Migration from France replaced half of Great Britain's ancestry in Late Bronze Age. Event occurred 1200-800 B.C. in southern Britain including England and Wales. Supports Celtic languages arriving from France in Late Bronze Age. Challenges Iron Age or early Bronze Age Celtic theories. Large-scale movement explains language shift in pre-state societies. Celtic languages too similar to descend from 4500-year-old ancestor. Little migration in Iron Age. English have more early European farmer DNA than 4000-year-old locals. Discontinuity in farmer ancestry detected 3000 years ago. Lactase persistence for cow milk rose sharply in Britain 1200-200 B.C. Milk tolerance increased millennium earlier than central Europe. Adaptation boosted child survival rates. Study analyzed DNA from 793 ancient individuals. Undergrad Michael Isakov discovered migration signal. Second study reconstructed 27-person Neolithic family tree over five generations. Family showed polygyny, polyandry, adoption, patrilineal and matrilineal descent.