for National Geographic News
Scientists have recreated part of the genetic code of an extinct, shrewlike creature that is thought to have been the most recent common ancestor of most placental mammals, including humans.
Placental mammals give birth to live young, and they descended from a common ancestor scientists simply call the "boreoeutherian ancestor." The creature scurried about the woodlands of Asia more than 70 million years ago.
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In recreating part of its genetic code, researchers say their goal wasn't to bring back the dead à la Jurassic Park (see sidebar). Rather, the scientists say their goal is to better understand human biology and evolution.
"The main reason we did this was to learn something about our [own] genome and the way genomes evolve within the mammal kingdom," said Mathieu Blanchette, an assistant professor in the school of computer science at McGill University in Montreal, Canada.
David Haussler, an investigator with the Howard Hughes Medical Institute at the University of California, Santa Cruz, joined Blanchette in his quest.
As Haussler explains, genomes read like a story, the text of which is written in a genetic alphabet of a's, c's, g's, and t'sshorthand for nitrogen-containing compounds known as bases: adenine, cytosine, guanine, and thymine.
The text of a genome contains instructions on how to build arms, legs, hearts, lungsan entire bodyand is stored in every mammalian cell.
Haussler noted that there is a common theme to the genome texts of placental mammals, since they are all derived from a common, ancient ancestor.
"A change here and a change there in this genome text converted it into a recipe for a human being. A different set of changes converted it into a recipe for a whale. A yet different set of changes into a recipe for a dog, and so forth," Haussler said.
To find out what those text changes were, Blanchette, Haussler, and their colleagues used a computer program to compare and contrast the known DNA sequences of 19 of the ancestor's living relatives, including species of pig, horse, cat, dog, bat, mouse, rabbit, gorilla, chimpanzee, and human.
The results allowed the scientists to reconstruct a DNA sequence common to each species, including their common ancestor, thought to be the mother of most placental mammals.
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