A group of researchers has re-created with remarkable accuracy part of the genome of the common ancestor of all placental mammals, a small shrew-like creature that prowled the forests of what is now Asia more than 80 million years ago. By comparing DNA sequences of 19 species of existing mammals, including humans, the researchers have reconstructed a large segment of DNA in the species from which all of today's placental mammals arose. They estimate that the reconstruction is 98 percent accurate.

The project, which was led by When geneticists hear that most DNA from the genome of a species extinct for many millions of years can be re-created with 98 percent accuracy, "jaws occasionally drop," said Haussler. "It sounds implausible. But there's enough information to reconstruct the ancestral genome on the basis of mammals that live today. We just need to sequence the genomes of these living mammals." The reconstructed ancestral genome will offer an invaluable vantage point from which to watch evolution at work.

According to Rasmus Nielsen, a geneticist at Cornell University who is familiar with the work, the paper is guaranteed to turn heads. "Previously it was thought that we would never really know what our ancestors looked like at the genetic level, but now it appears that we'll be able to tell," he said. "And now that we know it is possible, I think we'll see many more attempts to do this."

Efforts to extract DNA from fossils generally have been disappointing because DNA molecules break down over time. "Ancient DNA decays faster than [science fiction writers] would like," said Haussler. After a maximum of about 50,000 years, DNA sequences typically are too fragmented to be pieced together. Geneticists therefore have turned to a technique that has been called "[computerized] paleogenomics" to infer the DNA sequences of past organisms.

All of the placental mammals living today are descended from
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Contact: Jim Keeley
keeleyj@hhmi.org
301-215-8858
Howard Hughes Medical Institute
30-Nov-2004

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