What separates a chimpanzee from a human? Thanks to a draft of the chimp genome released this week, researchers may be able to find a genetic answer to that question.
Many of the new findings come from aligning and comparing a rough draft of the chimpanzee genome--announced, but not published, in 2003--with the human genome. One of the most prominent results, reported by the Chimpanzee Sequencing and Analysis Consortium in Nature this week, confirms the oft-cited statistic that on average only 1.23% of nucleotide bases differ between chimps and humans.
But other kinds of genomic variation turn out to be at least as important. Insertions and deletions--or indels--have dramatically changed the landscape of the human and chimp lineages since they diverged about 6 million years ago, the researchers found. Some insertions contain duplications of segments of DNA, and these duplications contribute more genetic difference than do single base pair substitutions, notes Evan Eichler, of the University of Washington (UW), Seattle, who led a team analyzing the duplications.
So which changes made us human? Many of the insertions contain gene duplications, which in other organisms have fostered evolutionary novelty by allowing one copy of a gene to adapt to a new function without disrupting the original. "It'll be very exciting to see how many indels actually made a difference in our own evolution," notes David Haussler of the University of California, Santa Cruz.
Another Nature paper addresses an ongoing controversy about whether the human Y chromosome will vanish within some 10 million years. By comparing human and chimp Ys, geneticist David Page of the Whitehead Institute in Cambridge, Massachusetts, and colleagues found that although the chimp has lost a number of genes on its Y chromosome, the human Y chromosome has not lost a gene in 6 million years. "It seems like the demise of the hypothesis of the demise of the Y," says geneticist Andrew Clark of Cornell University in Ithaca, New York.
This first pass through the genomes does not reveal a type of gene that definitively marks us as distinct from the apes. But experts predict that a comparison of the chimp and human genomes will reveal many more secrets in the coming years. "It's wonderful to have the chimp genome," says geneticist Mark Adams of Case Western Reserve University in Cleveland, Ohio. "It's the raw material ... to figure out what makes us unique."
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