LONDON (Reuters) - A 4,600-year-old grave in Germany containing the remains of two adults and their children provides the earliest evidence that even prehistoric tribes attached importance to the family unit, researchers said on Monday.

The researchers used DNA analysis and other techniques to determine that the group buried facing each other -- an unusual practice in Neolithic culture -- consisted of a mother, father and their two sons aged 8-9 and 4-5 years.

“By establishing the genetic links between the two adults and two children buried together in one grave, we have established the presence of the classic nuclear family in a prehistoric context in Central Europe -- to our knowledge the oldest authentic molecular genetic evidence so far,” Wolfgang Haak of the University of Adelaide said in a statement.

“Their unity in death suggests a unity in life.”

The remains were found in graves that held a total of 13 people, all of whom had been interned simultaneously, Haak and colleagues reported in the journal Proceedings of the National Academy of Sciences.

Several were buried face-to-face, with arms and hands interlinked in many cases. The remains included children ranging from newborns up to 10 years of age, and adults of around 30 years or older.

Tests showed that many had suffered massive injuries, suggesting they were victims of a violent raid. One female had a stone projectile point embedded in her back and another had skull fractures.

“Our study of the Eulau individuals shows that their deaths were sudden and violent, apparent in lesions caused by stone axes and arrows, with evidence of attempts of some of the individuals to defend themselves from blows,” the researchers wrote.

An analysis of dental remains also offered up insight into Stone Age society and showed that the females came from different regions than the males and their children.

This is evidence that men sought partners from different regions to avoid inbreeding and that it was customary for women to move to the location of the males, the researchers said.

“Such traditions would have been important to avoid inbreeding and to forge kinship networks with other communities,” Alistair Pike, an archaeologist at the University of Bristol, who co-led the study, said in a statement.