Scientists have used DNA testing to uncover the earliest evidence that Stone Age man lived in nuclear families.
An international team of researchers dated remains from four burial sites discovered in Germany in 2005.
The 4,600-year-old graves contained groups of adults and children buried facing each other, which was an unusual practice in Neolithic culture. One of the graves contained a female, a male and two children.
Using DNA analysis, the researchers established that the group were a mother, father and their two sons aged around eight or nine and four or five years old. They say this is the oldest molecular genetic evidence of a nuclear family in the world.
The burials, discovered and excavated at Eulau, Saxony-Anhalt, were also unusual for the way in which they were buried.
The remains of 13 individuals were found, and all had been interned simultaneously. Several pairs of individuals were buried face-to-face with arms and hands interlinked, and all the burials contained children ranging from new-borns to 10-year-olds to adults of about 30 or older.
Many showed injuries that indicated they were the victims of a violent raid.
One female had a stone projectile point embedded in one of her vertebra and another had skull fractures. Several bodies also had defence injuries to the forearms and hands.
The researchers used genetics and isotope techniques to reconstruct along with physical anthropology and archaeology.
In an article published this week in Proceedings of the National Academy of Sciences, lead author Dr Wolfgang Haak of the University of Adelaide said: "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.
“Their unity in death suggests a unity in life."
Dr Haak added: "However, this does not establish the elemental family to be a universal model or the most ancient institution of human communities.”
As well as establishing the biological relationships of the people buried at Eulau, the researchers were also able to shed light on their social organisation using strontium isotope analysis.
Strontium isotopes in the teeth were measured to give an indication of where these people spent their childhood, as strontium from food eaten is incorporated into teeth as they grow.
This revealed that females spent their childhoods in a different region from the males and children in the group.
The remains are now on permanent display in the Landesmuseum Sachsen-Anhalt in Germany.
PA