There are two species of bison — one of them with seven-foot horns — and some animals not typically associated with North America, including camels that stood taller than modern dromedaries.

Big cats, too, are well represented. Most famous is Smilodon fatalis, better known (but misleadingly so) as the saber-toothed tiger, a powerful predator named for its protruding seven-inch canines. More than 2,000 of them have been extracted from the tar pits.

And there was an even larger predator, the American lion, 25 percent bigger than the modern African lion. Imagine meeting one while jogging in Malibu.

These big animals and their relatively recent demise raise some big questions. How did they get here? What are their relationships to living species? And why did they all go extinct, and so close together in time?

One key to unraveling the mysteries is understanding just how different North America was at various times in the past. For example, during the last ice age, the maximum extent of glaciation was reached about 18,000 years ago. Glaciers covered all of Canada, the Great Lakes and New England. So much water was bound up in ice that sea levels were about 400 feet lower than they are today.

The lower sea levels exposed an enormous land bridge across the Bering Sea, known as Beringia. Thanks to light snowfall and winds coming off the Pacific Ocean, Beringia included largely snow- and ice-free grasslands that extended several hundred miles into each continent, and connected present-day Alaska and eastern Siberia. Animals could simply walk from one continent to the other.

Indeed, Beringia has served as a wildlife corridor at many times in the past — when sea levels allowed. That is how North American ancestors of today’s Arabian camel reached Eurasia about seven million years ago.

La Brea’s cats, on the other hand, are descendants of immigrants from Asia. The timing of their arrival and the identification of their ancestors are being deciphered not merely by the inspection of their bones, but also by analysis of the DNA within.

For example, it has not been clear from the fossil record alone whether the American lion is more closely related to the living African lion or to the extinct cave lion, whose bones have been found in Siberia, Alaska and western Canada. By analyzing DNA from many specimens of all three types of lions, Ross Barnett of Oxford University in England, Alan Cooper of the University of Adelaide in Australia, and numerous colleagues from the United States, Britain, Germany, Russia and Canada have determined that the American lion was more closely related to the cave lion than to the African lion. The American form appears to have evolved from a population that split off from a Beringian group and became isolated 200,000 to 300,000 years ago.

Analyses of ancient DNA by the same scientists have also helped resolve some uncertainty about the relationship of the saber-toothed Smilodon to other felines. It turns out that the name “tiger” is mistaken: Smilodon is not at all closely related to tigers or, indeed, to any living cat.

Instead, it belongs to an ancient lineage that broke off from the main line of cat evolution more than 11 million years ago. As for modern tigers, DNA analysis indicates that their closest living relatives are leopards and the African lion.

But the greatest remaining mystery about these magnificent ice age mammals is why so many of them disappeared. Fifty thousand years ago, the planet was populated with more than 150 genera of large mammals (weighing more than 100 pounds). By 10,000 years ago, nearly 100 had vanished.

Many ideas have been put forward to explain their disappearance, including climate change at the end of the ice age, or the impact or airburst of a comet. Evidence for the latter has been disputed, but one strong suspicion is that humans played a contributing role, at least for some mammals in some regions.

Archaeological and genetic evidence strongly indicates that humans first came to North America late in the last ice age, either overland via Beringia or by water along a coastal route. Whatever their mode of entry, the arrival of humans and the disappearance of large mammals have long appeared to be more than coincidence. There is concrete evidence of widespread hunting, including that of at least two species that became extinct.

But the pronounced changes in the environment that marked the end of the last ice age occurred at the same time, including abrupt warming of the planet, rapid retreat of the glaciers and widespread changes in vegetation. In that light, the La Brea fossils offer far more than a window into the past. They alert us to the catastrophic potential of combining climate change and human activity — a combination that is all too familiar today.

The La Brea Tar Pits, which still bubble with simmering asphalt, can be visited year round in Hancock Park in downtown Los Angeles. Ice age fossils are on display at the Page Museum (5801 Wilshire Boulevard; tarpits.org; (323) 934-7243), open from 9:30 a.m. to 5 p.m. every day but Independence Day, Thanksgiving, Christmas and New Year’s Day.

A version of this article appeared in print on January 24, 2012, on pageD2 of the New York edition with the headline: Preserved in Tar, Relics From Long Before Freeways