The science of the comparative study of the vocabularies of languages for measuring linguistic change through absolute time. By studying the rate of change, the length of time (time depth) during which two related languages developed independently may be calculated. Glottochronology relies on statistical comparison of the basic vocabulary shared by two or more related languages and on the assumption that the rate of vocabulary replacement is constant over sufficiently long periods of time. It is a way of arriving at a date of separation between two languages that have a common origin by studying the extent to which they have diverged from each other and provides archaeologists with approximate dates for the origination of subcultures diverging from each other. For instance, in Alaska the great difference between the Aleut language and the other Eskimo languages is thought to have been the result of the cultural isolation of the Aleuts from the 3rd millennium BC onwards. It is a controversial method.