By extracting gene fragments from insects preserved in amber for up to 40 million years, two independent teams of scientists have obtained the oldest DNA ever recovered from fossils.
Both groups of researchers succeeded in extracting fairly long sequences of nucleic acids, the building blocks of genes, from extinct insects embedded in amber found in the Dominican Republic.
They said the successful extraction of DNA from ancient amber-embedded specimens proved that substantial amounts of DNA could survive for many millions of years, far longer than many scientists had believed possible.
Analyses of the DNA found in the specimens has improved understanding of the evolutionary lines to which the extinct insects belonged. Popular Theory Challenged
One analysis, which focused on a newly discovered extinct termite called Mastotermes electrodominicus, was conducted by Dr. Rob DeSalle, Dr. Ward Wheeler and Dr. David Grimaldi of the American Museum of Natural History in New York City, and John Gatesy of Yale University.Continue reading the main story
Their analysis, which is being published today by the journal Science, challenges a formerly popular theory that termites evolved from cockroaches. Instead, analysis of the DNA fragment extracted from the extinct termite, embedded in amber 25 million to 30 million years old, supports a view that termites and roaches evolved separately from a common ancestor.
Another investigation, the results of which were published this month in the British journal Medical Science Research, examined genetic material recovered from a 40-million-year-old extinct stingless bee called Proplebeia dominicana. The study was conducted by Dr. George O. Poinar Jr., an entomologist at the University of California at Berkeley, with Dr. Raul J. Cano, a molecular biologist at the California Polytechnic Institute at San Luis Obispo, Dr. David W. Roubik of the Smithsonian Institution, and Hendrik N. Poinar, the son of Dr. Poinar.
In both sets of experiments, amber was cracked open under sterile conditions, and body parts of the insects were treated with a succession of chemical agents to isolate minute quantities of DNA, far too little to analyze or study. But by using a chemical copying technique called the polymerase chain reaction, the scientists were able to make great numbers of molecular copies of the target DNA, enough to study and identify.
The oldest DNA previously recovered was from a 17-million-year-old magnolia leaf found in 1990, which was compressed between layers of shale excavated near Moscow, Idaho. Dr. Edward M. Golenberg, who isolated the magnolia DNA, identified it as part of a gene involved in photosynthesis. The oldest animal DNA previously recovered was extracted from the carcass of a mammoth that had been preserved in ice for 40,000 years. Quest for Dinosaur DNA
But paleobiologists and science fiction buffs dream of obtaining DNA still older than that recovered from ancient termites and bees -- perhaps even DNA from dinosaurs.
"Sooner or later," Dr. Poinar said, "we're going to find amber containing some biting insect that filled its stomach with blood from a dinosaur before getting trapped in the resin that eventually turned into amber. The blood may contain actual dinosaur DNA. That will be an exciting discovery."
Just such a prospect, first articulated by Dr. Charles Pellegrino, inspired the best-selling novel "Jurassic Park" by Dr. Michael Crichton, in which live dinosaurs are created from gene grafts of dinosaur DNA extracted from fossil flies in amber. Dr. Poinar himself, and several other scientists, are assisting Steven Spielberg's organization in making a movie based on the novel.
But in real life, the problems in reconstructing a dinosaur (or any other long-extinct animal) from fragmentary gene sequences would be staggering if not completely insoluble, experts agree.
The magnitude of the task is illustrated by the DNA sequence extracted by Dr. Cano and his colleagues from the ancient stingless bee. This DNA fragment is only one ten-thousandth of 1 percent of an entire gene, of which vast numbers are required to make up a single chromosome in a chain of chromosomes. Moreover, the DNA preserved in amber or any other medium is always damaged or degraded by time and environmental influences.
Neither the Museum of Natural History team nor the California group could even guess where the DNA fragments they extracted fit within any specific chromosomes of the insects they studied.
"The most anyone could ever get from fossil DNA is some molecular sequences that might be grafted into the genes of some living creature," Dr. Poinar said. "But that would require a great advance in cell biology. Remember, no one has yet cloned animal cells." Science Fiction and Reality
Still, the notion of creating animals similar to dinosaurs by grafting dinosaur DNA into living DNA from some modern species is not totally ruled out, some scientists believe. " 'Jurassic Park' is science fiction, of course," said Dr. Wheeler of the Museum of Natural History, "but Crichton has been careful to keep his speculations within the bounds suggested by real technology."
Amber mined in the Dominican Republic formed many millions of years ago from the sap of an extinct tropical legume called Hymenaea, Dr. Grimaldi said. Once the sap hardened as a plastic-like polymer, it became nearly as impermeable as glass. The amber polymer completely dries the body of any animal trapped in it and locks out any oxygen that would promote decomposition. The sap acts as a preservative in another way: it contains substances that kill bacteria that would otherwise rot tissue.
Dominican amber, regarded as a treasure house of biological specimens dating from the Miocene, Oligocene and Eocene epochs, is too young for the age of dinosaurs. Amber old enough to have formed while the dinosaurs were still alive occurs mainly in Lebanon.Continue reading the main story