This common ancestor has no scientific name, Haussler said. It is known simply as the "boreoeutherian ancestor" because boreoeutheria are the largest clade, or group of species, of mammals.
The scientists are uncertain precisely when the boreoeutherian ancestor lived. But they say it definitely was more than 70 million years ago.
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Based on computer simulations and comparisons with living mammals, the researchers estimate their partial reconstruction of boreoeutherian ancestor's genome is 98 percent accurate.
The research was published in the December 2004 issue of the journal Genome Research.
The lead author of the study, Blanchette conducted his research while he was a postdoctoral student under Haussler at the University of California, Santa Cruz. Additional co-authors include Eric Green of the Human Genome Research Institute in Bethesda, Maryland, and Webb Miller of Pennsylvania State University.
Blanchette said knowing the genome of the ancestor to most placental mammals will allow scientists to see how individual genes have evolved and to better understand the role of genes in making an organism tick.
One example Blanchette points to is research led by Svante Paabo, a geneticist at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany. Paabo's investigation revealed that various mutations in a gene known as FOXP2 probably played a role in the evolution of speech in humans.
"By studying the history of this gene, [scientists] were able to understand the role of that gene," Blanchette said. "We hope to do that on a much larger scale, now on the whole genome."
For our boreoeutherian ancestor, Blanchette and colleagues have recreated a DNA sequence consisting of 1.1 million base pairs (the a's, c's, g's, and t's) around a region that flanks a gene linked to the disease cystic fibrosis.
The researchers chose the region because it had already been sequenced in a number of species as part of an effort led by study co-author Eric Green at the National Institutes of Health.
The complete genome of the boreoeutherian ancestor is estimated at three billion base pairs. That is roughly 3,000 times more base pairs than the segment Blanchette, Haussler, and their colleagues have recreated.
Blanchette said that once scientists sequence the complete genomes of several living placental mammalsa process that will take a few years and a lot of moneyscientists could conceivably reconstruct most of the ancestral genome with 98 percent accuracy.
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