Tight Genes

You've got them.

Scientists compared the genetic sequences of ethnically and geographically diverse people from around the world and found that the genes which code for the nervous systems, had some sequence differences (known as polymorphisms) among individuals. By analyzing human and chimpanzee polymorphism patterns, genetic probabilities and various other genetic tools, and geographical distributions, they found evidence that some of these genes are experiencing ongoing positive selection in humans. They calculated that one genetic variant of microcephalin arose approximately 37,000 years ago, which coincides with the emergence of culturally modern humans, and it increased in frequency too rapidly to be compatible with random genetic drift or population migration. This suggests that it underwent positive selection. An ASPM variant arose about 5800 years ago, coincident with the spread of agriculture, cities and the first record of written language. It too is found in such high frequencies in the population, that it indicates strong positive selection.