“I kept thinking the project was a disaster because I couldn’t find any difference,” Mary-Claire King once mused. Ironically, it was the absence of any difference that made the results of her Ph.D. project so remarkable.
While working at the University of California, Berkeley, in the early 1970s, the burgeoning geneticist compared the amino acid sequences as well as the immunological and physical properties of several proteins isolated from humans and chimpanzees. These three measures all indicated to King and her Ph.D. supervisor, Allan Wilson, that only a small genetic distance separated the two species. (An organism’s genetic material directly specifies each protein’s structure and, hence, physical and immunological properties.) In fact, King uncovered a 99 percent agreement in the amino acid sequences of several human and chimpanzee proteins.
While King felt disappointment with her inability to unmask any genetic or biochemical differences between humans and chimpanzees, the scientific community did not. In fact, *Science,* one of the world’s most prestigious science periodicals, published the article she co-wrote (with Wilson) on the molecular comparisons between human and chimpanzees as the cover piece for the April 11, 1975 issue - a rare honor indeed.
King and Wilson predicted, at that time, that gene regulation must account for the biological and behavioral differences between humans and chimpanzees. In other words, they did not regard the high percentage of genetic similarity between human and chimpanzee genes as particularly meaningful. (I discussed this point on 11/15/07.)
In the last few years, scientists have demonstrated that gene expression patterns between humans and chimpanzees differ, particularly with regard to brain tissue. (See Who Was Adam? for a detailed discussion of this work.) This new insight validates the early ideas of King and Wilson.
Two recent studies uncover added evidence for gene expression differences between our species and chimpanzees. One investigation compared gene expression patterns in humans and chimpanzees by looking at both mRNA and protein production and concluded that bona fide differences in gene expression exist for the two primates.
Another study identified differences in the promoter regions of a number of genes in humans and chimpanzees. Promoters are segments of DNA associated with genes that control gene expression. The genes most impacted are those associated with neural activity and nutrition-related functions.
Scientific advance continues to demonstrate that humans and chimpanzees do display significant genetic differences. These differences relate to how the genes are used, not the sequence differences in those genes. The two species differ genetically where it counts. Therefore, to say that a human is 99% chimpanzee is a meaningless statement.
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