By Julian Siddle
Science Reporter, BBC News
The graves contained mainly women and children
The oldest genetically identifiable nuclear family met a violent death, according to analysis of remains from 4,600-year-old burials in Germany.
Writing in the journal PNAS, researchers say the broken bones of these stone age people show they were killed in a struggle.
Comparisons of DNA from one grave confirm it contained a mother, father, and their two children.
The son and daughter were buried in the arms of their parents.
Dr Wolfgang Haak, from The Australian Centre for Ancient DNA, in Adelaide, conducted the DNA analysis. He says the scientific evidence supports the idea that they were indeed a family.
"We're really sure, based on hard biological facts not just supposing or assuming."
The bodies of the family were intertwined
In total, the four graves contain 13 bodies, eight children aged six months to nine years and five adults aged 25 to 60.
In two graves, DNA was well preserved, which allowed comparisons between the occupants. One of these contained the nuclear family, while the other grave contained three related children and an unrelated woman. The researchers suggest she may have been an aunt or stepmother.
These stone age people are thought to belong to a group known as the Corded Ware Culture, signified by their pots decorated with impressions from twisted cords. In their burial culture all bodies usually face south.
In the family grave the adults did face south, but the children they hold in their arms face towards them. The researchers say an exception to the cultural norm was made so as to express the biological relationship.
We don't know how hard daily life was back there and if there was any space for love
The care with which the bodies were laid out shows that whoever buried them must have known who they were says Dr Haak. He adds he was moved the first time he saw the grave.
"You feel some kind of sympathy for them, it's a human thing, somebody must have really cared for them. Normally you should be careful in archaeological research not to allow feelings in that make us base judgements on modern ideas, we don't know how hard daily life was back there and if there was any space for love."
Teeth hold clue
As well as looking at the DNA of each individual the researchers examined deposits of the element strontium in their teeth.
Found in rocks and soils, strontium is taken in from food as teeth grow in childhood. It can act as an indicator of where people came from.
The children and adult males had the same type of strontium - which was also found locally, but the nearest match to the women's teeth was at least 50km away, suggesting they had moved to the area.
Life in central Europe could be violent in the stone age
Dr Alistair Pike from Bristol University, who carried out the strontium analysis, says this indicates a culture of exogamy or marrying out.
"It's a bit like kings and queens in Europe in the past, creating an alliance by marrying out sons and daughters. This creates a bond between communities - useful if your harvest fails or if you need help fighting a war."
The most grisly aspect of the find is the manner of their death. Dr Pike says it was violent.
"They were definitely murdered , there are big holes in their heads, fingers and wrists are broken."
At least five of the individuals show the effects of a violent attack, one even had the tip of a stone weapon embedded in a vertebra.
They suffered a violent death
Wolfgang Haak says that as most of the people in the graves were women and children it is probable that most of the adults were elsewhere at the time of the attack, perhaps out fighting or working in their fields.
"They returned home to the village and found their loved ones dead. It's an assumption, but the most plausible explanation."
Researchers say such violence fits with what we know about life in central Europe at the time - the area had fertile soils, a stable climate and natural access routes. This made it a desirable place to live, but also created competition amongst its inhabitants, leading to violent confrontations when one community tried to displace another.