In the Genes : Sabertooth a Relative of Modern-Day Big Cats, DNA Tests Show
Fourteen thousand years ago a saber-toothed cat got stuck in the treacherous asphalt of the La Brea Tar Pits.
Now, for the first time, scientists have succeeded in stripping away the asphalt and reading some of the genetic information hidden in the marrow of its fossilized bones.
Earlier this month, an article in the Proceedings of the National Academy of Sciences announced that scientists had extracted and analyzed DNA--genetic information encoded in biochemical form--from sabertooth bones found in the tar pits. By comparing the genetic fragments to those of living animals, the scientists were able to show that Smilodon fatalis, the sabertooth that long ago prowled what is now Hancock Park, is related to such modern-day big cats as the African lion and the tiger.
The work was supervised by Stephen J. O’Brien, director of the Laboratory of Viral Carcinogenesis of the National Cancer Institute in Maryland and a world-renowned expert on the genetics of cheetahs and other cats.
George T. Jefferson, associate curator at the George C. Page Museum of La Brea Discoveries and a co-author of the paper, said the breakthrough, which demonstrated that the extinct animal’s DNA could be separated from the asphalt that helped preserve it, “opens the door for more work with the over 1 million bones conserved at the Page Museum.”
Jefferson said scientists from around the world have already expressed interest in studying the genetics of such extinct La Brea species as the dire wolf, the American lion, the Columbian mammoth, the mastodon and the bison.
The Page collection is one of the largest caches of Ice Age fossils in the world. It includes more than 1 million fossilized mammal and bird remains, and almost a million other specimens, including insect and pollen microfossils. More than 600 species of mammals, birds and plants have been found in the tar pits since excavation began at the beginning of the century.
The collection should prove especially valuable to scientists, Jefferson said, because the specimens are so well-preserved. (An additional virtue of the La Brea fossils, he said, is that the bones have been accurately dated.) The asphalt that trapped the animals was also responsible for preserving their bones and the genetic information within them. On the downside, the asphalt vastly complicated the process of extracting the DNA.
Co-author Dennis A. Gilbert explained that standard methods of analyzing DNA could not be used until the asphalt was stripped away. So the scientists went in search of a solvent. Their approach was a practical one, he said. “We used very unscientific things. We used kerosene and gasoline that we knew removed tar from your feet when you went to the beach.” To Gilbert’s delight, kerosene worked.
The project was a special pleasure for Gilbert, he said, because he grew up in the San Fernando Valley (he went to Granada Hills High School) and visited the tar pits as a child. He works as a geneticist for a Maryland firm.
The discovery of DNA from an extinct species raises the “Jurassic Park” question, which the scientists who did the sabertooth study addressed with nary a sneer. In “Jurassic Park,” Michael Crichton describes how it might be possible to make new dinosaurs if a sufficient amount of their preserved DNA was found. Is there any chance of fashioning a living sabertooth from the genetic bits and pieces found in the tar pits?
Unlikely, the scientists said. Fragments of only two genes were isolated. That is two out of hundreds of thousands that determine the genetic makeup of a complex organism such as a big cat, Gilbert said. And even if the entire genetic code were available, scientists would have to find a way to implant that information into a cell that could process the information, make the requisite parts and create a living organism.
Dianne N. Janczewski, principal author of the paper, is also pessimistic. There are subtleties about DNA and the way it interacts with itself and its environment that make it unlikely that humans will ever be able to recreate an extinct animal from its genetic code alone, she said. “The romantic side of me believes there’s a piece of the puzzle we’ll never know.”
A zoologist, Janczewski works with with the U.S. Agency for International Development in Virginia.
Until the discovery of sabertooth DNA, all scientists knew about the sabertooth was what they could infer from its bones.
That’s quite a bit, according to Christopher A. Shaw, collections manager at the Page Museum. About the size and weight of today’s African lion, the sabertooth was built differently, with powerful front limbs that suggest that it preyed on animals much larger than itself. Scientists infer from its short tail, and other evidence, that it hunted by stealth, stalking and pouncing rather than running its quarry down. Cats such as cheetahs that chase their prey tend to have long tails for balance, Shaw explained.
The sabertooth probably roared like a lion: Throat bones called hyoids show evidence of flexible connections similar to those of present-day cats that roar, instead of the relatively inflexible connections typical of cats that don’t. On the basis of its bones, Shaw even speculates that the sabertooth was a social animal. Specimens in the museum’s collection of bones affected by accident or disease suggest that some sabertooths lived for years after losing their sabers and other ancient disasters. Shaw speculates that such animals survived on the scraps of kills made by fitter members of the group.
Shaw noted that scientists have long preferred the term “saber-toothed cat” to the once-popular “saber-toothed tiger.” Tigers are big cats that live in jungles. The sabertooth’s habitat at La Brea was a sagebrush plain dotted with a few trees.
Now that scientists know they can read the DNA of the La Brea fossils, they are thinking about what to look at next. “It would be nice to see what pathogens were around then,” Gilbert said. If a particular virus were widely found in the genetic material of an extinct species, it might point to the reason the animal disappeared.
Like Gilbert, Janczewski said studying the sabertooth was a special pleasure for a person whose childhood imagination had been piqued by an extinct cat with nightmare teeth. “It’s neat to work on a cat that has such a mystique about it,” she said. She also discovered that saber-tooth science had far more cachet than, say, the genetics of house cats. “At cocktail parties, it’s definitely something I tell my friends.”
Scientific name: Smilodon fatalis.
Description: About the size and weight of today’s African lion, the sabertooth was a powerful predator most notable for its enormous curved teeth. It probably stalked and pounced on its prey, rather than running it down.
Numbers: The second most common mammal recovered from the La Brea tar pits; the dire wolf is the most common.
Extinction: About 11,000 years ago, for unknown reasons.
Unanticipated honor: California’s state fossil.
Biggest mistake: Wandering onto the asphalt of the La Brea Tar Pits.