In 1975, Mary-Claire King and the late Allan Wilson, both then at the University of California, Berkeley, showed that the genetic distance between humans and chimpanzees is simply too small to account for the dramatic anatomical and behavioral differences between the two species.1 No matter what method scientists used to measure genetic distance--protein electrophoresis, DNA hybridization, immunology, or amino acid sequencing--the result was always the same: Humans and chimpanzees are 98.7% genetically similar.

"The molecular similarity between chimpanzees and humans is extraordinary because they differ far more than sibling species in anatomy and way of life," King and Wilson argued. "Is it possible, therefore, that species diversity results from molecular changes other than sequence differences in proteins?" In particular, they suggested that differences between humans and chimps are perhaps based, not on dissimilar gene sets, but rather on gene expression divergencies. The thought was, maybe the degree to...

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