A grave with the remains of a mother and father huddled together with two sons has been dated to 4,600 years ago and marks the oldest genetic evidence for a nuclear family, researchers say.

The individuals were carefully arranged in their graves to denote they were part of a biological family, the researchers say. Wounds on the remains suggest the parents and kids were defending themselves against a violent raid, involving stone axes and arrows, at the time of their deaths.

The family grave is one of four burials discovered in 2005 near Eulau, Germany. All together, the burials hold 13 individuals, including adults ages 30 years and older, and children ranging from newborn to 10 years old at death.