People love to talk, and such chattiness may have catalyzed a divergence from chimpanzees. A clue to how that may have happened lies in deaf populations where sign language has facilitated marriage between individuals with the same type of recessive deafness, an example of assortative mating.

Walter Nance, the human genetics professor at Virginia Commonwealth University who recently modeled such relaxed selection on connexin 26 deafness,1 sees a bigger picture. "Assortative mating brings together rare genes with similar effects, and provides the opportunity for evolution and selection to act on rare combinations of genes."

Nance points to the FOXP2 gene. If early humans with dominant speech-enabling FOXP2 variants could communicate and that led to sex, then the same "linguistic homogamy" that today has increased the prevalence of one form of hereditary deafness might have selected and accelerated speech acquisition.

Says Douglas Baynton, historian at the University of Iowa...

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