The ancient cemetery of Pommerœul, Belgium, was classified as Gallo-Roman in the 1970s', yielding 76 cremation graves and one inhumation. However, subsequent radiocarbon analyses dated the inhumation to the Late Neolithic (4th-3rd millennium calBC). We report osteoarchaeological analysis indicating that the inhumation was composed of bones from multiple individuals, afterwards buried as "one". Ancient DNA analyses also finds evidence of multiple individuals and revealed another surprise: the cranium is post-Neolithic and genetically related to a pair of siblings from another Belgian Gallo-Roman site. This composite burial may have been created in Late Neolithic times, with Gallo-Romans adding the cranium, or alternatively the burial may have been fully assembled in the Gallo-Roman periods. This exceptional burial documents unexpected burial practices for both prehistoric and Roman times.