THE GENETICS AND EVOLUTION OF HANDEDNESS

At some point in hominid evolution, a mutation may have produced a ''d extral'' (D) allele, strongly biasing handedness in favor of the right hand and control of speech toward the left cerebral hemisphere. An al ternative (chance [C]) allele is presumed directionally neutral, altho ugh there are probably other genes that influence asymmetries and that may create a weak bias toward right-handedness (and other asymmetries ). Simulations show that the D allele could have spread quite quickly through a population, given even a minuscule advantage of CD heterozyg otes over CC and DD homozygotes in terms of reproductive fitness. This heterozygotic advantage would also explain the apparent stability in the relative proportions of left-handers and right-handers. This putat ive, uniquely human allele may have emerged with the evolution of Ho,n o sapiens in Africa some 150,000 to 200,000 years ago.