In Jurassic Park, ambitious scientists recreated dinosaurs from ancient DNA. Because genetic material more than about 50,000 years old cannot be reliably recovered using current techniques, the story lies squarely in the realm of science fiction. But new research suggests that scientists may soon be able to achieve the next best thing using a computer: assembling the complete genetic profile of a common ancestor of all placental mammals that lived more than 80 million years ago.

David Haussler, a Howard Hughes Medical Institute investigator at the University of California at Santa Cruz, and his colleagues wrote computer software to simulate evolution of DNA in different mammalian lineages. They first fed it information about a section of the genome called the CFTR locus of a variety of species, including the pig, horse, cat, dog, bat, mouse, rabbit, gorilla, chimpanzee and humans. The program then looked for changes in both individual nucleotides and sequences of DNA that were inserted or deleted. Eventually they arrived at a reconstruction of the CFTR locus of the mother of all placental mammals. By running the program over and over, the team was able to test the accuracy of the DNA reconstruction and determined that it was about 98 percent. Comparisons with genetic information from species not initially used, such as the opossum, confirmed the high rate of accuracy. "Previously it was thought that we could never really know what our ancestors looked like at the genetic level, but now it appears that we'll be able to tell," remarks Rasmus Nielsen of Cornell University. "And now that we know it is possible, I think we'll see many more attempts to do this."

Complete genetic information from more animals would be required to reconstruct the entire genome of the common ancestor, which was a small, furry nocturnal animal. Currently, nearly complete genomes are available for five mammals and Haussler predicts that about 20 would be required for an accurate complete reconstruction. "We will be able to trace the molecular evolution of our genome over the past 75 million years," he says. "It's a very exciting new way to think about our origins, a kind of DNA-based archaeology to understand how we came to be."

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