The history of the Anglo-Saxons is the history of a cultural identity. It developed from divergent groups in association with the people's adoption of Christianity and was integral to the founding of various kingdoms. Threatened by extended Danish Viking invasions and military occupation of eastern England, this identity was re-established; it dominated until after the Norman Conquest. Anglo-Saxon material culture can still be seen in architecture, dress styles, illuminated texts, metalwork and other art. Behind the symbolic nature of these cultural emblems, there are strong elements of tribal and lordship ties. The elite declared themselves kings who developed "burhs", and identified their roles and peoples in Biblical terms. Above all, as Helena Hamerow has observed, "local and extended kin groups remained...the essential unit of production throughout the Anglo-Saxon period." The effects persist in the 21st century, as a 2015 study found the genetic makeup of British populations today shows divisions of the tribal political units of the early Anglo-Saxon period.