For over 1,000 years, megalithic tombs were a defining feature of the Paris Basin, but around 3100 B.C.E., construction suddenly stopped. Researchers now believe that a massive population decline across northern Europe created a “demographic vacuum” that allowed new demographics and cultures to move in. The study, published in Nature Ecology & Evolution, found that DNA evidence from a 5,000-year-old megalithic tomb near Paris reveals two distinct phases of burials with different genetic origins. One phase came from farming populations across the continent, while the other was linked to neolithic Iberia (Spain and southern France). The researchers suggest that infectious disease, environmental stress, and demographic contraction led to the population collapse, allowing neighboring groups like Iberian farmers to repopulate the area.
Source: https://www.popularmechanics.com/science/archaeology/a70929116/human-populations-disappeared