Urheimat  In historical linguistics, an Urheimat (from German "ur-" "original" and "Heimat", home, homeland) is the area of origin of the speakers of a proto-language, the (reconstructed or known) parent language of a group of languages assumed to be genetically related. Depending on the age of the language family under consideration, its homeland may be known with near-certainty (in the case of historical or near-historical migrations) or it may be very uncertain (in the case of deep prehistory). The reconstruction of a prehistorical homeland makes use of a variety of disciplines, including archaeology and archaeogenetics. The concept of a (single, identifiable) "homeland" of a given language family implies a purely genealogical view of the development of languages. This assumption is often reasonable and useful, but it is by no means a logical necessity, as languages are well known to be susceptible to areal change such as substrate or superstrate influence. Over a sufficient period of time, in the absence of evidence of intermediary steps in the process, it may be impossible to observe linkages between languages that have a shared Urheimat: given enough time, natural language change will obliterate any meaningful linguistic evidence of a common genetic source. This general concern is a manifestation of the larger issue of "time depth" in historical linguistics. For example, the languages of the New World are believed to be descended from a relatively "rapid" peopling of the Americas (relative to the duration of the Upper Paleolithic) within a few millennia (roughly between 20,000 and 15,000 years ago), but their genetic relationship has become completely obscured over the more than ten millennia which have passed between their separation and their first written record in the early modern period. Similarly, the Australian Aboriginal languages are divided into some 28 families and isolates for which no genetic relationship can be shown. The "Urheimaten" reconstructed using the methods of comparative linguistics typically estimate separation times dating to the Neolithic or later. It is undisputed that fully developed languages were present throughout the Upper Paleolithic, and possibly into the deep Middle Paleolithic (see origin of language, behavioral modernity). These languages would have spread with the early human migrations of the first "peopling of the world", but they are no longer amenable to linguistic reconstruction. The Last Glacial Maximum (LGM) has imposed linguistic separation lasting several millennia on many Upper Paleolithic populations in Eurasia, as they were forced to retreat into "refugia" before the advancing ice sheets.